299 results on '"152.14"'
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2. Visual attention in naturalistic scenes across the lifespan
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Nicholls, Victoria
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152.14 - Abstract
Visual attentional skills continue to develop through childhood and do not reach maturity until adolescence. On the other end of the spectrum, older adults' visual attentional skills are declining with age. The development and decline of these skills can lead to difficulties in day to day activities such as throwing or catching a ball, cycling, crossing a road, or even maintaining stability when walking. Alongside this, children and older adults are among the most vulnerable groups in road crossing situations, with older adults accounting for almost 50% of road crossing fatalities in the EU. A link has been suggested between visual attentional control skills and the vulnerability of older adults and children to pedestrian accidents but little has been done to investigate this link. In this PhD, I set out to investigate in fine- grained detail the involvement of attentional control skills in road crossing decisions in children, younger, and older adults. To this aim four experiments were run. The first experiment tested younger adults and children from five to 15 years old in a road crossing situation where participants had to watch videos of road traffic and decide when they could safely cross the road. The participants' eye movements were recorded. I found that younger children made riskier crossing decisions compared to older children and young adults. Younger children were also less able to inhibit attentional capture by distractors and were less able to disengage overt attention from moving targets when the visual load was high. In the second experiment, I used a similar paradigm with young and older adults. My findings revealed that older adults were less able to inhibit attentional capture by distractors compared to younger adults. Despite this attentional bias, older adults made safe crossing decisions. This experiment involved only one direction of traffic and more complex situations (several traffic directions, different speeds, large field of view) might be more taxing for older people and impact their abilities to make safe crossing decisions. As such, in the third experiment I used a virtual reality set-up in order to test scenarios of varying complexity. I also tracked the participants' eye movements across a wide field of view (180°). My results showed that older adults were able in simple situations to make safe crossing decisions and they chose larger gaps between vehicles than younger adults. In more complex situations such as when cars travel faster, older adults made more risky crossing decisions. In experiments one and two, participants looked predominantly at the point where cars appeared on the road and did not overtly follow the cars down the road. This finding suggested a dissociation between overt and covert attention in the context of road-crossing. In order to explore this dissociation and its potential deficit in children and older people, I developed a technique using in conjunction eye-tracking and steady state visually evoked potential (SSVEP). In this paradigm, participants overtly tracked a moving object and covertly monitored the appearance of a new object at the appearance point. I found a drop in the SSVEP power signal prior to the appearance of the second moving object while the participants' eyes were still overtly tracking the first object. This result suggests during smooth pursuit a decrease in attentional resources allocated to the foveated object when there is a shift of covert attention towards a second object. In future studies, I aim to use this paradigm to explore more precisely the dynamic of overt and covert attention in more realistic scenario and with children and older participants. This research used novel approaches to address the socially relevant and timely question of pedestrian safety. To this aim, I used a variety of methods ranging from eye-tracking to image processing, EEG and VR, and I developed new techniques tailored to the questions at hand. For the first time, I directly investigated the relationships between visual exploration, road crossing decisions and changes in attentional control through the lifespan. My findings show that children below the age of 10 are less able to inhibit attentional capture by distractors, which increased the risk of unsafe crossing decisions. In similar, simple situations, older adults also show an attentional bias towards distractors, but they maintain the ability to make safe crossing decisions. VR experiments with systematic manipulations of the complexity of the road crossing scene revealed that older adults make riskier crossing decisions in specific situations such as when cars travel quickly, or from different directions. This research furthers our understanding of attentional control through the lifespan as well as providing insights for pedestrian safety. As such, it provides avenues for the development of training and safety guidelines for pedestrians. more...
- Published
- 2021
Catalog
3. Colour constancy : cues, priors and development
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Wedge-Roberts, Rebecca Jane
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152.14 - Abstract
Colour is crucial for detecting, recognising, and interacting with objects. However, the reflected wavelength of light ("colour") varies vastly depending on the illumination. Whilst adults can judge colours as relatively invariant under changing illuminations (colour constancy), much remains unknown, which this thesis aims to resolve. Firstly, previous studies have shown adults can use certain cues to estimate surface colour. However, one proposed cue - specular highlights - has been little researched so this is explored here. Secondly, the existing data on a daylight prior for colour constancy remain inconclusive so we aimed to further investigate this. Finally, no studies have investigated the development of colour constancy during childhood so the third aim is to determine at what age colour constancy becomes adult-like. In the introduction, existing research is discussed, including cues to the illuminant, daylight priors, and the development of perceptual constancies. The second chapter contains three experiments conducted to determine whether adults can use a specular highlight cue and/ or daylight prior to aid colour constancy. Results showed adults can use specular highlights when other cues are weakened. Evidence for a daylight prior was weak. In the third chapter the development of colour constancy during childhood was investigated by developing a novel child-friendly task. Children had higher constancy than adults, and evidence for a daylight prior was mixed. The final experimental chapter used the task developed in Chapter 3 to ask whether children can use specular highlights as a cue for colour constancy. Testing was halted early due to the coronavirus pandemic, yet the data obtained suggest that children are negatively impacted by specular highlights. Finally, in the general discussion, the results of the six experiments are brought together to draw conclusions regarding the use of cues and priors, and the development of colour constancy. Implications and future directions for research are discussed. more...
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- 2021
4. In the face of consciousness : how emotion, orientation, and gaze modulate face perception
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Lanfranco Guevara, Renzo Carlo, Rabagliati, Hugh, and Carmel, David Podhortzer
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152.14 ,consciousness ,face perception ,emotion processing ,visual perception ,EEG ,psychophysics ,interocular suppression ,unconscious processing ,perceptual awareness - Abstract
Human faces convey essential information for social behaviour, such as information about others’ mental states and intentions. Crucially, many studies have claimed that several facial features such as configural facial information, emotional expressions, and gaze direction modulate how faces gain access to perceptual awareness. However, the procedures employed in said studies suffer from multiple methodological issues and limitations. In a series of experiments, I tested whether configural facial features, emotional expressions, and gaze direction modulate how faces gain access to awareness. To achieve this, I used stringent procedures that allow measurement of perceptual sensitivity and decision criterion to the location and identity of faces. I used these measures to assess how long it takes faces to reach awareness as they overcome Continuous Flash Suppression – an interocular suppression technique that can render images invisible for several seconds. Using classical and Bayesian analyses, I found that configural face processing (which occurs for upright, but not inverted faces) promotes faces’ access to awareness. Similarly, faces making eye contact gain access to awareness faster than faces looking away. Contrary to past claims, however, I found that faces expressing negative emotional expressions (anger or fear) do not enter awareness faster than neutral expressions. In another series of experiments, I measured the minimal exposure durations required for configural facial processing, emotion processing, metacognition, and conscious access. To this end, I used a newly developed LCD tachistoscope that can present images with sub-millisecond precision and examined both behavioural (psychophysical) and neural (electroencephalography) markers of processing. I found that configural face processing promotes faces’ access to awareness by showing that upright faces require shorter exposure durations than inverted faces to be seen. Crucially, only around four milliseconds of exposure were required to find this advantage. Fearful expressions, however, do not gain access to awareness faster than neutral expressions. Evidence from neural markers expanded this by showing that the exposure duration required for configural facial processing is the same as that required for faces to reach conscious access. Finally, around six milliseconds of exposure were required for emotion processing. Together, these findings shed light on the factors that affect access of faces to awareness: configural facial information and gaze direction can modulate faces’ access to perceptual awareness; and such modulation is due to perceptual sensitivity rather than decision criterion. Furthermore, the perceptual processing of faces follows a hierarchical pattern: configural information precedes and facilitates access to awareness, and emotion processing follows awareness. more...
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- 2020
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5. Interpersonal influences in human visual attention : from behaviour to EEG
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Avendan~o Diaz, Juan
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152.14 - Abstract
Human evolution has shaped us into social animals, who are continually immersed in social interactions, constantly performing tasks with others and sharing our reality with them (Dunbar, 2003; Tomasello, Carpenter, Call, Behne, & Moll, 2005). For many of these interactions to be successful, it is necessary to pay attention to the same spatial locations with other individuals. Surprisingly however, this specific low-level aspect of our social life (i.e., attending to the same spatial locations with others) is not well understood. The present PhD work aims at contributing to this understanding by investigating whether paying attention towards the same spatial location with another person modulates one’s attention performance, along with its social, cognitive, and neural implications. In this line, the classic visuospatial sustained attention paradigm (e.g., Eimer, 1996; Mangun & Hillyard, 1988; Mangun & Buck, 1998) was adapted so it could be independently performed by two people (a dyad) sitting next to each other, to examine how visual attention performance (reaction times, RTs) is interpersonally modulated when an experiment partner is paying attention to the same or a different spatial location (aka., dual attention paradigm). In this paradigm (Experiment 1), participants performed a visual go/no-go task, responding to visual targets while attending to the same vs. different spatial location than the experiment partner. A typical attention effect was present in RTs (i.e., faster responses to targets appearing at the attended locations compared with those at the unattended locations) when the dyad attended to different locations. This attention effect, however, was significantly reduced when the participants shared the attentional locus (aka., dual attention effect). This pattern was reversed when single participants performed the task in isolation (Experiment 2), suggesting that the reduction in the attention benefit was socially driven between individuals (interpersonally). Additional experiments showed that the dual attention effect persisted under an increased perceptual load (Experiment 3), was not modulated by the group membership status attributed to the task partner (i.e., social closeness; Experiment 4), and disappeared once the partner was performing the task from a separate room (i.e., physical closeness; Experiment 5). Finally, an electroencephalography (EEG) study (Experiment 6) investigated the neural underpinnings of the dual attention effect, focusing on the information processing stage(s) influenced by dual attention. The aim was to understand whether the dual attention effect took place at a sensory level vs. a cognitive control stage. Event-related potentials (ERPs) and neural oscillations suggested that the effect was driven by a cognitive control process, and also showed an enhancement in the early sensory level information processing in the brain. Both the N2b ERP component and mid-frontal theta oscillations pointed towards a stronger need for control when sharing the attentional locus with another person in the dual attention task, while the P1 component yielded an enhancement in the attention effect in the attention sharing condition. The P1 effect may be top-down driven through alpha band long-range communication from prefrontal to posterior areas. Likely higher order processing related accounts were proposed for the current findings (e.g., linked to response inhibition, or mentalising/monitoring others). The current thesis made the first attempt to place dual attention as a bridge between the general shared attention perspective (Stheynberg 2015) and the overt behavioural interplay characterising joint attention and joint action. In addition, the present results could have ubiquitous real-life implications, and may give us some clues about how to optimize daily performance in dual-attention- like environments (e.g., classrooms/working spaces). more...
- Published
- 2020
6. Colour constancy for glossy objects under complex lighting environments
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Morimoto, Takuma and Smithson, Hannah E.
