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What makes identification of familiar faces seemingly effortless? Recent studies using unfamiliar face stimuli suggest that selective processing of information conveyed by horizontally oriented spatial frequency components supports accurate performance in a variety of tasks involving matching of facial identity. Here, we studied upright and inverted face discrimination using stimuli with which observers were either unfamiliar or personally familiar (i.e., friends and colleagues). Our results reveal increased sensitivity to horizontal spatial frequency structure in personally familiar faces, further implicating the selective processing of this information in the face processing expertise exhibited by human observers throughout their daily lives.

Face perception is impaired in a variety of ways in older adults, but the mechanisms underlying these changes remain unclear. A central theme of this dissertation is that task performance is constrained by two factors intrinsic to the observer: sources of random variability -- internal noise -- and the efficiency with which task-relevant stimulus information is utilized. This thesis uses several behavioural, psychophysical methods to examine how age-related changes in one or both of these factors affect face processing. Chapter 2 used the classification image (CI) method to characterize the spatial sampling patterns of younger and older observers performing a face discrimination task. Compared to younger adults, older adults used information in the eye/brow region less consistently and instead relied on relatively less informative regions such as the forehead. The differences in CIs accounted for the lower absolute efficiency that was found in older observers. Chapter 3 estimated internal noise and calculation efficiency by measuring threshold-vs.-noise (TvN) curves and response consistency in a face discrimination task. Compared to younger observers, older observers had higher additive internal noise and lower calculation efficiency, but the magnitude of multiplicative internal noise did not differ between age groups. Previous studies have shown that younger adults have a bias to rely on horizontal structure to discriminate and identify faces, and the magnitude of this so-called horizontal bias is correlated with identification accuracy. The experiments in Chapter 4 measured horizontal bias in younger and older adults, and found that age differences in horizontal bias account for some, but not all, of the age difference in face identification accuracy. In summary, my work demonstrates that additive (but not multiplicative) internal noise is greater in older adults, and that they are less efficient at sampling information that is conveyed by structure at different locations and orientations in a face. Dissertation Doctor of Science (PhD) Our experience of the visual environment results from perceptual processes in the brain. Many of these processes change with age, such as our ability to identify someone from a photograph of their face. Performance is influenced by both random variability, or "noise", within the observer and how efficiently we use task-relevant information in the visual environment. By systematically manipulating the amount of available stimulus information I assessed the contribution of these factors to older adults' judgements of facial identity, and characterized the information on which these decisions are based. These experiments are the first to consider how face perception in older adults is constrained by the combined effects of internal noise and the efficiency with which the visual system utilizes various sources of information. The results provide a number of directions for future research in the fields of face perception and age-related changes in complex pattern vision.

The present dissertation investigates perceptual grouping and shape completion in motion in younger and older observers. The first set of experimental results, Chapter 2, provides a set of exploratory experiments which characterize the nature of a motion grouping task, uncovering accurate direction discrimination when the stimulus contains a cue for global grouping, but compelling illusory motion when the stimulus does not contain that cue. Chapter 3 builds on the findings of Chapter 2 by measuring biases in motion integration that lead to the illusory motion. Chapter 3 extends this paradigm into older adult observers, and explores various stimulus variants and tasks to add to the characterization of this effect. In Chapter 4 the concepts of grouping and motion perception are examined in the context of shape completion by measuring the accuracy of aspect ratio, size, and area discrimination in younger and older adults. Chapter 4 concludes that older adults are worse at discriminating characteristics of shapes, but are similarly affected by incomplete or occluded stimuli as younger adults. On the whole, the dissertation demonstrates several novel findings in the perceptual organization literature, and investigates many of these phenomena into older adults populations. Throughout the dissertation, a central theme concerns the concept of behavioural measurement of perceptual processes, and consequently many of the phenomena are studied using several types of measurements and tasks to ensure a complete picture of the perceptual experience. The current dissertation presents a novel and important addition to the current literature in perceptual grouping and shape completion. Dissertation Bachelor of Arts (BA) The visual experience of a human observer is the result of perceptual processing of the brain. The current dissertation focuses on a subset of these processes that concern the grouping of segmented parts into a single shape, while those segmented parts are in motion. This type of visual experience occurs regularly, in situations where part of a visual scene is occluded such as when viewing a moving object through a window occluded by window blinds. In general, we measure an observer's ability to identify the sources of motion, and the ability to infer the shape of the grouped object. In each experiment, the physical stimulus is varied while observers complete tasks using this stimulus. The dissertation characterizes perceptual organization processes using a number of different tasks, stimuli and paradigms in both younger and older adults. Each experimental result characterizes the perceptual abilities of younger and older observers, contributing to the literature of visual perceptual organization in aging.

