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DNA polymorphism in mosquito fish (Gambusia affinis), as revealed by RAPD (randomly amplified polymorphic DNA) and allozyme analysis, was compared to relative amounts of DNA strand breakage in blood and liver tissues. Mosquito fish were exposed to radionuclide contamination in situ and to X-rays in the laboratory. The types of RAPD metrics used were the number of RAPD bands per individual and the frequency of certain RAPD bands. In a previous study, it was noted that in some instances the number of RAPD bands and the frequency of certain RAPD bands were elevated in radionuclide-contaminated sites relative to reference sites. In the present study, it was found that the median molecular length (MML) of the DNA (which is inversely proportional to the amount of DNA strand breakage) was correlated in several cases to the number of RAPD bands per individual. In addition, for those RAPD bands that occurred at a higher frequency in mosquito fish from radionuclidecontaminated sites, DNA strand breakage was often lower for those fish with than without these RAPD bands. RAPD data obtained on mosquito fish exposed to X-rays in the laboratory paralleled those from the field. Furthermore, analysis showed that heterozygotes for the allozyme locus nucleoside phosphorylase were more prevalent in radionuclidecontaminated sites and had fewer DNA strand breaks than did homozygotes. These results provide additional evidence that changes in population genetic structure of mosquito fish exposed to a genotoxicant (radiation) can be detected at the DNA level.

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Observers continually make unconscious inferences about the state of the world based on ambiguous sensory information. This process of perceptual decision-making may be optimized by learning from experience. We investigated the influence of previous perceptual experience on the interpretation of ambiguous visual information. Observers were pre-exposed to a perceptually stabilized sequence of an ambiguous structure-from-motion stimulus by means of intermittent presentation. At the subsequent re-appearance of the same ambiguous stimulus perception was initially biased toward the previously stabilized perceptual interpretation. However, prolonged viewing revealed a bias toward the alternative perceptual interpretation. The prevalence of the alternative percept during ongoing viewing was largely due to increased durations of this percept, as there was no reliable decrease in the durations of the pre-exposed percept. Moreover, the duration of the alternative percept was modulated by the specific characteristics of the pre-exposure, whereas the durations of the pre-exposed percept were not. The increase in duration of the alternative percept was larger when the pre-exposure had lasted longer and was larger after ambiguous pre-exposure than after unambiguous pre-exposure. Using a binocular rivalry stimulus we found analogous perceptual biases, while pre-exposure did not affect eye-bias. We conclude that previously perceived interpretations dominate at the onset of ambiguous sensory information, whereas alternative interpretations dominate prolonged viewing. Thus, at first instance ambiguous information seems to be judged using familiar percepts, while re-evaluation later on allows for alternative interpretations. ispartof: PLoS One vol:7 issue:1 pages:1-15 ispartof: location:United States status: published

When the two eyes view incompatible images that subtend the entire visual field, perception alternates between the two images unpredictably: at seemingly random times and locations, observers experience sudden changes in the awareness of the unchanging visual stimulation. Here we focus on the very first spontaneous breakout from the very first suppression phase after onset of the two eyes' competing whole-field stimuli. We call such spontaneous local breakout an "initial percept-switch nucleation." We employed homogeneous visual input to examine where, and how, spontaneous local initial percept-switch nucleations originate, demonstrating that their spatial distribution contains locally random inhomogeneities, which are eye- and observer-dependent. We were able to predict the occurrence probability of the percept nucleations by adaptation buildup of the neurons associated with the representation of one eye's image. Intriguingly, the neuronal processes related to both cross-inhibition and local eye dominance could not predict nucleation probability; this is because nucleation inhomogeneity appeared to be different from another previously reported local inhomogeneity known as "onset bias" signifying the local first dominance-choice inhomogeneity upon stimulus onset. Collectively, we reveal a governing role of local adaptation in the neurons associated with early visual processing of one eye's image, in the origination of new phases in awareness