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152.14 ,Human Colour Vision ,Vision Science ,Experimental Psychology - Abstract
Colour constancy refers to our visual ability to identify the colour of objects under different illuminations. As a result of research over decades, our understanding of the mechanisms underlying this ability has advanced considerably. However, a significant limitation of past studies has been that they have mostly employed oversimplified experimental stimuli that only partially reflect the properties of surfaces and illuminations in the natural environment. Stimuli were mostly matte, two-dimensional, and uniformly illuminated by single illumination. In the real world, in contrast, we know that objects sometimes include gloss (perceptual attribute of specular reflection), and their shape is usually three-dimensional rather than flat. Moreover, an object placed in the natural environment receives illumination from every direction as light is reflected from other objects in the scene, and the spectra of those lights might vary from one direction to another. In this thesis, I report five research projects. The first three are all experiment based, and they were designed to find mechanisms that underpin human colour constancy for three-dimensional glossy objects that are placed under complex lighting environments. We conducted first an experiment where observers were asked to judge surface and illuminant colour changes for glossy objects under point light sources. The second and third experiments explored our ability to identify surface colours of glossy objects under environmental illumination that introduces a complex spatial pattern of specular highlights on the surfaces. All experiments used computer-rendered objects that take account of the complex interaction between material and illumination in the real world. Collectively these experiments suggest that observers are able to extract surface colour, even with complex stimuli in which simple heuristics are unlikely to be reliable. Instead, the results suggest that observers may build up complex internal models from which they can segregate colours of surface and illumination. The fourth chapter is based on field work that was conducted in collaboration with Prof. Sergio Nascimento at University of Minho, Portugal. We report newly collected hyperspectral environmental illuminations recorded in eight outdoor scenes and four indoor scenes. The aim was to characterise the directional spectral variation that occurs in natural environments and its application to more precise simulation of objects using computer graphics. Subsequent fifth chapter was motivated by the striking variation of illuminant spectra in natural environments. We revisited a classical problem in human colour vision, called illuminant metamerism where two distinct surfaces appear the same under a specific illumination. We showed theoretically that this metamerism is less of an issue than previously thought thanks to the presence of directional variation of illumination. more...
- Published
- 2020
7. The phenomenology of visual experience
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Tan, Li Li, Sliwa, Paulina, and Crane, Tim
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152.14 ,visual perception ,visual experience ,phenomenology ,high-level perception ,visual recognition - Abstract
Our visual experiences of the world around us deliver information about the visible features of the objects we see. What these features are, however, is still up for debate. On the one hand, most philosophers agree that if our visual systems are functioning normally, "low-level" features such as colours, shapes, sizes, and movements undoubtedly figure in the phenomenal character of our visual experiences, or "what it is like" for us to visually experience the world. On the other, many have argued for the expansionist view that various "high-level" features figure in visual experience over and above low-level features. These include kind features (e.g. hibiscus-ness, armchair-ness), biological features (e.g. animacy), and facial expressions (e.g. surprise). The aim of this thesis is to defend the opposing restrictivist view that the visible features of objects that figure in our visual experiences are limited to low-level ones. The first part of the thesis argues that this debate between expansionists and restrictivists should be resolved by identifying an independently-motivated criterion to determine the features that figure in visual experience. A criterion based on visual discrimination is suggested and developed, which has the result that high-level features do not figure in visual experience over and above low-level features. The second part considers the expansionist strategy of showing that high-level features must be introduced to explain how visual categorisation and recognition work. It is argued that this strategy does not succeed, and that a restrictivist account of visual recognition turns out to be better than the expansionist one. The third part focuses on the expansionist claim that objects visually appear to us to be mind-independent. It is argued that this claim is false, because the best account of mind-independence as a perceptible feature requires us to appeal to our proprioceptive sense in addition to visual perception. more...
- Published
- 2019
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8. Role of foveal vision in static and dynamic environments
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Clayden, Adam Christopher, Fisher, Robert, and Nuthmann, Antje
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152.14 ,foveal ,unity ,dynamic ,static ,environments ,saccade ,optic flow ,scotoma ,TEA ,T.E.A. ,compensation effect - Abstract
The visual field has multiple regions, with visual acuity being highest in the centre before declining rapidly outward toward the periphery. This central region, otherwise known as the fovea, is typically defined as the central 2.0o of vision. Although comparatively small with respect to other visual field regions, being able to discern objects of interest in fine detail is only possible in this region. Due to this, people make ballistic eye movements (saccades) towards the fine details and depending on the task, may stabilise their gaze in the form of a fixation to discriminate parts of this newly attended area. A typical everyday task that can exhibit such behaviours is visual search (scanning a visual environment for objects or features among distractors), and much of this thesis is concerned with the importance of foveal vision with respect to visual search tasks. Seven experiments are presented in this thesis, with chapters 1 - 4 containing a review of the literature, methodologies and a glossary. In Chapter 5, search experiments with targets of varying sizes were conducted to assess the role of foveal vision on search performance. This chapter builds on a debate on whether foveal vision was necessary for the successful completion of a visual search task. In addition, a novel algorithm was developed to embed targets at a specified change in local contrast to automate target placement. The presented results show that the time taken to find targets with and without foveal vision is nearly identical even when target size is small. Chapter 6 modulates search difficulty by manipulating salience to investigate the effect of target size and salience on search performance. Coined the Compensation Effect, the results of Chapter 6 show that the above-mentioned variables were able to compensate for one another, resulting in an improved search performance. This effect occurred regardless of visual field degradation. In the same chapter, another experiment revealed the stage in the search process where performance costs originated from, which was the final stage of search concerned with target verification. Finally, Chapter 7 transitions from static images to dynamic scenes which simulate self-motion. Additional algorithms were developed, including an extension to the existing gaze-based decomposition of search time (Malcolm & Henderson 2009). Chapter 7 investigated the role of foveal vision in visual search whilst optical flow was present. Unlike previous results, the final experiments of this thesis revealed the necessity of foveal vision for the attainment of a normal search performance. The results of this thesis demonstrate that the importance of foveal vision with respect to visual search is modulated by the stimulus environment, with it being relatively unimportant for static scenes, but important for dynamic scenes. more...
- Published
- 2019
9. A common framework for visual crowding in typical and amblyopic vision
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Kalpadakis-Smith, Alexandra V.
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152.14 - Abstract
Amblyopia is a developmental disorder of vision characterised by reduced acuity in one eye despite optical correction. When associated with strabismus, foveal vision is impaired by crowding: objects that are readily recognised in isolation become indistinguishable in clutter. In typical vision, crowding is minimal in the fovea and increases in the visual periphery. According to pooling accounts, the increase of crowding in the periphery arises from the integration of adjacent objects to promote perceptual homogeneity where sampling is insufficient and neurons have large receptive fields. It is unclear whether amblyopic crowding represents the same process. In this thesis I characterise amblyopic crowding, and investigate whether it can be understood within the same pooling framework as peripheral crowding. First, I show that amblyopic crowding systematically shifts the appearance of crowded objects to promote perceptual homogeneity, matching the perceptual effects in the periphery. A model simulating pooled responses of populations of visual neurons accurately characterises these effects in amblyopic and peripheral crowding, suggesting a common underlying mechanism. Second, I investigate the pattern of amblyopic crowding across the visual field and its neural correlates. I show that amblyopic crowding is elevated relative to typical vision in both fovea and periphery. At a group level, the increase of crowding in amblyopia and the typical periphery matches the increase of fMRI population receptive field (pRF) estimates in V1, V2, and V3, but at an individual level there is no correlation. Finally, I investigate the effects of higher-level grouping processes by examining whether uniformity in global clutter configuration modulates amblyopic crowding. I find that in most cases, clutter disrupts recognition in amblyopia regardless of global configuration, suggesting that contrary to the periphery, amblyopic crowding is largely unaffected by higher-level grouping processes. Therefore, on the whole pooling provides a successful framework for both amblyopic and peripheral crowding. more...
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- 2019
10. I expect, therefore I see : individual differences in visual awareness
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Andermane, Nora
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152.14 ,BF0241 Special senses. Vision. Visual perception - Abstract
Predictive processing theories posit that awareness of the visual world emerges as the brain engages in predictive inference about the causes of its sensory input. At each level of the processing hierarchy top-down predictions are corrected by bottom-up sensory prediction error to form behaviourally optimal inferences about the state of the visual world. Research suggests there may be individual differences in predictive processing mechanisms such that some individuals are more reliant on prior knowledge, whereas others assign more weight to sensory evidence. Predictive processing biases are thought to manifest in a range of typical and atypical perceptual experiences including proneness to perceptual illusions, sensory sensitivity in autism, and hallucinations in psychosis. The overarching aim of this thesis was to investigate whether in the general population predictive processing biases predict individual differences in visual awareness. Change blindness was selected as the central paradigm of investigation, as it can be conceptualised as a failure to incorporate a novel change into the current prediction about the state of the visual world. The empirical work in Chapter 2 aimed to characterise individual differences in visual change detection using naturalistic scenes and to identify the perceptual and cognitive measures that predict noticing ability. There were reliable individual differences in change detection that generalised to ecologically valid displays. The ability to notice visual changes was predicted by the strength and stability of perceptual predictions, as measured by the accuracy of visual short-term memory and attentional control in the face of distractors. In Chapter 3 I used voxel-based-morphometry to investigate whether inter-individual variability in brain structure predicts individual differences in visual awareness. The latter was assessed by the change blindness task as well as its strongest predictor measures (visual short-term memory, attentional capture, and perceptual rivalry). Regions of interest (ROIs) were selected in the parietal and visual cortices based on previous evidence that these areas are causally involved in the awareness of visual stimuli. This study aimed to discover whether the average grey matter density in the ROIs predict susceptibility to CB. The ROI-based analyses revealed the average grey matter density in left posterior parietal cortex predicted visual short-term memory accuracy but none of the other hypothesised relationships were significant. Chapter 4 aimed to measure individual differences in the reliance on prior knowledge by employing the Mooney face detection task. In this task participants disambiguated faces in two-tone degraded images before and after the presentation of the original versions of the images. Better change detection was predicted by Mooney face detection without any prior knowledge of the images, a measure of ‘perceptual closure' or an ability to generate a gestalt of a scene. The attention to detail subscale of the autism spectrum also predicted superior change detection. Reliance on prior knowledge in visual perception (assessed by improvement in Mooney face detection after seeing original images) did not consistently predict atypical perceptual experiences associated with the autism spectrum or schizotypy. Chapter 5 was an investigation into, firstly, whether there is a general predictive processing bias, which manifests across different methods of inducing prior knowledge, or whether such a bias is paradigm-specific and, secondly, whether reliance on priors predicts perceptual experiences and traits. All prior manipulations in this study lead to an increased tendency to see the expected stimulus in a binocular rivalry display, except adaptation, which lead to a suppression of visual awareness. Attentional control, perceptual priming, expectancy, and imagery loaded onto a common factor, suggesting that the strength of selective attention is closely linked with the facilitatory effect of expectation. The strength of adaptation predicted superior change detection and perceptual priming predicted the propensity to experience perceptual illusions. Taken together, these findings suggest that there are reliable individual differences in visual change detection, and these are predicted by the strength of visual short-term memory representations, attentional control, perceptual closure ability, as well as the strength of low-level adaptation. Possessing expectations facilitates the entry of the corresponding percept into awareness, irrespective of the method of prior induction. The facilitatory effect that priors exert on visual awareness across different methods is closely linked with the ability to exert attentional control. This suggests that the effects of expectations on awareness may be attentional. However, predictive processing biases were method-specific in that a facilitatory effect using one prior induction method will not necessarily predict the magnitude of the effect using a different method. Some prior effects (e.g., perceptual priming, imagery, and adaptation) yielded correlations with perceptual experiences and traits in the general population. As the research in this thesis is correlational, future studies will need to delineate the effects of expectation, attention, and adaptation on visual awareness and explore the neural representations of these mechanisms. more...