Perceptual learning occurs because observers become more sensitive to informative aspects of the stimuli. Learning the informative aspects of one stimulus set does not transfer to another stimulus set of the same class. In this dissertation, the argument will be made that if observers learn how to discover informative aspects, learning will be more generalizable. However, discovery requires that the informative aspects are not easily apparent. To this end, stimulus orientation structure can be manipulated to contain informative structure in one orientation band, and non-informative structure in the other orientation band. Such a manipulation was inspired by research on face perception: Faces are best identified when decisions are based more on the horizontal relative to the vertical facial structure. Hence, the first three chapters focus on understanding the horizontal bias during face identification, and the final two chapters introduce a novel stimulus set for which horizontal bias may be learned. Chapter 2 identifies a neural marker of horizontal bias that is correlated with face identification accuracy, suggesting that we can predict how well observers identify faces based on their neural sensitivity to horizontal relative to vertical structure. Chapter 3 shows that when face identification accuracy declines due to healthy ageing, so too do behavioural and neural horizontal bias, but Chapter 4 shows that perceptual learning can increase horizontal bias in healthy older adults. Chapter 5 uses texture stimuli and shows that observers can learn to discover informative horizontal structure embedded in uninformative vertical structure. Chapter 6 extends these findings to show that adequate practice results in learning that generalizes to novel textures for which the orientation-selective processing is relevant. The results presented inform our understanding of the neural representations associated with orientation-selective processing, and suggest that observers can learn to discover informative structure conveyed by a particular orientation band. Thesis Doctor of Philosophy (PhD)

The research discussed here examines how normal healthy aging affects spatial suppressive mechanisms in a variety of visual tasks using both static and dynamic stimuli. Prior research has suggested that younger adults demonstrate a center-surround antagonistic pattern in which they show spatial summation at low contrast and spatial suppression at high contrast in brief motion direction discrimination tasks. Older adults have been shown to have reduced spatial suppression at high contrast and this is thought to be related to reduced GABAergic inhibition in the visual cortex. The results obtained from this program of research suggest that age-related changes in optical and neural visual mechanisms do not affect spatiotemporal mechanisms for static stimuli when the target is presented with the mask (embedded masking). However, when the mask appears immediately before (forward masking) or after (backward masking) the target, older adults require more contrast to detect the target (Chapter 2). In addition, spatial suppression is not reduced for older adults in a task with moving stimuli presented at long durations, even with increasing speed (Chapter 3). In Chapter 4, we used static stimuli presented at brief durations to induce a sudden motion onset and found that although there was no significant age difference in spatial suppression, there was a trend showing reduced levels of spatial suppression in older adults. These results taken together suggest that inhibitory neural mechanisms in the visual cortex may mediate spatial suppression for briefly presented stimuli only. Thesis Doctor of Philosophy (PhD)

Human vision has an incredible ability to translate light reaching the retinae into a coherent, three-dimensional representation of the outside world in a fraction of a second. Much research has been devoted to understanding how local orientation information is integrated to form global contours and shapes -– a crucial step in visual processing. This dissertation describes experiments examining how contour and shape perception are affected in healthy aging. Chapter 2 examined contour grouping at low contrast and in the absence of distracters. Unlike younger subjects, older subjects did not benefit from co-alignment of local orientations with the contour’s outline, suggesting that grouping by orientation co-alignment is impaired in older age in low contrast. Chapters 3 and 4 examined the effects of aging on the ability to detect and discriminate high-contrast contours embedded in a dense field of distracters, as real life situations often require detecting objects among clutter, such as a snake hiding among tall grass. Results showed that older adults require significantly more time to discriminate contours in clutter, especially for less salient contours. Moreover, increasing the relative density of background clutter had a greater detrimental effect on older, compared to younger, subjects. However, aging did not seem to affect the ability to group contours across a range of spatial distances, or the sensitivity of contour integration to orientation misalignment. Lastly, Chapter 5 examined the influence of local orientation information on the perception of a contour's shape. Results revealed that older and younger subjects perceived the shape of a sampled contour in the same way, even when the contour's orientation and position information were in conflict. These findings indicated that the integration of orientation and position information in shape perception does not change with age. Doctor of Philosophy (PhD)

Older adults tend to show overall recognition deficits and qualitatively different patterns in the particular expressions that are most difficult to identify (Ruffman et al., 2008). In the current study, 23 younger (18-33 years old) and 23 older (60-80 years old) adults performed a 4AFC (angry, fearful, happy, sad) facial expression categorization task varying orientation (upright/inverted) and stimulus duration (100, 500, 1000 ms). For both groups, happiness was the easiest expression to identify and fear and sadness were the most difficult. Compared to younger adults, older adults were more affected by stimulus orientation, and generally benefit less from increased stimulus duration. For upright faces, there was no age difference in response accuracy but response latency was longer in older subjects. For inverted faces, older adults showed lower accuracy and longer latencies for expressions of anger, fear, and sadness. Recognition of inverted happy faces was spared in older adults for accuracy, but not response latency. These findings could not be explained by impaired detection sensitivity, as no systematic age differences were found for perceived intensity ratings. Finally, the expressions that were most to least difficult to identify was the same in each age group at both orientations. Overall, these results suggest that older individuals process expressive faces in a qualitatively similar way to their younger counterparts, but are less efficient at extracting the diagnostic information. Master of Science (MSc)