When sensory input allows for multiple, competing perceptual interpretations, observers’ perception can fluctuate over time, which is called bistable perception. Imaging studies in humans have revealed transient responses in a right-lateralized network in the frontalparietal cortex (rFPC) around the time of perceptual transitions between interpretations, potentially reflecting the neural initiation of transitions.Weinvestigated the role of this activity in malehumanobservers, with specific interest in its relation to the temporal structure of transitions, which can be either instantaneous or prolonged by periods during which observers experience a mix of both perceptual interpretations. Using both bistable apparent motion and binocular rivalry, we show that transition-related rFPC activity is larger for transitions that last longer, suggesting that rFPC remains active as long as a transition lasts. We also replicate earlier findings that rFPC activity during binocular rivalry transitions exceeds activity during yoked transitions that are simulated using video replay. However, we show that this established finding holds only when perceptual transitions are replayed as instantaneous events. When replay, instead, depicts transitions with the actual durations reported during rivalry, yoked transitions and genuine rivalry transitions elicit equal activity. Together, our results are consistent with the view that at least a component of rFPC activation during bistable perception reflects a response to perceptual transitions, both real and yoked, rather than their cause. This component of activity could reflect the change in sensory experience and task demand that occurs during transitions, which fits well with the known role of these areas in attention and decision making

In bistable vision, one constant ambiguous stimulus leads to 2 alternating conscious percepts. This perceptual switching occurs spontaneously but can also be influenced through voluntary control. Neuroimaging studies have reported that frontal regions are activated during spontaneous perceptual switches, leading some researchers to suggest that frontal regions causally induce perceptual switches. But the opposite also seems possible: frontal activations may themselves be caused by spontaneous switches. Classically implicated in attentional processes, these same regions are also candidates for the origins of voluntary control over bistable vision. Here too, it remains unknown whether frontal cortex is actually functionally relevant. It is even possible that spontaneous perceptual switches and voluntarily induced switches are mediated by the same top-down mechanisms. To directly address these issues, we here induced ‘‘virtual lesions,’’ with transcranial magnetic stimulation, in frontal, parietal, and 2 lower level visual cortices using an established ambiguous structure-from-motion stimulus. We found that dorsolateral prefrontal cortex was causally relevant for voluntary control over perceptual switches. In contrast, we failed to find any evidence for an active role of frontal cortex in passive bistable vision. Thus, it seems the same pathway used for willed top-down modulation of bistable vision is not used during passive bistable viewing.

Background: About 30% of the population has difficulties detecting the sign and the magnitude of binocular disparity in the absence of eye movements, a phenomenon called stereo-anomaly. The stereo-anomaly tests so far are based on disparity only (e.g. red-green stereograms), which means that other depth cues cannot be used and even provide conflicting depth information. Objective: Here we investigate whether stereo-anomaly also occurs using a "true-3D" display which provides other depth cues that are all consistent with one particular distance in depth. Secondly, we examine differences in depth perception between red-green (anaglyphic) and true-3D displays. Finally, we test the displays' relative viewing comfort. Method: Sixteen observers (four of which were stereo-anomalous) judged the distance in depth between a fixation square and one or two bars. They were presented on an anaglyphic and a true-3D display, both in the fovea and 5 deg in the visual periphery. Observers were asked about the viewing comfort of both displays. Results: Stereo-anomalous observers also showed difficulties in perceiving depth with the true-3D display. Yet the true-3D display increased the perceived depth range compared to the red-green display for practically all observers at both eccentricities. All observers reported greater viewing comfort for the true-3D display. Conclusion: Stereo-anomaly is a robust phenomenon. True-3D displays improve depth perception and viewing comfort, most likely because retinal blur provides depth information consistent with disparity. Application: The true-3D display shows potential for clearly and comfortably displaying objects at different depth planes. © 2010 Elsevier B.V. All rights reserved.