- Published
- 2019
11. Less than meets the eye : the diagnostic information for visual categorization
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Zhan, Jiayu
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152.14 ,BF Psychology ,Q Science (General) - Abstract
Current theories of visual categorization are cast in terms of information processing mechanisms that use mental representations. However, the actual information contents of these representations are rarely characterized, which in turn hinders knowledge of mechanisms that use them. In this thesis, I identified these contents by extracting the information that supports behavior under given tasks - i.e., the task-specific diagnostic information. In the first study (Chapter 2), I modelled the diagnostic face information for familiar face identification, using a unique generative model of face identity information combined with perceptual judgments and reverse correlation. I then demonstrated the validity of this information using everyday perceptual tasks that generalize face identity and resemblance judgments to new viewpoints, age, and sex with a new group of participants. My results showed that human participants represent only a proportion of the objective identity information available, but what they do represent is both sufficiently detailed and versatile to generalize face identification across diverse tasks successfully. In the second study (Chapter 3), I modelled the diagnostic facial movement for facial expressions of emotion recognition. I used the models that characterize the mental representations of six facial expressions of emotion (Happy, Surprise, Fear, Anger, Disgust, and Sad) in individual observers. I validated them on a new group of participants. With the validated models, I derived main signal variants for each emotion and their probabilities of occurrence within each emotion. Using these variants and their probability, I trained a Bayesian classifier and showed that the Bayesian classifier mimics human observers' categorization performance closely. My results demonstrated that such emotion variants and their probabilities of occurrence comprise observers' mental representations of facial expressions of emotion. In the third study (Chapter 4), I investigated how the brain reduces high dimensional visual input into low dimensional diagnostic representations to support a scene categorization. To do so, I used an information theoretic framework called Contentful Brain and Behavior Imaging (CBBI) to tease apart stimulus information that supports behavior (i.e., diagnostic) from that which does not (i.e., nondiagnostic). I then tracked the dynamic representations of both in magneto-encephalographic (MEG) activity. Using CBBI, I demonstrated a rapid (~170 ms) reduction of nondiagnostic information occurs in the occipital cortex and the progression of diagnostic information into right fusiform gyrus where they are constructed to support distinct behaviors. My results highlight how CBBI can be used to investigate the information processing from brain activity by considering interactions between three variables (stimulus information, brain activity, behavior), rather than just two, as is the current norm in neuroimaging studies. I discussed the task-specific diagnostic information as individuals' dynamic and experienced-based representation about the physical world, which provides us the much-needed information to search and understand the black box of high-dimensional, deep and biological brain networks. I also discussed the practical concerns about using the data-driven approach to uncover diagnostic information. more...
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- 2019
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12. Observations on invisibility : an investigation on the role of expectation and attentional set on visual awareness
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Tompkins, Matthew L. and Davies, Anne Aimola
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152.14 ,Psychology, Experimental ,Metacognition ,Magic ,Perception ,Cognition ,Illusion ,Magicians - Abstract
This thesis is concerned with the psychology of magic and illusion. In particular it is focused on three illusions, which can be conceptualized as types of invisibility: (1) Illusions of omission - failures to see, (2) Illusions of commission - seeing things that are not really present, and (3) metacognitive illusions - people's false beliefs about their own cognitive and perceptual systems. The work presented in this thesis is set out to explore these illusions through behavioural visual experiments inspired by sleight-of-hand magic tricks. Across three distinct paradigms using stimuli ranging in complexity from static line drawings, to recorded videos, to live events, I demonstrate that manipulations of observers' expectations and attentional set can result in perceptions of visual events that are variously accurate representations, illusions of omission, or illusions of commission. I also demonstrate that these illusions are often associated with failures of visual metacognition, in that they are generally considered to be surprising and counterintuitive. In addition to these empirical elements of the project, I also consider historical and contemporary connections between experimental psychology and magic tricks. I show that, in some instances, magicians' misdirection techniques anticipated developments by experimental psychology by hundreds of years, and that the idea of investigating the mechanisms of magic tricks and illusions played a key role in the development of experimental psychology as a scientific discipline. Through this combination of historical analyses novel experiments, I show that the integration of magic and experimental psychology has a great potential to drive future research in human cognition and perception. more...
- Published
- 2018
13. Higher-level representations of natural images
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Miflah, Hussain Ismail Ahamed
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152.14 ,higher level processing ,natural images ,Psychology ,Brain and behaviour ,Eye movement - Abstract
The traditional view of vision is that neurons in early cortical areas process information about simple features (e.g. orientation and spatial frequency) in small, spatially localised regions of visual space (the neuron's receptive field). This piecemeal information is then fed-forward into later stages of the visual system where it gets combined to form coherent and meaningful global (higher-level) representations. The overall aim of this thesis is to examine and quantify this higher level processing; how we encode global features in natural images and to understand the extent to which our perception of these global representations is determined by the local features within images. Using the tilt after-effect as a tool, the first chapter examined the processing of a low level, local feature and found that the orientation of a sinusoidal grating could be encoded in both a retinally and spatially non-specific manner. Chapter 2 then examined these tilt aftereffects to the global orientation of the image (i.e., uprightness). We found that image uprightness was also encoded in a retinally / spatially non-specific manner, but that this global property could be processed largely independently of its local orientation content. Chapter 3 investigated if our increased sensitivity to cardinal (vertical and horizontal) structures compared to inter-cardinal (45° and 135° clockwise of vertical) structures, influenced classification of unambiguous natural images. Participants required relatively less contrast to classify images when they retained near-cardinal as compared to near-inter-cardinal structures. Finally, in chapter 4, we examined category classification when images were ambiguous. Observers were biased to classify ambiguous images, created by combining structures from two distinct image categories, as carpentered (e.g., a house). This could not be explained by differences in sensitivity to local structures and is most likely the result of our long-term exposure to city views. Overall, these results show that higher-level representations are not fully dependent on the lower level features within an image. Furthermore, our knowledge about the environment influences the extent to which we use local features to rapidly identify an image. more...
- Published
- 2018
14. The illusion of action-specific scaling effects : action capacity does not directly influence spatial perception
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Collier, E. S., Lawson, Rebecca, Walker, Peter, and Makin, Alexis
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152.14 - Published
- 2018
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15. Exploring how object shape and binocular vision interact to make or break camouflage
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Cammack, Philip and Harris, Julie
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152.14 ,BF469.C26 ,Depth perception ,Movement, Psychology of ,Camouflage (Biology) - Abstract
Depth perception is a major component of 3D vision. There are many cues to depth; one particularly sensitive aspect is the vivid perception of depth created from having eyes with overlapping visual fields (binocular vision). As the eyes are located at different points in space, they see different views of the scene - these slight differences (called binocular disparity) can be used to obtain depth information. However, extracting depth from disparity requires complex visual processing. So why use binocular vision? Julesz (1971) proposed an explanation - camouflaged animals can fool the perception of some cues to 3D shape, but camouflage is ineffective against binocular vision. We would expect that animals with binocular vision could see the 3D shape of animals, despite their camouflage. Whilst commonly accepted, this hypothesis has not been tested in detail. In this thesis, we present experiments designed to establish how depth from binocular vision interacts with camouflage and object shape. Two main questions were addressed: First, we explored how the visual system represented depth information about 3D objects from binocular disparity. Objects with smooth depth edges (hill-shaped) were perceived with less depth than sharper edged objects. A computational model that segregated the object, then averaged the disparity over the segregated region emulated human performance. Finally, we found that disparity and luminance cues interacted to alter perceived depth. Secondly, we investigated if binocular vision could overcome camouflage. We found that camouflaged objects defined by luminance were detected faster when also defined by depth from disparity, thus reduces the effect of camouflage. Smooth objects were detected slower than sharp objects: an effect that was replicated in the real world, suggesting a camouflage technique to counter binocular vision. In summary, binocular vision is useful because it can detect camouflaged objects. However, smoother shapes take longer to spot, forming binocular (or stereoscopic) camouflage. more...
- Published
- 2017
16. Investigating neural substrates of visual motion sensitivity in deaf individuals
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Levine, Alexandra Toba and Heidi, Baseler
- Subjects
152.14 - Abstract
The aim of this thesis has been to explore neural substrates of enhanced far-peripheral visual motion processing in congenitally deaf adults. To do this, psychophysical measures were used as well as novel fMRI stimulus delivery methods to record responses to stimu- lation in the far-peripheral visual field. For the first time, far-peripheral visual field mapping measured an extended representa- tion of the visual field (72 ◦) in early visual cortex in deaf and hearing individuals. Using this method, unique evidence of plasticity within the cortical surface area distribution of visual field representations in the primary visual cortex was found in congenitally deaf adults, biased towards the far-peripheral visual field. Furthermore, neural responses to far-peripheral stimuli were measured in visual motion processing areas V5/MT+ and V6, and in auditory regions. Results show novel and dis- tinctive differences in response profiles in auditory, but not visual regions between deaf and hearing participants, indicating crossmodal plasticity in deaf participants, specific to coherent but not incoherent global optic flow field motion stimuli. Most importantly, the aim of the thesis was to relate neural measures to behavioural per- formance of motion perception. The results show evidence that unimodal plasticity in V1 and activation in visual motion areas V5/MT+ and V6 are not related to performance in two visual motion tasks (local motion detection and global motion direction discrimina- tion), but that response inhibition and excitation levels in auditory regions are related to motion processing performance in deaf and hearing individuals. In summary, the findings described in this thesis show for the first time that congenital deafness leads to plastic changes within primary visual cortex. In addition, auditory but not visual motion regions are recruited differentially between deaf and hearing individu- als, depending on the motion type, and this activation shows a trending relationship with visual motion performance in both groups. more...
- Published
- 2017
17. Is visual crowding a multi-level phenomenon?
- Author
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Reuther, Josephine
- Subjects
152.14 ,Visual perception - Abstract
Visual crowding is a dramatic breakdown of object recognition that has been studied extensively as a gateway to determine the mechanisms that underlie normal object recognition. Despite numerous proposed models and mechanisms, a unifying account has yet to be found. Proposed mechanisms reach from purely hierarchical, bottom-up accounts that place crowding early in the processing stream, to high-level attention-based accounts that allow for recurrent processing and top-down feed-back. The aim of the current work was to investigate a range of factors that would help to differentiate between these accounts. Firstly, object-category was studied as a factor that would be expected to modulate crowding if the phenomenon were to affect object recognition during several stages along the processing stream. Secondly, knowledge was studied as a possible source of top-down feedback that, if found to have a direct influence on crowding, would provide evidence against a purely hierarchical account for object recognition. Thirdly, anticipation of flanker-presence was studied as a factor modulating volitional attention-allocation. Observing an influence of anticipation on object recognition under the influence of crowding, would provide support that crowding may be the result of a limitation to focus attention. Finally, object-familiarity was studied as another factor that may modulate crowding via top-down feedback. Of these factors, only object-familiarity was found to have an influence on visual crowding. However, instead of being the result of top-down feedback, hard-wired pathways developed based on repeated exposure might explain the effect of object-familiarity. In summary, none of the studied factors provided univocal evidence to suggest that crowding were to occur at multiple levels of object recognition, or that crowding were to be influenced by higher-level effects. Hence, it may be concluded that a purely hierarchical bottom-up account is sufficient to account for the effects visual crowding exerts on normal object recognition. more...
- Published
- 2017
18. An investigation into the positive visual experience design for the elderly
- Author
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Men, Delai
- Subjects
152.14 - Abstract
Visual impression is the first impact on human perception through receiving and assessing the external information observed by the eyes. Therefore, product appearance is closely linked to the human’s visual experience and can affect human’s mood throughout the process of information perception and product manipulation. Generally, visual ability declines as age increases, which can lead to visual impairment and unpleasant moods among the elderly. For the elderly, there is a need to improve the quality of the visual experience in design. The aim of this research is to explore feasible methods to discover the consistency between subjective desires and objective environmental reactions in order to provide the optimal pleasurable visual experience conditions for elderly individuals in design. The objectives are 1) To examine the common elderly visual experience features in relation to physiological and psychological effects, specifically reflected in vision, perception, and emotion; 2) To determine a positive visual experience framework for elderly-focused design based on visual experience consideration; 3) To develop positive visual experience assessments and test methods specially designed for the elderly in order to gather information and data about subjective desire and objective reaction to objects; 4) To analyze the results of consistency between subjective preference and objective attraction in order to form a positive visual experience for elderly individuals. This research describes the research process and its findings. Initially, the introduction of the thesis presents the research motivations and background, with its aim and objectives defined. Related literature supporting the research is then examined for further understanding of aging, visual experience and relevant emotional issues, and research methodologies. Based on this understanding of the elderly visual experience and visual experience behavior, along with observations of purchasing processes, interviews with the elderly, and interviews with experts, a set of assessment tools (VPTs) was developed for assessing the elderly visual experience factors from the two perspectives of subjective preference and objective attraction. Based on statistical analysis, the elderly general cognitive features, and a series of positive visual experience factors that affect the emotional mood of the elderly were determined. The results of the investigation contribute to design for the elderly with positive visual experience factors identified for enhancing the elderly satisfaction in their visual experiences and manipulating product design to create a positive emotional state. This VPTs assessment tool can also be recommended as a reference for investigating into other various target groups’ visual experience features. more...