Motion perception is required for nearly all aspects of daily living. Previous work has shown that the ability to perceive a variety of different types of motion is significantly affected by increasing age. The research described in this thesis further probes the complex effects of aging on motion perception using a number of different stimulus configurations and performance measures. In order to relate psychophysical performance to neurophysiological recordings, we used two different notched-noise masking techniques to estimate the directional selectivity of masking in younger and older adults. We found evidence to suggest that the directionally-selective mechanisms of older adults are more broadly tuned than their younger counterparts, which is consistent with the animal literature that links decreased neural inhibition with broader directional tuning. Behaviourally however, task-specific contrast sensitivity may play a role in explaining that result. We also uncovered a previously unobserved difference between horizontal and vertical motion perception which bears further study. We also tested peripheral motion perception both under focused and divided attentional conditions. Older adults were impaired compared to younger adults when asked to make judgments about motion patterns in the peripheral vision field, but they were not differentially impaired under divided attention conditions. The findings presented in this thesis paint a more complex picture of how increasing age impairs motion perception than previously described. Specifically, our observations show that age effects interact with stimulus contrast, attention, and motion direction, and these interactions each present an interesting avenue for further exploration. Doctor of Philosophy (PhD)

Audio-visual temporal processing, and integration of auditory and visual information, is necessary for perceiving the world around us. Although previous research has indicated a slowing of temporal processing in older adults for unisensory stimuli, little work has examined the role of aging on multisensory temporal processing. The goal of this thesis is to use temporal-order judgment (TOJ) tasks to investigate age-related changes in audio-visual temporal processing. Overall, our results indicate that older adults do not demonstrate impairments on simple audio-visual TOJs, but they do exhibit deficits on more complex TOJ tasks. We also found no influence of spatial cues on TOJs for younger or older adults. Finally, we found age differences in complex TOJ tasks could not be explained by changes in the ability of older adults to detect a gap between sequential visual stimuli. The work in the thesis suggests that although there may be slowing in audio-visual temporal processing in complex situations, there are circumstances where audio-visual temporal processing is spared. By categorizing multisensory processing deficits in the elderly, we can aim to improve quality of life by preventing fails and perpetuating social interactions. Master of Science (MSc)

Holistic processing has been deemed a crucial part of human face processing. There are three tasks that are indexes of holistic processing and each is used by many researchers for the purposes of demonstrating that either their participants have intact holistic processing or that holistic processing is impaired or missing. The tasks that demonstrate holistic processing are the face inversion, composite face, and the whole-part tasks. In this dissertation, I evaluate the hypothesis that holistic processing is important for face identification. A secondary hypothesis that is evaluated is whether the three indexes of holistic processing are related and whether they are tapping the same underlying process. Chapter 2 tests the first hypothesis in a large group of young adults and shows that the composite face effect (an index of holistic processing) is not related to accuracy on two identification tasks. Chapter 3 tested both hypotheses and showed that none of the holistic indexes are related to one another and they are unrelated to face identification accuracy. In Chapter 4, a large group of older adults are tested on the composite face task and a face identification task, similar to Experiment 2 from Chapter 2. Unlike the results for young adults, older adults show a significant positive correlation between the composite face effect and identification accuracy even though older adults perform worse on the identification task. Doctor of Philosophy (PhD)

As an object approaches an observer’s eye, tau, defined as the inverse relative expansion rate of the object’s image on the retina (Lee, 1976) approximates time-to-collision (TTC). Many studies have suggested that human observers use TTC information, but evidence for the use of tau remains inconclusive. Here we present two studies that investigated the use of tau in object-motion and observer-motion situations. In Study I, we dissociated several monocular variables that potentially contributed to TTC perception, and found that participants were most sensitive to TTC information when performing a relative TTC estimation task; and less sensitive to non-time variables such as distance-to-collision, speed and object size. Additionally, when we manipulated sources of information to specify conflicting time-of-arrivals, TTC specified by tau was weighted more than TTC derived from distance and speed. Thus, we concluded that even in the presence of monocular depth information, observers strongly utilize tau when estimating TTC. In Study II, observers estimated TTC of a looming target in the presence or absence of background expansion. Results demonstrated that participants overestimated TTC in situations where the surroundings of the target’s contours expanded at a reduced rate. Moreover, simulated self-motion was unnecessary to induce this bias, as results were comparable in situations where this relative expansion was limited to the target’s immediate surroundings. Therefore, we also concluded that a relative tau variable, based on the relative rate of expansion, is utilized whenever expansion beyond the object’s immediate boundaries is less than the target’s absolute rate of expansion. Master of Science (MSc)