A previous study has shown that diverting attention from binocular rivalry to a visual distractor task results in a slowing of rivalry alternation rate between simple orthogonal orientations. Here, we investigate whether the slowing of visual perceptual alternations will occur when attention is diverted to an auditory distractor task, and we extend the investigation by testing this for two kinds of binocular rivalry stimuli and for the Necker cube. Our results show that doing the auditory attention task does indeed slow visual perceptual alternations, that the slowing effect is a graded function of attentional load, and that the attentional slowing effect is less pronounced for grating rivalry than for house/face rivalry and for the Necker cube. These results are explained in terms of supramodal attentional resources modulating a high-level interpretative process in perceptual ambiguity, together with a role for feedback to early visual processes in the case of binocular rivalry.

Stereo-vision is generally considered to provide information about depth in a visual scene derived from disparities in the positions of an image on the two eyes; a new study has found evidence that retinal-image coding relative to the head is also important.

When observing bistable stimuli, the percept can change in the absence of changes in the stimulus itself. When intermittently presenting a bistable stimulus, the number of perceptual alternations can increase or decrease, depending on the duration of the period that the stimulus is removed from screen between stimulus presentations (off-period). Longer off-periods lead to stabilization of the percept, while short off-periods produce perceptual alternations. Here we compare fMRI brain activation across percept repetitions and alternations when observing an intermittently presented ambiguously rotating structure from motion sphere. In the first experimental session, subjects were requested to voluntarily control the percept into either a repeating or an alternating perceptual regime at a single off-period. In a consecutive session, subjects observed the sphere uninstructed, and reported alternations and repetitions. The behavioral data showed that there were marked individual biases for observing the sphere as either repeating or alternating. The fMRI data showed activation differences between alternating and repeating perceptual regimes in an extensive network that included parietal cortex, dorsal premotor area, dorsolateral prefrontal cortex, supplementary motor area, insula, and cerebellum. However, these activation differences could all be explained by intersubject differences in the bias for one of the two perceptual regimes. The stronger the bias was for a particular perceptual regime, the less activation and vice versa. We conclude that widespread activation differences between perceptual regimes can be accounted for by differences in the perceptual bias for one of the two regimes

When the sensory system is subjected to ambiguous input, perception alternates between interpretations in a seemingly random fashion. Although neuronal noise obviously plays a role, the neural mechanism for the generation of randomness at the slow time scale of the percept durations (multiple seconds) is unresolved. Here significant nonzero serial correlations are reported in series of visual percept durations (to the author’s knowledge for the first time accounting for duration impurities caused by reaction time, drift, and incomplete percepts). Serial correlations for perceptual rivalry using structure-from-motion ambiguity were smaller than for binocular rivalry using orthogonal gratings. A spectrum of computational models is considered, and it is concluded that noise in adaptation of percept-related neurons causes the serial correlations. This work bridges, in a physiologically plausible way, widely appreciated deterministic modeling and randomness in experimental observations of visual rivalry.

The neural mechanisms underlying attentional selection of competing neural signals for awareness remains an unresolved issue. We studied attentional selection, using perceptually ambiguous stimuli in a novel multisensory paradigm that combined competing auditory and competing visual stimuli. We demonstrate that the ability to select, and attentively hold, one of the competing alternatives in either sensory modality is greatly enhanced when there is a matching cross-modal stimulus. Intriguingly, this multimodal enhancement of attentional selection seems to require a conscious act of attention, as passively experiencing the multisensory stimuli did not enhance control over the stimulus. We also demonstrate that congruent auditory or tactile information, and combined auditory–tactile information, aids attentional control over competing visual stimuli and visa versa. Our data suggest a functional role for recently found neurons that combine voluntarily initiated attentional functions across sensory modalities. We argue that these units provide a mechanism for structuring multisensory inputs that are then used to selectively modulate early (unimodal) cortical processing, boosting the gain of task-relevant features for willful control over perceptual awareness