- Published
- 2016
19. Physiological, psychophysical and psychological correlates of the ageing visual cortex
- Author
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McKernan Ward, Laura
- Subjects
152.14 - Published
- 2016
20. The psychophysical characterisation of dorsal and ventral stream functions with equivalent noise paradigm normal clinical populations
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Joshi, Mahesh Raj
- Subjects
152.14 - Published
- 2016
21. The visual processing of human faces and bodies as visual stimuli in natural scenes
- Author
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Kroll, V. R.
- Subjects
152.14 - Abstract
How faces are recognized and detected has been the focus of an extensive corpus of research. As such, it is now well established that human faces can be detected rapidly in a visual scene and that they automatically capture a viewer’s attention over other objects. However, under natural viewing conditions the human face is attached to a substantial cue, the human body. The evidence to-date of a similar attentional processing advantage for human bodies is less clear. This is remarkable given the social significance and evidence of neural specificity for these stimuli. Additionally, most previous investigations of preferential attention towards faces and bodies have presented these stimuli in simple displays, namely uniform colour backgrounds (Bindemann, Scheepers, Ferguson & Burton, 2010). Therefore, this thesis aimed to address the relationship between attention and face and body processing in natural scenes directly by assessing the consequences of numerous experimental manipulations in both a visual search paradigm and additional singleton paradigm. The first line of enquiry examined participants’ ability to detect face and body stimuli in comparison to other objects in natural scenes. Subsequent experiments examined whether faces and bodies captured attention when they were task-irrelevant. In line with previous research, the main findings indicate that human faces do have attentional advantages and capture attention in both natural and grey scenes. They also indicate that human bodies (without the head) do not have detection advantages over other objects, nor do they capture attention in a bottom-up manner. Any biases or detection advantages observed for body targets are because they larger in size than other objects or because they are odd stimuli in that scene. Human full-body targets (including the face), which are perceived on a day-to-day basis, capture attention partly because they include face and partly because they are large objects in the scene. These findings modify claims of person perception suggesting that the detection of a full-body in natural scenes is facilitated by attention capture by faces, any advantages from bodies are the result of attention capture by their large size, rather than some attentional advantage. Future investigating into face and body processing should use natural backgrounds to gain a more realistic insight in to face and body processing in the real world. more...
- Published
- 2016
22. Exploring orientation with geovisualisations and virtual nested environments
- Author
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Allison, Craig and Redhead, Edward
- Subjects
152.14 - Abstract
Spatial orientation is the ability to maintain knowledge of our position with respect to other cues within an environment. This is an essential skill, forming the foundation of other abilities, including spatial navigation. Previous research has identified that virtual environments impede participants’ ability to orient accurately. Research exploring the role of environment type, specifically nested environments, has further identified a situation which hinders orientation ability. This thesis seeks to link these research bodies, exploring orientation ability within virtual nested environments. Across a series of experiments, it was found that participants struggled to accurately orient within these environments, especially when a link to the external environment was unavailable. The addition of orienting cues within the environment, however, reduced this difficulty. Participants provided with additional cues recorded significantly lower orientation error. This effect is apparent following either active exploration or a passive video tour. Subsequent studies illustrated that other factors such as anxiety, as manipulated via the use of stereotype threat, also influenced orientation accuracy within a nested environment. Geovisualisations were explored to examine whether orientation difficulties are observed in symbolised, rather than realistic, virtual environments. Participants reported orientation difficulties and demonstrated an inability to accurately track their position within symbolised space. Results suggest that geovisualisation users, similar to users of virtual nested environments, require increased support to efficiently orient. Results support that orientation within digital nested environments is difficult due to the lack of consistent visual cues within the multiple aspects of the environment. more...
- Published
- 2016
23. Cultural differences in scene perception
- Author
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Alotaibi, Albandari
- Subjects
152.14 ,BF Psychology - Abstract
Do individuals from different cultures perceive scenes differently? Does culture have an influence on visual attention processes? This thesis investigates not only what these influences are, and how they affect eye movements, but also examines some of the proposed mechanisms that underlie the cultural influence in scene perception. Experiments 1 & 2 showed that Saudi participants directed a higher number of fixations to the background of images, in comparison to the British participants. British participants were also more affected by background changes, an indication of their tendency to bind the focal objects to their contexts. Experiments 3 & 4 revealed a higher overall number of fixations for Saudi participants, along with longer search times. The intra-group comparisons of scanpaths for Saudi participants revealed less similarity than within the British group, demonstrating a greater heterogeneity of search behaviour within the Saudi group. These findings could indicate that the British participants have the advantage of being more able to direct attention towards the goals of the task. The mechanisms that have been proposed for cultural differences in visual attention are due to particular thinking styles that emerge from the prevailing culture: analytic thinking (common in individualistic cultures) promotes attention to detail and a focus on the most important part of a scene, whereas holistic thinking (common in collectivist cultures) promotes attention to the global structure of a scene and the relationship between its parts. Priming methodology was used in Experiments 5, 6 & 7 to cue these factors, although it did not reveal any significant effects on eye movement behaviours or on accuracy at recognition of objects. By testing these explanations directly (Experiment 8), findings have mainly suggested the holistic-analytic dimension is one of the main mechanisms underlying cultural diversity in scene perception. Taken together, these experiments conclude that the allocation of visual attention is also influenced by an individual’s culture. more...
- Published
- 2016
24. Not a slave to the rhythm : the perceptual consequences of rhythmic visual stimulation
- Author
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Kerlin, Jess Robert
- Subjects
152.14 ,BF Psychology - Abstract
We investigated whether rhythmic visual stimulation leads to changes in visual perception attributable to the entrainment of endogenous alpha-band oscillations. First, we report evidence that the attentional blink phenomenon is not selectively modified by alpha-band rhythmic entrainment. Next, we provide evidence that changes in single target identification following rhythmic stimulation are poorly explained by rhythmic entrainment, but well explained by alternative factors. We report failures to replicate the results of two previous visual entrainment studies supporting the hypothesis that alpha-band rhythmic stimulation leads to matching rhythmic fluctuations in target detection. Finally, we examined whether temporal acuity during an RSVP sequence is dependent on rhythmic entrainment by studying the role of object change on temporal acuity, finding novel results inconsistent with the predictions of the rhythmic entrainment model. We conclude that visual perception is robust against entrainment to task-irrelevant rhythmic visual inputs and that endogenous and externally driven oscillations in the visual system may be functionally distinct. more...
- Published
- 2016
25. Operation of eye-movement control mechanisms during the perception of naturalistic scenes
- Author
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Walshe, Ross Calen, Nuthmann, Antje, and Pickering, Martin
- Subjects
152.14 ,saccades ,computational modelling ,perceptual decision making - Abstract
Understanding of visual scenes takes place within very brief episodes known as fixations. To explore the extent of the scene, the eye shifts between fixation locations at intervals of roughly 300 ms. Currently, it is a matter of open inquiry as to what factors influence the timing of these movements. This thesis focuses on understanding the mechanisms that govern the rapid adjustment of fixation and saccade timings when novel stimulus information is encountered during a fixation. In part I, I use an experimental technique known as the fixation-contingent scene quality paradigm to control the quality of incoming visual scene information. This approach is used to assess how fixation timing adapts to moment-by-moment changes in the quality level of the stimulus. I find that quality changes tend to result in an increase in fixation durations and this occurs whether the quality is increased or decreased. Using distributional analytic techniques, I argue that these results reflect the combined influence of a rapid surprise related process and a slower acting encoding related influence. In part II, I study how fixation durations are influenced by the underlying saccade programming mechanisms. An important assumption within the eye-movement control literature is that there exists a threshold called the point-of-no-return. Once this point has been reached, a saccade may no longer be modified or cancelled. I adapt a classic psychophysical technique known as the double-step procedure to study the point-of-no-return within scene viewing tasks. I also provide a measurement of the saccadic dead time, the last point in time that a saccade may be modified. In Part III, a formal model of fixation durations in high-level tasks is presented. I build on recent modelling work and develop a formal account for the early-surprise late-encoding modulation account of fixation durations in scene viewing tasks. The model is tested against data observed in Part I of the thesis. I demonstrate that the model does a very good job of predicting these distributions with relatively few assumptions. In summary, I use experimental techniques in combination with computational modelling to reveal how a composite of low-level (saccade programming) and high-level (information processing) considerations can, and must, be taken into consideration when understanding eye-movement control behaviour in scene viewing tasks. more...
- Published
- 2016
26. Ensemble perception of hue
- Author
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Maule, John
- Subjects
152.14 ,BF0241 Special senses. Vision. Visual perception - Abstract
In order to rapidly get the gist of new scenes or recognise objects, the brain must have mechanisms to process the large amount of visual information which enters the eye. Previous research has shown that observers tend to extract the average feature from briefly seen sets of multiple stimuli that vary along a dimension (e.g., size), a phenomenon called ensemble perception. This thesis investigates ensemble perception of hue. Paper 1 (Maule, Witzel & Franklin, 2014) demonstrates that human observers have memories biased towards the mean hue of a rapidly-presented ensemble of colours. Paper 2 (Maule & Franklin, 2015) further shows that observers are able to identify the mean hue from a distractor fairly reliably, provided the range of hues is manageable. Paper 3 provides evidence that, while observers' settings of the mean hue converge quite closely on the true mean across many trials, the precision of those settings is low and does not support claims that ensemble perception can surpass the limits of visual working memory. Paper 4 found that adults with autism have an enhanced ability to discriminate members from non-members of multi-hue ensembles, and a similar ability to extract the mean hue compared to typical adults, but are worse at averaging small sets. Finally, paper 5 investigated colour afterimages in adults with autism and whether they are affected by top-down gist of a scene. It was found that afterimages were no different in autism compared to a typical group. Overall these studies provide the first comprehensive exploration of ensemble perception of hue, showing that observers can extract and estimate the mean hue of a rapidly-presented multi-colour ensemble with a small hue variance. The ability to average hue may be driven by a sub-sampling mechanism, but results from autistic adults suggests that it can be modulated by processing style. more...
- Published
- 2016
27. The time course of the influence of colour terms on visual processing
- Author
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Forder, Lewis
- Subjects
152.14 ,BF0241 Special senses. Vision. Visual perception - Abstract
This thesis explores whether colour terms (e.g., “red”, “blue”, “purple”, etc.) influence visual processing of colour, and if so, the time course of any effect. Broadly, this issue relates to debate concerning whether language affects the way we perceive the world (i.e., the theory of linguistic relativity). Three of the experiments conducted used the event-related potential method (ERP), taking electrophysiological measurements of visual processing and visual cognition in human participants. The ERP provides high-resolution information about the timing of neural activity in the brain and can therefore be used to effectively investigate the time course of a potential influence of colour terms on visual processing. The first study, using a behavioural approach, identified that colour terms can influence the detection of colours and colour-associated objects suppressed from awareness by continuous flash suppression. The second study found that a cross-linguistic difference in colour lexicons affected a post-perceptual ERP component (the P2-N2 complex), but not sensory ERP components occurring early in visual processing. However, the third study found that differences in colour naming within a language do affect an early sensory ERP component (the P1). The final study used ERPs to identify a post-perceptual neural marker (in the posterior P2 component) for the unique ‘pure' hues (red, yellow, green, and blue), which had previously only been defined and identified linguistically. All of the studies provide evidence that colour terms affect colour processing, and the specific time course of this effect is identified as being task-dependent. These findings have implications for broader debate about the influence of language on visual cognition and perception. more...