We employed a parametric psychophysical design in combination with functional imaging to examine the influence of metric changes in perceptual incongruence on perceptual alternation rates and cortical responses. Subjects viewed a bistable stimulus defined by incongruent depth cues; bistability resulted from incongruence between binocular disparity and monocular perspective cues that specify different slants (slant rivalry). Psychophysical results revealed that perceptual alternation rates were positively correlated with the degree of perceived incongruence. Functional imaging revealed systematic increases in activity that paralleled the psychophysical results within anterior intraparietal sulcus, prior to the onset of perceptual alternations. We suggest that this cortical activity predicts the frequency of subsequent alternations, implying a putative causal role for these areas in initiating bistable perception. In contrast, areas implicated in form and depth processing (LOC and V3A) were sensitive to the degree of slant, but failed to show increases in activity when these cues were in conflict

When faced with inconclusive or conflicting visual input human observers experience one of multiple possible perceptions. One factor that determines perception of such an ambiguous stimulus is how the same stimulus was perceived on previous occasions, a phenomenon called perceptual memory. We examined perceptual memory of an ambiguous motion stimulus while applying transcranial magnetic stimulation (TMS) to the motion-sensitive areas of the middle temporal cortex (hMT+). TMS increased the predominance of whichever perceptual interpretation was most commonly reported by a given observer at baseline, with reduced perception of the less favored interpretation. This increased incidence of the preferred percept indicates impaired long-term buildup of perceptual memory traces that normally act against individual percept biases. We observed no effect on short-term memory traces acting from one presentation to the next. Our results indicate that hMT+ is important for the long-term buildup of perceptual memory for ambiguous motion stimuli

Adaptation is one of the key constituents of the perceptual alternation process during binocular rivalry, as it has been shown that preadapting one of the rivaling pairs before rivalry onset biases perception away from the adapted stimulus during rivalry. We investigated the influence of retinotopic and spatiotopic preadaptation on binocular rivalry. We show that for grating stimuli, preadaptation only influences rivalry when adaptation and rivalry locations are retinotopically matched. With more complex house and face stimuli, effects of preadaptation are found for both retinotopic and spatiotopic preadaptation, showing the importance of spatiotopic encoding in binocular rivalry. We show, furthermore, that adaptation to phase-scrambled faces results in retinotopic effects only, demonstrating the importance of form content for spatiotopic adaptation effects, as opposed to spatial frequency content. Are the spatiotopic adaptation influences on rivalry caused by direct spatiotopic stimulus interactions, or instead are they due to altered feedback from the adapted spatiotopic representations to the retinotopic representations that are involved in rivalry? By using rivaling face and grating stimuli that minimize rivalry between spatiotopic representations while still engaging these representations in stimulus encoding, we show that at least part of the preadaptation effects with face stimuli depend on feedback information.

The mechanisms underlying conscious visual perception are often studied with either binocular rivalry or perceptual rivalry stimuli. Despite existing research into both types of rivalry, it remains unclear to what extent their underlying mechanisms involve common computational rules. Computational models of binocular rivalry mechanisms are generally tested against Levelt's four propositions, describing the psychophysical relation between stimulus strength and alternation dynamics in binocular rivalry. Here we use a bistable rotating structure-from-motion sphere, a generally studied form of perceptual rivalry, to demonstrate that Levelt's propositions also apply to the alternation dynamics of perceptual rivalry. Importantly, these findings suggest that bistability in structure-from-motion results from active cross-inhibition between neural populations with computational principles similar to those present in binocular rivalry. Thus, although the neural input to the computational mechanism of rivalry may stem from different cortical neurons and different cognitive levels the computational principles just prior to the production of visual awareness appear to be common to the two types of rivalry.

Presenting the eyes with spatially mismatched images causes a phenomenon known as binocular rivalry—a fluctuation of awareness whereby each eye's image alternately determines perception. Binocular rivalry is used to study interocular conflict resolution and the formation of conscious awareness from retinal images. Although the spatial determinants of rivalry have been well-characterized, the temporal determinants are still largely unstudied. We confirm a previous observation that conflicting images do not need to be presented continuously or simultaneously to elicit binocular rivalry. This process has a temporal limit of about 350 ms, which is an order of magnitude larger than the visual system's temporal resolution. We characterize this temporal limit of binocular rivalry by showing that it is independent of low-level information such as interocular timing differences, contrast-reversals, stimulus energy, and eye-of-origin information. This suggests the temporal factors maintaining rivalry relate more to higher-level form information, than to low-level visual information. Systematically comparing the role of form and motion—the processing of which may be assigned to ventral and dorsal visual pathways, respectively—reveals that this temporal limit is determined by form conflict rather than motion conflict. Together, our findings demonstrate that binocular conflict resolution depends on temporally coarse form-based processing, possibly originating in the ventral visual pathway.

When the two eyes are presented with conflicting stimuli, perception starts to fluctuate over time (i.e., binocular rivalry). A similar fluctuation occurs when two patterns are presented to a single eye (i.e., monocular rivalry), or when they are swapped rapidly and repeatedly between the eyes (i.e., stimulus rivalry). Although all these cases lead to rivalry, in quantitative terms these modes of rivalry are generally found to differ significantly. We studied these different modes of rivalry with identical intermittently shown stimuli while varying the temporal layout of stimulation. We show that the quantitative differences between the modes of rivalry are caused by the presence of monocular interactions between the rivaling patterns; the introduction of a blank period just before a stimulus swap changed the number of rivalry reports to the extent that monocular and stimulus rivalries were inducible over ranges of spatial frequency content and contrast values that were nearly identical to binocular rivalry. Moreover when monocular interactions did not occur the perceptual dynamics of monocular, binocular, and stimulus rivalries were statistically indistinguishable. This range of identical behavior exhibited a monocular (~50 ms) and a binocular (~350 ms) limit. We argue that a common binocular, or pattern-based, mechanism determines the temporal constraints for these modes of rivalry.

We investigated the spatio-temporal dynamics of the steady-state-visual-evoked field during perceptual switches in binocular rivalry using MEG. Several authors have previously used frequency tagging in MEG studies on binocular rivalry and have claimed to have found a widespread network of synchronized areas that are entrained by the stimulus, reaching up to frontal regions. Moreover, it has been claimed that the results prove that becoming aware of a stimulus is established by increased intra- and interhemispheric synchronization of brain areas, separated by large distances. Our results dovetailed nicely with previous findings such as power and coherence modulations as a function of perceptual state. However, while we also found stimulus-entrained activity across the entire scalp, a phase analysis revealed that the spatially extended nature of the frequency tag was produced by a limited set of occipital sources. Furthermore, we provide evidence that the coherence results from earlier studies are ambiguous in that they likely measured coherence between different sensors whose signals were dominated by the same sources. We conclude that the claims about widespread synchronized networks to consciously perceive flicker stimuli are currently unconvincing.

When visual input is inconclusive, does previous experience aid the visual system in attaining an accurate perceptual interpretation? Prolonged viewing of a visually ambiguous stimulus causes perception to alternate between conflicting interpretations. When viewed intermittently, however, ambiguous stimuli tend to evoke the same percept on many consecutive presentations. This perceptual stabilization has been suggested to reflect persistence of the most recent percept throughout the blank that separates two presentations. Here we show that the memory trace that causes stabilization reflects not just the latest percept, but perception during a much longer period. That is, the choice between competing percepts at stimulus reappearance is determined by an elaborate history of prior perception. Specifically, we demonstrate a seconds-long influence of the latest percept, as well as a more persistent influence based on the relative proportion of dominance during a preceding period of at least one minute. In case short-term perceptual history and long-term perceptual history are opposed (because perception has recently switched after prolonged stabilization), the long-term influence recovers after the effect of the latest percept has worn off, indicating independence between time scales. We accommodate these results by adding two positive adaptation terms, one with a short time constant and one with a long time constant, to a standard model of perceptual switching.