- Published
- 2016
28. The perception of surface properties : translucence and gloss
- Author
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Chadwick, Alice Caitlin
- Subjects
152.14 - Abstract
The human visual system is sensitive to differences in gloss and translucence, two optical properties which are found in conjunction in many natural materials. They are driven by similar underlying physical properties of light transport - the degree to which light is scattered from the surface of a material, or within the material. This thesis aimed to address some fundamental questions about how gloss and translucence are perceived. Two psychophysical methods (maximum likelihood difference scaling, and conjoint measurement) were used throughout, as they provided an appropriate way of investigating how perceptual experiences related to physical variables. In the introduction, I review the literature on the perception of gloss and translucence. Study 1 investigated the relationship between variables controlling light transport in translucent volumes and percepts of translucence. The results show that translucence perception is not based on estimates of light transport properties per se, but probably uses spatially-related statistical pseudocues in conjunction with other cues. Study 2 examined a similar issue, but the translucent material was presented as a layer enveloping a solid object. Behavioural responses were similar for these translucent materials, which were perceived as glossy layers of coating. Study 3 further explored established findings that perceived translucence shows inconstancy under changes in viewing condition. Perceived translucence was dependent in a complex way on both light-scattering in the material and illumination direction in both volumes and layers of translucent materials. Study 4 used similar layers of subsurface light-scattering and -absorbing material and applied them to multiple base materials. Opacity and a lack of mirror-like reflections enabled observers to make the most accurate independent judgements of darkness and cloudiness. Study 5 explored observers' sensitivity to spatial variation of scatter across a surface using similar layers of coating, and the way in which observers might weight cues differently to answer subtly different questions (judgements of 'shininess' vs. 'cleanliness'). Layer thickness and variation of scatter significantly affected perceived shine and cleanliness, with layer thickness influencing decisions more than variation. Scatter variation contributed to decisions significantly more for judgements of cleanliness than shine. Study 6 investigated how tactile surface roughness influenced perceived gloss. Previous findings have shown that tactile compliance and friction influence perceived gloss, and that friction interacts with visual gloss. Our results showed that surface roughness and visual gloss both affected perceived gloss, but there was no interaction, suggesting that different types of haptic information are combined with visual information differently. Finally, study 7 explored the potential cortical basis of perceived translucence. Through testing a neuropsychological patient, we showed that perceived translucence is dependent on cortical areas not responsible for colour or texture discrimination. The thesis concludes with a discussion of additional recent findings, the implications of the research reported in this thesis, and proposals for future research. more...
- Published
- 2016
29. Psychophysical investigations of speed processing in the human visual system
- Author
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Hassan, Omar
- Subjects
152.14 - Abstract
A range of models of motion processing has been proposed (e.g. motion energy, gradient, Bayesian, ratio) but there is currently no consensus as to how the human visual system computes the speed of a moving image and there is insufficient data to adequately characterise the effects of even a few image parameters such as contrast, luminance and eccentricity upon perceived speed. A series of experiments was conducted in order to inform models of speed encoding and provide insight into the mechanisms underlying spatio-temporal processing in the visual system. Measurements of the ramp after-effect suggest that the after-effect is determined by ramp amplitude rather than gradient and thus offer little support for the existence of gradient motion detectors. However, the findings of luminance-dependent ramp after-effects do provide support for the idea that the after-effect is mediated by ON- and OFF- pathways in the visual system. Measurements of biases in speed perception indicated that speed encoding in the periphery is essentially similar to central vision whilst the results of a further study of speed biases indicated that at lower luminance there is significantly less reduction in perceived speed and greater increase in perceived speed (at low and high speeds respectively) and a concomitant reduction in the speed at which the bias is reversed. This luminance-dependent pattern of results is consistent with ratio-type models of speed encoding but inconsistent with all extant Bayesian models. Overall the results of these experiments offer little support for gradient or Bayesian models. Whilst the results do not rule out other speed-encoding models, only ratio-class models can currently account for the entire pattern of results reported in this series of experiments. more...
- Published
- 2016
30. The perception of transformed auditory and visual pattern structure : an exploration of supramodal pattern space
- Author
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Thorpe, Michael J. A., Eysenck, Michael, Aksentijevic, Aleksandar, and Ockelford, Adam
- Subjects
152.14 - Abstract
The present thesis is broadly concerned with the processing of structural information. More specifically, it investigates the possibility that auditory pitch patterns share, at some level, supramodal structural representations and processes with visuo-spatial patterns. The motivation for the research was provided by a number of areas of psychological research that are brought together and discussed in this thesis, and which inform the development of a new theoretical framework that conceives of a supramodal pattern space (SPS). According to the SPS framework, auditory and visual patterns can be represented in equivalent ‘1!-D’ supramodal pattern spaces. A series of experiments was devised to test the assumptions of the SPS framework, by means of analysing the perception of two types of structural transformation: inverse and retrograde. The main hypothesis that was tested in all experiments predicted a processing advantage for inverse transformations when patterns corresponded to 1!-D supramodal pattern space. Support for the hypothesis was provided by experiments adopting a short-term recognition paradigm. However, contrasting results were revealed by experiments adopting a structural priming paradigm, which did not support the hypothesis. It was concluded that different processing strategies were used depending on the task demands. The findings were discussed with relation to theories of sequential pattern learning, melodic perception and brain organisation. more...
- Published
- 2016
31. Incidental learning of trust from identity-contingent gaze cues : boundaries, extensions and applications
- Author
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Strachan, James and Tipper, Steven
- Subjects
152.14 - Abstract
Monitoring the trustworthiness of social interaction partners is a cornerstone of social cognition. However, the mechanics of learning about trust during online interactions as a result of a person’s behaviour can be difficult to explore. The current experiments use a gaze cueing paradigm where faces provide either valid (always shift their gaze towards the location of a subsequent target), or invalid cues (always shift their gaze to a different location). Following gaze cueing, participants rate valid faces as more trustworthy than invalid faces. We show that this incidental trust learning is sensitive to the emotional expression of the face, is specific to assessments of trust, occurs outside of conscious awareness, and is driven primarily by a decrease in trust for invalid faces (Chapter 2), perhaps reflecting a cheater detection module. Memory for incidentally learned trust is surprisingly durable, is affected by the familiarity of the cueing faces (Chapter 3), and does not affect memory for the faces’ physical features, nor does the trustworthiness of the face generalise to other stimuli (Chapter 4). Furthermore, learning is modulated by top-down knowledge of social group membership − when group identity is made experimentally salient, participants default to a group-level representation as a heuristic for social judgements (Chapter 5), while using naturally occurring group memberships (i.e. race) results in better learning for in-group members than out-group (Chapter 6). Finally, while there is evidence that trust learning is driven by learning about eye-gaze behaviour, this cannot be explained purely by disruptions to visuomotor fluency (Chapter 7), which suggests that this phenomenon is part of an active social monitoring framework that relies on physical changes or behaviours in a face to affect subsequent social judgements. more...
- Published
- 2016
32. Control and development of time-based visual selection
- Author
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Zupan, Zorana
- Subjects
152.14 ,BF Psychology - Abstract
Attention plays an integral role in healthy cognitive functioning, and failures of attention can lead to unfavourable and dangerous consequences. As such, comprehending the nature of attentional mechanisms is of fundamental theoretical and practical importance. One way in which humans can attentionally prioritise new information is through top-down inhibition of old distractors, known as the preview benefit (Watson & Humphreys, 1997). In the preview benefit, time is used to efficiently guide visual selection in space. Given that this ability is based on limited resources, its deployment in everyday life may be hindered by a multitude of factors. This thesis will explore the endogenous and exogenous factors that can facilitate or constrain the preview benefit, and determine its developmental trajectory. Understanding the nature of this mechanism (endogenous and exogenous factors) in adults can elucidate the contexts in which visual selection can efficiently filter old distractors. In turn, a developmental perspective can unravel the hidden aspects of this ability and inform when children are endowed to use temporal information for efficient attentional selection. Chapter 1 introduces the theoretical problems and topics of attentional research in adults and children. Chapter 2 addresses the question of endogenous control of top-down inhibition in time-based visual selection – when can top-down inhibition be controlled by the observer? Chapter 3 examines the exogenous influence of complex stimuli on time-based visual selection. Chapters 4 and 5 focus on the development of time-based visual selection for stationary and moving stimuli, respectively, in children aged 6 to 12 years. These chapters also examine the relative association of the efficiency of the preview benefit with the development of executive functions across different age-groups. Overall, the findings suggest that there exist remarkable endogenous and exogenous constraints in how time guides selection. This may account for why in certain contexts, attentional selection can fail to be efficient. Moreover, time-based visual selection shows striking quantitative and qualitative changes over developmental time, and most importantly, children have a long developmental trajectory in learning to ignore moving items. Unlike children, adults’ time-based visual selection is coupled with individual differences in executive functions, highlighting an acquired functional connection. The findings are discussed in terms of their theoretical implications for time-based visual selection, the development of children’s attentional control for distractors, and impact routes for educational and clinical practice, and policy makers. more...
- Published
- 2015
33. Complexity, specificity, and the timescales of developing expectations in visual perception
- Author
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Gekas, Nikos, Series, Peggy, and Lengyel, Máté
- Subjects
152.14 ,visual perception ,inference ,expectation ,psychophysical - Abstract
Perception is strongly influenced by our expectations, especially under situations of uncertainty. A growing body of work suggests that perception is akin to Bayesian Inference in which expectations can be viewed as ‘prior’ beliefs that are combined via Bayes’ rule with sensory evidence to form the ‘posterior’ beliefs. In this thesis, I aim to answer open questions regarding the nature of expectations in perception, and, in particular, what the limits of complexity and specificity in developing expectations are, and how expectations of different temporal properties develop and interact. First, I conducted a psychophysical experiment to investigate whether human observers are able to implicitly develop distinct expectations using colour as a distinguishing factor. I interleaved moving dot displays of two different colours, either red or green, with different motion direction distributions. Results showed that statistical information can transfer from one group of stimuli to another but observers are also able to learn two distinct priors under specific conditions. In a collaborative work, I implemented an online learning computational model, which showed that subjects’ behaviour was not in disagreement with a near-optimal Bayesian observer, and suggested that observers might prefer simple models which are consistent with the data over complex models. Next, I investigated experimentally whether selective manipulation of rewards can affect an observer’s perceptual performance in a similar manner to manipulating the statistical properties of stimuli. Results showed that manipulation of the reward scheme had similar effects on perception as statistical manipulations in trials where a stimulus was presented but not in the absence of stimulus. Finally, I used a novel visual search task to investigate how expectations of different timescales (from the last few trials to hours to long-term statistics of natural scenes) interact to alter perception. Results suggested that recent exposure to a stimulus resulted in significantly improved detection performance and significantly more visual ‘hallucinations’ but only at positions at which it was more probable that a stimulus would be presented. These studies provide new insights into the approximations that neural systems must make to implement Bayesian inference. Complexity does not seem to necessarily be a prohibitive factor in learning but the system also factors the provided evidence and potential gain in regards to learning complex priors and applying them in distinct contexts. Further, what aspects of the statistics of the stimuli are learned and used, and how selective attention modulates learning can crucially depend on specific task properties such as the timeframe of exposure, complexity, or the observer’s current goals and beliefs about the task. more...
- Published
- 2015
34. An investigation of the gaze contingent tilt after-effect
- Author
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Parwaga, Sandeep, Duke, Philip, and De Lillo, Carlo
- Subjects
152.14 - Abstract
In order to act on the world around us, the brain needs to encode the location of visible objects. This begins with representing the location of visual features on the retina. However, a retinal representation alone does not provide information about the direction of visual features with respect to the head or body whenever the eyes, head and body move. Therefore, successful action requires more than a retinal representation of location. One possibility is that the locations of visible objects may be encoded in retino-centric, head-centric and body-centric frames of reference. To investigate this, we used a well-known visual phenomenon: the tilt aftereffect (TAE). We investigated whether visual feature tilt is represented beyond a retino-centric representation, using a gaze contingent adaptation paradigm. The results of seven experiments yielded four key findings: 1) The TAE was contingent on gaze, suggesting that the TAE is not just retino-centric. We found evidence of a head-centric representation of tilt, but no evidence of a body-centric or world-centric representation of tilt. 2) The gaze contingent TAE showed different characteristics compared to the conventional TAE. While the conventional TAE was sensitive to test stimulus duration, the gaze contingent TAE was not. The gaze contingent TAE was also significantly smaller in aftereffect magnitude. Both TAEs shared similar characteristics with respect to stimulus contrast. 3) We found no evidence that either TAE was modulated by attention. 4) Our findings support the conclusion that the conventional TAE is the result of adaptation of two mechanisms: a tilt sensitive mechanism and a gaze direction encoding mechanism. This result fits with neurophysiological findings of neurons jointly sensitive to tilt and gaze direction (Trotter & Celebrini, 1999). Concluding, our results in this thesis provide psychophysical evidence that our impression of the world is based on head-centric visual representations. more...
- Published
- 2015
35. Detecting a visual object in the presence of other objects : the flanker facilitation effect in contour integration
- Author
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Gillespie, Christopher, Vishwanath, Dhanraj, and Harris, Julie
- Subjects
152.14 - Abstract
When an observer views a complex visual scene and tries to identify an object, his or her visual system must decide what regions of the visual field correspond to the object of interest and which do not. One aspect of this process involves the grouping of the local contrast information (e.g., orientation, position and frequency) into a smooth contour object. This thesis investigated whether the presence of other flanking objects affected this contour integration of a central target contour. To test this, a set of Gaborized contour shapes were embedded in a randomised Gabor noise field. The detectability of the contours was altered by adjusting the alignment of the Gabor patches in the contour (orientation jitter) until a participant was unable to distinguish between a field with and without a target shape (2-AFC procedure). By varying the magnitude of this jitter, detection thresholds were determined for target contours under various experimental conditions. These thresholds were used to investigate whether contour integration was sensitive to shared shape information between objects across the visual field. This thesis determined that the presence of flanking contours of a similar shape (as the target) facilitated the detection of a noisy target contour. The specific results suggest that this facilitation does not involve a simple template matching or shape priming but is associated with integration of shape level information in the detection of the most likely smooth closed contour. The magnitude of this flanker facilitation effect was sensitive to a number of factors (e.g., numerosity, relative position of the flankers, and perimeter complexity/compactness). The implication of these findings is that the processing of highly localised contrast and orientation information originating from a single object is subject to modulation from other sources of shape information across the whole of the visual field. more...
- Published
- 2015
36. How vision influences perceived duration : an empirical and computational investigation
- Author
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Rowland, Edward
- Subjects
152.14 ,Vision ,Perception ,Time ,Psychophysics ,Modelling ,fMRI ,Illusions ,flash-lag effect ,Neuroscience(all) ,Sensory Systems - Abstract
Time perception is often thought of as arising from a centralised mechanism. Recently, there has been a shift away from this perspective, with evidence showing that adaptation to particular visual properties at specific locations and stimulus predictability affects perceived event duration, implying that sensory systems carry out duration processing. This thesis investigates the following three questions: (1) Does adaptation induced duration compression affect other visual processes? (2) Can sensory systems encode duration using their basic response properties? (3) What is the relationship between predictability, perceived duration and neural response to a visual stimulus? The first question was investigated using a visual illusion called Flash-Lag, commonly thought to possess a fixed time component. After pilot investigations comparing psychophysical techniques, results from a behavioural experiment show that adapting to a high temporal frequency stimulus reduces the time component of the Flash-Lag illusion, implying a role for duration in positional and/or motion computations. We demonstrate a model using labelled lines and varying temporal response of neurons can encode the duration of a temporally normalised input. This model exhibits effects similar to those observed in literature including adaptation induced duration compression, central tendency and perceived duration scaling with the magnitude of various stimulus properties. Finally, a new paradigm is developed to test if stimulus duration decreases with stimulus predictability. Although behavioural results show no effect of predictability, event related fMRI shows significant differences in BOLD signal. Area MST demonstrates reduced response to expected events in a duration judgement task, but not an orientation judgement task, suggesting response is reduced by predictability, dependent on the task. These results show that duration may be encoded in sensory systems and is used in perceptual tasks. Furthermore, it is proposed that duration is estimated using ramping or climbing activity within neural populations in the dorsal visual pathway. more...
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- 2015
37. Investigating the visual tasks of pedestrians and how one of these tasks, obstacle detection, is influenced by lighting
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Uttley, Jim and Fotios, Steve
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152.14 - Abstract
Current guidelines for pedestrian road lighting are not based on empirical evidence. One approach to providing suitable evidence is to examine the effect of lighting on the visual tasks of pedestrians. This first requires an understanding of what these visual tasks are. An eye-tracking study was carried out in which pedestrians walked a real, outdoor route during the day and after-dark. A novel dual-task method was used to identify the critical visual tasks of the pedestrians. Reaction times to a concurrent audio response task were used to indicate instances when attention may have been diverted towards something significant in the visual environment. Analysis of the eye-tracking videos at these critical times found that the path and other people were the two most significant items looked at. Observation of the path is important for detection and avoidance of obstacles and trip hazards. Good road lighting should therefore facilitate obstacle detection. An obstacle detection experiment was therefore carried out examining the effect of illuminance and Scotopic/Photopic (S/P) ratio on obstacle detection. The experiment improved the realism and ecological validity of previous research by introducing a dynamic fixation target, realistic apparatus scales and real walking (on a treadmill) whilst carrying out an obstacle detection task. Results showed that obstacle detection only improved with illuminance increases up to 2.0 lux. A higher S/P ratio (2.0) provided better detection performance than a low S/P ratio (1.2), but only at the lowest illuminance used of 0.2 lux. The data is used to discuss optimal design criteria for pedestrian road lighting based on obstacle detection. However, other purposes of road lighting, such as creating a feeling of reassurance and enabling accurate interpersonal judgements to be carried out, should also be considered when designing pedestrian road lighting. more...
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- 2015
38. Adaptation to multiple radial optic flows
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Dassy, Brice
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152.14 ,BF Psychology - Abstract
There is long-standing evidence suggesting that our visual system can adapt to new visual environments, like a single radial optic flow generated when driving (Brown, 1931; Denton, 1966). In fact, as we move through the environment multiple optic flows can be generated. For example, when driving, we are often exposed to more than one radial optic flow at the same time. In this thesis I investigate whether the visual system can simultaneously adapt to two radial motion optic flows. More specifically, I explored this issue in three ways. First, I investigated whether the visual system could – through a fast low-level process – adapt to two optic flows present at two specific locations in space. Second, I probed whether the visual system could – through a perceptual learning process – learn to associate two radial optic flows with their locations in space. Third, I examined whether the visual system could – through a perceptual learning process – learn to associate each of two radial optic flows with preceding eye-movements. With regard to the first issue, the results from Experiments 1 – 6 suggested following exposure to two radial motion stimuli, a fast low-level process in the visual system could adapt to a radial flow pattern at one location in space: the radial flow pattern generated by the most recently presented radial motion stimulus. With respect to the second issue, the results from Experiments 7 – 10 indicated that the visual system could not learn to associate specific locations with two different radial motion stimuli. Finally, regarding the third issue, the results from Experiment 11 suggest that the visual system can associate specific eye-movements with two different radial motion stimuli. Taken together, these results suggest constraints on the way in which the visual system can adapt to radial motion, and emphasize the importance of self-movement in generating adaption to new visual environments. more...
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- 2015
39. Looking through the crowded mask : investigating the effect of distractor number and position in object substitution masking
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Camp, Sarah-Jayne
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152.14 - Abstract
Object substitution masking (OSM) is a phenomenon wherein a surrounding mask (typically four dots) that onsets with a target but lingers after its offset significantly reduces target perceptibility. OSM was originally postulated to occur only when spatial attention was spread (Di Lollo et al., 2000). Specifically, it was claimed that OSM only occurred when the target was presented in the context of large set-size displays (Di Lollo et al., 2000). However, more recent research has raised questions over the relevance of set size in OSM. Two separate investigations (Argyropoulos et al., 2013; Filmer et al., 2014) found that strong masking by OSM could be produced even with a set size of one. It was argued that the “set size” effects in OSM were actually an artifact of constrained performance. That is, once performance was brought within a measurable range, OSM was reported to be independent of set size. Further research however has suggested that perhaps this rejection of the role of set size in OSM was premature. Pilling (2013) found that increased set size did in fact lead to greater OSM magnitude. Therefore it seems that an explanation of constrained performance cannot fully account for the experimental findings. This thesis begins by investigating the disparity between these results by further exploring the role of set size in OSM. The first chapter provides an overview of some of the constraints for perceptual awareness by examining experimental phenomena that prevent visual awareness. The experimental phenomena of visual masking and specifically OSM are focused on with particular focus given to the role of attention in OSM. Chapter 2 is the first experimental chapter. This chapter investigates the role of set size in OSM using five experiments. Chapter 3 explores if visual crowding can be used as an alternative explanation for the set size effects in OSM with five experiments. Chapter 4 attempts to investigate the neural underpinnings of OSM, and the interaction between OSM and crowding using an EEG method. This thesis proposes, based on its findings, that the nominal set size effect in OSM is actually an effect of crowding, a factor which tends to co-vary with set size in most studies. Further experiments in this thesis showed that the interaction between crowding and OSM was one in which OSM affected crowding rather than the converse process. That is, with the use of OSM, the window at which flankers crowd the target becomes extended. These findings show parallels with the previously reported phenomenon of “supercrowding” which has been reported with classical masking. Given this, these results challenge claims regarding the position of OSM and crowding in the object processing hierarchy (e.g. Breitmeyer, 2014). This thesis contributes to the ongoing investigation of OSM, provides implications for its existing theories and for accounts of object processing more generally as well as highlighting future directions for research in this field. more...