At the onset of bistable stimuli, the brain needs to choose which of the competing perceptual interpretations will first reach awareness. Stimulus manipulations and cognitive control both influence this choice process, but the underlying mechanisms and interactions remain poorly understood. Using intermittent presentation of bistable visual stimuli, we demonstrate that short interruptions cause perceptual reversals upon the next presentation, whereas longer interstimulus intervals stabilize the percept. Top-down voluntary control biases this process but does not override the timing dependencies. Extending a recently introduced low-level neural model, we demonstrate that percept-choice dynamics in bistable vision can be fully understood with interactions in early neural processing stages. Our model includes adaptive neural processing preceding a rivalry resolution stage with cross-inhibition, adaptation, and an interaction of the adaptation levels with a neural baseline. Most importantly, our findings suggest that top-down attentional control over bistable stimuli interacts with low-level mechanisms at early levels of sensory processing before perceptual conflicts are resolved and perceptual choices about bistable stimuli are made.

The spatial differences between the two retinal images, called binocular disparities, can be used to recover the three-dimensional (3D) aspects of a scene. The computation of disparity depends upon the correct identification of corresponding features in the two images. Understanding what image features are used by the brain to solve this binocular matching problem is an important issue in research on stereoscopic vision. The role of colour in binocular vision is controversial and it has been argued that colour is ineffective in achieving binocular vision. In the current experiment subjects were required to indicate the amount of perceived depth. The stimulus consisted of an array of fronto-parallel bars uniformly distributed in a constant sized volume. We studied the perceived depth in those 3D stimuli by manipulating both colour (monochrome, trichrome) and luminance (congruent, incongruent). Our results demonstrate that the amount of perceived depth was influenced by colour, indicating that the visual system uses colour to achieve binocular matching. Physiological data have revealed cortical cells in macaque V2 that are tuned both to binocular disparity and to colour. We suggest that one of the functional roles of these cells may be to help solve the binocular matching problem.

We examined whether binocular saccadic eye movements are determined solely by disparity-defined slant or whether they are influenced by both disparity-defined and perceived slant. The Werner illusion was used to distinguish a plane’s disparity-defined slant from its perceived slant. Three subjects viewed a horizontally elongated test strip that was flanked vertically by two planes. The perceived slant of the test strip depended on the slant of the flanking planes. Subjects estimated the perceived slant of the test strip by adjusting the angle between two lines in a symbolic top view. The saccadic eye movements between targets on the test strip were recorded both with visual feedback (“later saccades”) and without visual feedback (“first saccades”). We calculated vergence differences for saccades between targets on the test strip (and for fixation on these targets). For each geometrical test strip slant we examined whether the vergence differences could be explained as an effect of perceived slant. This study shows that saccadic eye movements are determined predominantly by the disparity-defined slant, but they can be affected by perceived slant, particularly when multiple saccades are being made.

Occlusion geometry gives rise to interocular shifts in the positions of binocularly viewed contour junctions. Since these shifts do not give rise to normal binocular disparities, they have been called ' pseudodisparities' . Previous work has shown that the unmatched contour segments of a partially occluded contour at occlusion junctions can be used to recover the geometry of the occluding surface through the construction of ' illusory' contours. Here, experiments were performed to determine whether such junction shiftscould enhance stereoscopic depth detection when the relative disparity between the contours was below threshold. Our results showed that stereoscopic depth detection does not improve when pseudodisparity is present. We conclude that the visual system is less sensitive to pseudodisparity than to conventional disparity information. We suggest that the primary role of pseudodisparity is to overcome conditions of camoue age.

The spatial differences between the images seen by the two eyes, called binocular disparities, can be used to recover the volumetric (three-dimensional) aspects of a scene. The computation of disparity depends upon the correct identi®cation of corresponding features in the two images. Understanding what image features are used by the brain to solve this matching problem is one of the main issues in stereoscopic vision. Many cortical neurons in visual areas V1 (ref. 2), MT (refs 3, 4) and MST (refs 5, 6) that are tuned to binocular disparity are also tuned to orientation, motion direction and speed. Although psychophysical work has shown that motion direction can facilitate binocular matching, the psychophysical literature on the role of orientation is mixed^8,9 , and it has been argued that speed differences are ineffective in aiding correspondence^7. Here we use a different psychophysical paradigm to show that the visual system uses similarities in orientation, motion direction and speed to achieve binocular correspondence. These results indicate that cells that multiplex orientation, motion direction, speed and binocular disparity may help to solve the binocular matching problem.