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- 2015
40. How processing of visual symmetry relates to preference for regular patterns, and the role of attention on preference formation
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Rampone, Giulia
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152.14 - Abstract
The process of preference formation is influenced by many factors. These include intrinsic stimulus attributes as well as contextual factors, which are not directly related with the stimulus itself. In this PhD thesis I present seven studies that give significant new insights about how human preference is affected by intrinsic stimulus properties, as well as contextual factors. The common denominator is the use of abstract shapes forming regular patterns (reflectional symmetry). The presence of symmetry within the stimuli predicts preference, and there is behavioural evidence of an association between symmetry and positive valence. The first study shows the neural basis of this association (Part 1, Chapter 2). The thesis proceeds with four studies (Part 2, Chapters 3 – 6) exploring the role of (exogenous) attention on preference evaluation of abstract shapes that already contain intrinsic valence (symmetry/random). Previous research has demonstrated that attention plays a major role on preference formation. These studies showed that exogenous orienting of attention led to more positive evaluation of stimuli at cued locations, although this effect was sensitive to endogenous control. In the last part (Part 3), Chapter 7 explored preference devaluation of abstract symmetry with increasing visual eccentricity, as a potential consequence of reduced perceived regularity at farther locations. Random shapes, which do not lend themselves to coherent interpretation at fovea, were similarly evaluated at all eccentricities. Chapter 8 did not focus on preference. With a novel design, it explored how symmetry detection speed in the periphery was affected by the way attention was deployed in visual space. Overall these findings confirm the role of symmetry in aesthetic appreciation of abstract shapes. Importantly, they highlight a role of attention orienting and gazing in preference modulation. Defining what factors determine preference is key to understanding human behaviour and decisions. This thesis provides a significant contribution towards this goal. more...
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- 2015
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41. Associative recognition : exploring the contributions of recollection and familiarity
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Murray, Jamie G., Donaldson, David, and Watt, Roger
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152.14 ,Episodic memory ,Recollection ,Familiarity ,Unitization ,Threshold ,ERPs ,Mid-Frontal old/new effect ,Left Parietal old/new effect ,Recognition (Psychology) ,Memory ,Neuropsychology ,Cognitive function - Abstract
Episodic memory refers to the storage and retrieval of information about events in our past. According to dual process models, episodic memory is supported by familiarity which refers to the rapid and automatic sense of oldness about a previously encoded stimulus, and recollection which refers to the retrieval of contextual information, such as spatial, temporal or other contextual details that bring a specific item to mind. To be clear, familiarity is traditionally assumed to support recognition of item information, whereas recollection supports the recognition of associative information. Event Related Potential (ERP) studies provide support for dual process models, by demonstrating qualitatively distinct patterns of neural activity associated with familiarity (Mid-Frontal old/new effect) and recollection (Left-Parietal old/new effect). In the current thesis, ERPs were used to address two important questions regarding associative recognition – namely, the function of the neural signal supporting recollection and whether familiarity can contribute to the retrieval of novel associative information. The first series of experiments was aimed at addressing how recollection operates by employing a recently developed continuous source task designed to directly measure the accuracy of retrieval success. To date, the function of recollection has been fiercely debated, with some arguing that recollection reflects the operation of a continuous retrieval process, whereby test cues always elicit some information from memory. Alternatively, recollection may reflect the operation of a thresholded process that allows for retrieval failure, whereby test cues sometimes elicit no information from memory at all. In the current thesis, the Left Parietal effect was found to be sensitive to the precision of memory responses when recollection succeeded, but was entirely absent when recollection failed. The result clarifies the nature of the neural mechanism underlying successful retrieval whilst also providing novel evidence in support of threshold models of recollection. The second series of experiments addressed whether familiarity could contribute to the retrieval of novel associative information. Recent associative recognition studies have suggested that unitization (whereby multi-component stimuli are encoded as a single item rather than as a set of associated parts) can improve episodic memory by increasing the availability of familiarity during retrieval. To date, however, ERP studies have failed to provide any evidence of unitization for novel associations, whereas behavioural support for unitization is heavily reliant on model specific measures such as ROC analysis. Over three separate associative recognition studies employing unrelated word pairs, the magnitude of the Mid-Frontal old/new effect was found to be modulated by encoding instructions designed to manipulate the level of unitization. Importantly, the results also suggest that different encoding strategies designed to manipulate the level of unitization may be more successful than others. Finally, the results also revealed that differences in behavioural performance and modulation of the Mid-Frontal old/new effect between unitized and non-unitized instructions is greater for unrelated compared to related word pairs. In essence, the results suggest that unitization is better suited to learning completely novel associations as opposed to word pairs sharing a pre-existing conceptual relationship. Overall, the data presented in this thesis supports dual process accounts of episodic memory, suggesting that at a neural level of analysis, recollection is both thresholded and variable, whilst also supporting the assumption that familiarity can contribute to successful retrieval of novel associative information. The results have important implications for our current understanding of cognitive decline and the development of behavioural interventions aimed at alleviating associative deficits. more...
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- 2014
42. Cue combination of colour and luminance in edge detection
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Sharman, Rebecca J.
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152.14 ,BF Psychology - Abstract
Much is known about visual processing of chromatic and luminance information. However, less is known about how these two signals are combined. This thesis has three aims to investigate how colour and luminance are combined in edge detection. 1) To determine whether presenting colour and luminance information together improves performance in tasks such as edge localisation and blur detection. 2) To investigate how the visual system resolves conflicts between colour and luminance edge information. 3) To explore whether colour and luminance edge information is always combined in the same way. It is well known that the perception of chromatic blur can be constrained by sharp luminance information in natural scenes. The first set of experiments (Chapter 3) quantifies this effect and demonstrates that it cannot be explained by poorer acuity in processing chromatic information, higher contrast of luminance information or differences in the statistical structure of colour and luminance information in natural scenes. It is therefore proposed that there is a neural mechanism that actively promotes luminance information. Chapter 4 and Experiments 5.1 and 5.3 aimed to investigate whether the presence of both chromatic and luminance information improves edge localisation performance. Participant performance in a Vernier acuity (alignment) task was compared to predictions from three models; ‘winner takes all’, unweighted averaging and maximum likelihood estimation (a form of weighted averaging). Despite several attempts to differentiate the models we failed to increase the differences in model predictions sufficiently and it was not possible to determine whether edge localisation was enhanced by the presence of both cues. In Experiment 5.4 we investigated how edges are localised when colour and luminance cues conflict, using the method of adjustment. Maximum likelihood estimation was used to make predictions based on measurements of each cue in isolation. These predictions were then compared to observed data. It was found that, whilst maximum likelihood estimation captured the pattern of the data, it consistently over-estimated the weight of the luminance component. It is suggested that chromatic information may be weighted more heavily than predicted as it is more useful for detecting object boundaries in natural scenes. In Chapter 6 a novel approach, perturbation discrimination, was used to investigate how the spatial arrangement of chromatic and luminance cues, and the type of chromatic and luminance information, can affect cue combination. Perturbation discrimination requires participants to select the grating stimulus that contains spatial perturbation. If one cue dominated over the other it was expected that this would be reflected by masking and increased perturbation detection thresholds. We compared perturbation thresholds for chromatic and luminance defined line and square-wave gratings in isolation and when presented with a mask of the other channel and other grating type. For example, the perturbation threshold for a luminance line target alone was compared to the threshold for a luminance line target presented with a chromatic square-wave target. The introduction of line masks caused masking for both combinations. Introduction of an achromatic square-wave mask had no effect on perturbation thresholds for chromatic line targets. However, the introduction of a chromatic square-wave mask to luminance line targets improved perturbation discrimination performance. This suggests that the perceived location of the chromatic edges is determined by the location of the luminance lines. Finally, in Chapter 7, we investigated whether chromatic blur is constrained by luminance information in bipartite edges. Earlier in the thesis we demonstrated that luminance information constrains chromatic blur in natural scenes, but also that chromatic information has more influence than expected when colour and luminance edges conflict. This difference may be due to differences in the stimuli or due to differences in the task. The luminance masking effect found using natural scenes was replicated using bipartite edges. Therefore, the finding that luminance constrains chromatic blur is not limited to natural scene stimuli. This suggests that colour and luminance are combined differently for blur discrimination tasks and edge localisation tasks. Overall we can see that luminance often dominates in edge perception tasks. For blur discrimination this seems to be because the mechanisms differ. For edge localisation it might be simply that luminance cues are often higher contrast and, when this is equated, chromatic cues are actually a good indicator of edge location. more...
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- 2014
43. Towards a better understanding of sensory substitution : the theory and practice of developing visual-to-auditory sensory substitution devices
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Wright, Thomas D.
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152.14 ,BF0241 Special senses. Vision. Visual perception - Abstract
Visual impairment is a global and potentially devastating affliction. Sensory substitution devices have the potential to lessen the impact of blindness by presenting vision via another modality. The chief motivation behind each of the chapters that follow is the production of more useful sensory substitution devices. The first empirical chapter (chapter two) demonstrates the use of interactive genetic algorithms to determine an optimal set of parameters for a sensory substitution device based on an auditory encoding of vision (“the vOICe”). In doing so, it introduces the first version of a novel sensory substitution device which is configurable at run-time. It also presents data from three interactive genetic algorithm based experiments that use this new sensory substitution device. Chapter three radically expands on this theme by introducing a general purpose, modular framework for developing visual-to-auditory sensory substitution devices (“Polyglot”). This framework is the fuller realisation of the Polyglot device introduced in the first chapter and is based on the principle of End-User Development (EUD). In chapter four, a novel method of evaluating sensory substitution devices using eye-tracking is introduced. The data shows both that the copresentation of visual stimuli assists localisation and that gaze predicted an auditory target location more reliably than the behavioural responses. Chapter five explores the relationship between sensory substitution devices and other tools that are used to acquire real-time sensory information (“sensory tools”). This taxonomy unites a range of technology from telescopes and cochlear implants to attempts to create a magnetic sense that can guide further research. Finally, in chapter six, the possibility of representing colour through sound is explored. The existence of a crossmodal correspondence between (equi-luminant) hue and pitch is documented that may reflect a relationship between pitch and the geometry of visible colour space. more...
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- 2014
44. Electrophysiological correlates of processing unattended objects in visual cognition
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Wakui, Elley
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152.14 - Abstract
Research is divided as to what degree visually unattended objects are processed (Lachter et al., 2008; Carrasco, 2011). The hybrid model of object recognition (Hummel, 2001) predicts that familiar objects are automatically recognised without attention. However under perceptual load theory (Lavie, 1995), when objects are rendered unattended due to exhausted attentional resources, they are not processed. The present work examined the visual processing of images of everyday objects in a short-lag repetition-priming paradigm. In Experiments 1-3 attention was cued to the location of one of two objects in the first (prime) display, with the unattended sometimes repeated in the second (probe) display. ERP repetition effects were observed which were insensitive to changes in scale (Experiment 1) but sensitive to slight scrambling of the image (Experiment 2). Increasing perceptual load did not modulate these view-specific repetition effects (Experiment 3), consistent with the predictions of automatic holistic processing. In Experiments 4-7 a letter search task was used to render the flanking object image unattended under high load. In Experiment 5 distractor processing was observed in ERP even under high load. In Experiments 4, 6 and 7 a pattern of view sensitive/insensitive and load sensitive/insensitive repetition effects on RT (Experiment 4) and ERP amplitude (Experiments 6, 7) were observed that were difficult to interpret under either the hybrid model or perceptual load theory, but may reflect fast view-based and slow view-independent processing of objects. Overall, the properties of the view-sensitive repetition effects were generally consistent with those associated with the automatic/pre-attentive processing of the holistic route of the hybrid model. However, differences between the processing of objects rendered unattended via a spatial cue or perceptual load indicate that the bottom-up driven hybrid model and perceptual load theory may benefit from the consideration of the interaction of top-down biasing of processing (Tsotsos et al., 2008). more...
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- 2014
- Full Text
- View/download PDF
45. Predictive feedback to the primary visual cortex during saccades
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Edwards, Grace
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152.14 ,BF Psychology ,Q Science (General) - Abstract
Perception of our sensory environment is actively constructed from sensory input and prior expectations. These expectations are created from knowledge of the world through semantic memories, spatial and temporal contexts, and learning. Multiple frameworks have been created to conceptualise this active perception, these frameworks will be further referred to as inference models. There are three elements of inference models which have prevailed in these frameworks. Firstly, the presence of internal generative models for the visual environment, secondly feedback connections which project prediction signals of the model to lower cortical processing areas to interact with sensory input, and thirdly prediction errors which are produced when the sensory input is not predicted by feedback signals. The prediction errors are thought to be fed-forward to update the generative models. These elements enable hypothesis driven testing of active perception. In vision, error signals have been found in the primary visual cortex (V1). V1 is organised retinotopically; the structure of sensory stimulus that enters through the retina is retained within V1. A semblance of that structure exists in feedback predictive signals and error signal production. The feedback predictions interact with the retinotopically specific sensory input which can result in error signal production within that region. Due to the nature of vision, we rapidly sample our visual environment using ballistic eye-movements called saccades. Therefore, input to V1 is updated about three times per second. One assumption of active perception frameworks is that predictive signals can update to new retinotopic locations of V1 with sensory input. This thesis investigates the ability of active perception to redirect predictive signals to new retinotopic locations with saccades. The aim of the thesis is to provide evidence of the relevance of generative models in a more naturalistic viewing paradigm (i.e. across saccades). An introduction into active visual perception is provided in Chapter 1. Structural connections and functional feedback to V1 are described at a global level and at the level of cortical layers. The role of feedback connections to V1 is then discussed in the light of current models, which hones in on inference models of perception. The elements of inferential models are introduced including internal generative models, predictive feedback, and error signal production. The assumption of predictive feedback relocation in V1 with saccades is highlighted alongside the effects of saccades within the early visual system, which leads to the motivation and introduction of the research chapters. A psychophysical study is presented in Chapter 2 which provides evidence for the transference of predictive signals across saccades. An internal model of spatiotemporal motion was created using an illusion of motion. The perception of illusory motion signifies the engagement of an internal model as a moving token is internally constructed from the sensory input. The model was tested by presenting in-time (predictable) and out-of-time (unpredictable) targets on the trace of perceived motion. Saccades were initiated across the illusion every three seconds to cause a relocation of predictive feedback. Predictable in-time targets were better detected than the unpredictable out-of-time targets. Importantly, the detection advantage for in-time targets was found 50 – 100 ms after saccade indicating transference of predictive signals across saccade. Evidence for the transfer of spatiotemporally predictive feedback across saccade was supported by the fMRI study presented in Chapter 3. Previous studies have demonstrated an increased activity when processing unpredicted visual stimulation in V1. This activity increase has been related to error signal production as the input was not predicted via feedback signals. In Chapter 3, the motion illusion paradigm used in Chapter 2 was redesigned to be compatible with brain activation analysis. The internal model of motion was created prior to saccade and tested at a post-saccadic retinotopic region of V1. An increased activation was found for spatiotemporally unpredictable stimuli directly after eye-movement, indicating the predictive feedback was projected to the new retinotopic region with saccade. An fMRI experiment was conducted in Chapter 4 to demonstrate that predictive feedback relocation was not limited to motion processing in the dorsal stream. This was achieved by using natural scene images which are known to incorporate ventral stream processing. Multivariate analysis was performed to determine if feedback signals pertaining to natural scenes could relocate to new retinotopic eye-movements with saccade. The predictive characteristic of feedback was also tested by changing the image content across eye-movements to determine if an error signal was produced due to the unexpected post-saccadic sensory input. Predictive feedback was found to interact with the images presented post-saccade, indicating that feedback relocated with saccade. The predictive feedback was thought to contain contextual information related to the image processed prior to saccade. These three chapters provide evidence for inference models contributing to visual perception during more naturalistic viewing conditions (i.e. across saccades). These findings are summarised in Chapter 5 in relation to inference model frameworks, transsacadic perception, and attention. The discussion focuses on the interaction of internal generative models and trans-saccadic perception in the aim of highlighting several consistencies between the two cognitive processes. more...
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- 2014
46. Psychophysical studies of interactions between luminance and chromatic information in human vision
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Clery, Stéphane and Harris, Julie M.
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152.14 ,Colour perception ,Luminance perception ,Shape perception - Abstract
In this thesis, I investigated how human vision processes colour and luminance information to enable perception of our environment. I first tested how colour can alter the perception of depth from shading. A luminance variation can be interpreted as either variation of reflectance (patterning) or variation of shape. The process of shape-from-shading interprets luminance variation as changes in the shape of the object (e.g. the shading on an object might elicit the perception of curvature). The addition of colour variation is known to modify this shape-from-shading processing. In the experiments presented here I tested how luminance driven percepts can be modified by colour. My first series of experiments confirmed that depth is modulated by colour. I explored a larger number of participants than previously tested. Contrary to previous studies, a wide repertoire of behaviour was found; participants experienced variously more depth, or less depth, or no difference. I hypothesised that the colour modulation effect might be due to a low-level contrast modulation of luminance by colour, rather than a higher-level depth effect. In a second series of experiments, I therefore tested how the perceived contrast of a luminance target can be affected by the presence of an orthogonal mask. I found that colour had a range of effects on the perception of luminance, again dependant on the participants. Luminance also had a similar wide range of effects on the perceived contrast of luminance targets. This showed that, at supra-threshold levels, a luminance target's contrast can be modulated by a component of another orientation (colour or luminance defined). The effects of luminance and colour were not following a particular rule. In a third series of experiments, I explored this interaction at detection levels of contrast. I showed cross-interaction between luminance target and mask but no effects of a colour mask. more...
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- 2014
47. The role of peripheral vision in flow parsing
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Rogers, Cassandra
- Subjects
152.14 ,BF Psychology - Abstract
Identifying moving objects while we are moving is an important everyday skill. This ability allows us to monitor our surroundings, successfully interact with objects or people, and avoid potential hazards. Self-movement generates optical flow on the retina that complicates the recognition of moving objects from retinal motion alone. Rushton and Warren (2005) proposed a purely visual solution to this problem. They suggest that, in order to assess scene-relative object movement, the brain identifies and parses out (globally subtracts) patterns of visual flow that are consistent with self-movement. Existing research has demonstrated evidence of this flow parsing process in central vision (i.e. Warren & Rushton, 2008). This thesis aims to characterise the role of peripheral visual flow in this process. Research from the wider self-motion literature has often distinguished between central and peripheral vision. Some researchers have claimed that peripheral vision is specialised for self-motion perception, whilst more recent studies have challenged this assertion. This thesis investigates whether peripheral visual motion, traditionally considered to be a strong cue to self-movement, also contributes to flow parsing. The experimental work employed a simulated self-movement paradigm to isolate retinal motion from other non-visual cues. Thus, observers remained stationary whilst computer generated stimuli moved to produce patterns of retinal motion associated with actual self-movement. In the first set of experiments, I demonstrate that peripheral flow simulating forward or backward self-movement gives rise to characteristic flow parsing effects. This finding generalises to rotational observer motion (Chapter 3). Chapter 4 considers whether peripheral flow contributes to parsing for judgements of object size change. Finally, Chapter 5 investigates whether there is a benefit of peripheral information under conditions where central flow is potentially ambiguous. The results indicate that peripheral visual flow contributes to the flow parsing process. The contribution of flow in the near periphery appears to be maximally important. more...
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- 2014
48. Psychophysical correlates of age-related visual decline
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Arena, Amanda, Hutchinson, Claire, and Shimozaki, Steven
- Subjects
152.14 - Abstract
Much of what we know about the physical world is afforded to us by vision. With age, our ability to perceive visual information can become compromised, affecting our autonomy and quality of life. Given the rapid aging of populations worldwide, it is imperative to develop a comprehensive understanding of the effects of age on visual function. Therefore, the aim of this thesis was to use psychophysical methods to examine the effects of 'healthy' aging on visual perception, with a particular emphasis on furthering our knowledge of age-related reductions in the perception of motion. In a series of experiments, observers ranging from 18-82 years of age were tested and compared on psychophysical tasks which measured orientation and direction sensitivity, first- and second-order global motion perception, contrast sensitivity, and visual attention as measured by the Useful Field of View. Four key findings are presented within this thesis: (1) older observers demonstrate reductions in orientation and direction sensitivity that can be attributed to increased internal noise in the aged motion pathway, (2) age related impairments in global motion perception are mediated by reductions in spatial integration, (3) older female populations may be particularly susceptible to age-related impairments in motion perception, and (4) there may be a selective impairment in the processing of second-order radial motion in the aged. The results of these experiments indicate that whilst some aspects of motion processing remain preserved with age, older observers demonstrate marked impairments on a number of motion tasks. more...
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- 2013
49. Some aspects of visual discomfort
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O'Hare, Louise, Hibbard, Paul B., and Vishwanath, Dhanraj
- Subjects
152.14 ,Visual discomfort ,Natural images - Abstract
Visual discomfort is the adverse sensations, such as headaches and eyestrain, encountered on viewing certain stimuli. These sensations can arise under certain viewing conditions, such as stereoscopic viewing and prolonged reading of text patterns. Also, discomfort can occur as a result of viewing stimuli with certain spatial properties, including stripes and filtered noise patterns of particular spatial frequency. This thesis is an exploration of the stimulus properties causing discomfort, within the framework of two theoretical explanations. Both of the explanations relate to the stimuli being difficult for the visual system to process. The first is concerned with discomfort being the result of inefficient neural processing. Neural activity requires energy to process information, and stimuli that demand a lot of energy to be processed might be uncomfortable. The second explanation revolves around uncomfortable stimuli not being effective in driving the accommodative (focussing) response. Accommodation relies on the stimulus as a cue to drive the response effectively - an uninformative cue might result in discomfort from an uncertain accommodative response. The following research investigates both these possibilities using a combination of psychophysical experimentation, questionnaire-based surveys on non-clinical populations, and computational modelling. The implications of the work for clinical populations are also discussed. more...
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- 2013
50. How fast can we see? : the latency development in human infants to pattern, orientation, and direction-reversal visual evoked potentials
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Lee, Jin and Braddick, Oliver
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152.14 ,Ophthamology ,Paediatrics ,Neuroscience ,Developmental psychology ,Experimental psychology ,Perception ,Biology ,visual evoked potentials ,pattern reversal ,motion ,orientation ,direction - Abstract
The goal of this thesis is to track latency changes in three visual evoked potentials (VEP) stimuli as an indication of overall brain development, in order to provide a normative baseline to differentiate visual and neurological development from pathological processes. VEP- neural electrical activity recorded from the scalp surface and synchronized with visual stimulus transitions- is one of the common techniques in understanding infant vision development. Past work has concentrated on responses to pattern reversal and to the latency of the initial positive peak. Here we compare the timing of responses to pattern, orientation, and direction-reversal VEPs, and transient peak latencies to those calculated from the gradient of steady-state phase against reversal rate. The three stimuli were tested in 81 adults at 1- 16 r/s and 137 infants (3.6- 79.0 weeks) at 2- 8 r/s. Initial responses to orientation and direction were as fast as for contrast- around 100 ms, consistent with other findings that V1 is orientation selective. Cortical processing for both OR and DR yielded longer latencies (200 ms) by the calculated method, perhaps reflecting more involvement of higher visual processing in comparison to PR. Orientation and direction latencies also had a delayed onset and longer developmental period to reach maturity. Infants reached adult transient PR latency values by 15 weeks, for OR by 50 weeks, and for DR by 10 weeks. For the calculated latency, infants reached both adult PR and DR latencies by 30 weeks while OR showed little change across age. We successfully confirmed that (1) phase-based calculation of latency is effective, easy to use, and taps into a different cortical pathway; (2) motion processing has an additional, faster, subcortical pathway; (3) a parallel processing of initial contrast and orientation; and (4) later visual processing is not only developmentally delayed for all three stimuli but also more vulnerable to perinatal brain damage. These latency differences provided a baseline for clinical evaluations where identification of delayed latencies should aid early diagnosis and guide therapies for adults and infants. more...
- Published
- 2013
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