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Prediction error is a basic component of predictive-coding theory of brain processing. According to the theory, each stage of brain processing of sensory information generates a model of the current sensory input; subsequent input is compared against the model and only if there is a mismatch, a prediction error, is further processing performed. Recently, Smout and colleagues found that a signature of prediction error, the visual (v) mismatch negativity (MMN), for a fundamental property of visual input-its orientation-was absent without endogenous attention on the stimuli. This is remarkable because the weight of evidence for MMNs from audition and vision is that they occur without endogenous attention. To resolve this discrepancy, we conducted an experiment addressing 2 alternative explanations for Smout and colleagues' finding: that it was from a lack of reproducibility or that participants' visual systems did not encode the stimuli when attention was on something else. We conducted a similar experiment to that of Smout and colleagues. We showed 21 participants sequences of identically oriented Gabor patches, standards, and, unpredictably, otherwise identical, Gabor patches differing in orientation by ±15°, ±30°, and ±60°, deviants. To test whether participants encoded the orientation of the standards, we varied the number of standards preceding a deviant, allowing us to search for a decrease in activity with the number of repetitions of standards-repetition suppression. We diverted participants' attention from the oriented stimuli with a central, letter-detection task. We reproduced Smout and colleagues' finding of no vMMN without endogenous attention, strengthening their finding. We found that our participants showed repetition suppression: They did encode the stimuli preattentively. We also found early processing of deviants. We discuss various explanations why the earlier processing did not extend into the vMMN time window, including low precision of prediction., Competing Interests: The authors have declared that no competing interests exist., (Copyright: © 2023 Male, O’Shea. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

Research shows that the visual system monitors the environment for changes. For example, a left-tilted bar, a deviant, that appears after several presentations of a right-tilted bar, standards, elicits a classic visual mismatch negativity (vMMN): greater negativity for deviants than standards in event-related potentials (ERPs) between 100 and 300 ms after onset of the deviant. The classic vMMN is contributed to by adaptation; it can be distinguished from the genuine vMMN that, through use of control conditions, compares standards and deviants that are equally adapted and physically identical. To determine whether the vMMN follows similar principles to the auditory mismatch negativity (MMN), in two experiments we searched for a genuine vMMN from simple, physiologically plausible stimuli that change in fundamental dimensions: orientation, contrast, phase, and spatial frequency. We carefully controlled for attention and eye movements. We found no evidence for the genuine vMMN, despite adequate statistical power. We conclude that either the genuine vMMN is a rather unstable phenomenon that depends on still-to-be-identified experimental parameters, or it is confined to visual stimuli for which monitoring across time is more natural than monitoring over space, such as for high-level features. We also observed an early deviant-related positivity that we propose might reflect earlier predictive processing., (© 2020 The Authors. Psychophysiology published by Wiley Periodicals LLC on behalf of Society for Psychophysiological Research.)

When dissimilar images are presented one to each eye, we do not see both images; rather, we see one at a time, alternating unpredictably. This is called binocular rivalry, and it has recently been used to study brain processes that correlate with visual consciousness, because perception changes without any change in the sensory input. Such studies have used various types of images, but the most popular have been gratings: sets of bright and dark lines of orthogonal orientations presented one to each eye. We studied whether using cardinal rival gratings (vertical, 0°, and horizontal, 90°) versus oblique rival gratings (left-oblique, -45°, and right-oblique, 45°) influences early neural correlates of visual consciousness, because of the oblique effect: the tendency for visual performance to be greater for cardinal gratings than for oblique gratings. Participants viewed rival gratings and pressed keys indicating which of the two gratings they perceived, was dominant. Next, we changed one of the gratings to match the grating shown to the other eye, yielding binocular fusion. Participants perceived the rivalry-to-fusion change to the dominant grating and not to the other, suppressed grating. Using event-related potentials (ERPs), we found neural correlates of visual consciousness at the P1 for both sets of gratings, as well as at the P1-N1 for oblique gratings, and we found a neural correlate of the oblique effect at the N1, but only for perceived changes. These results show that the P1 is the earliest neural activity associated with visual consciousness and that visual consciousness might be necessary to elicit the oblique effect.

Monocular rivalry was named by Breese in 1899. He made prolonged observation of superimposed orthogonal gratings; they fluctuated in clarity with either one or the other grating occasionally being visible alone. A year earlier, Tscherning observed similar fluctuations with a grid of vertical and horizontal lines and with other stimuli; we draw attention to his prior account. Monocular rivalry has since been shown to occur with a wide variety of superimposed patterns with several independent rediscoveries of it. We also argue that Helmholtz described some phenomenon other than monocular rivalry in 1867.

Astoundingly, looking at a photograph with one eye can yield an experience of depth of the depicted objects similar to that from viewing the real objects with both eyes. Édouard Claparède (1873-1940) was one of the first to report this phenomenon in a French paper published in 1904. I give some historical and theoretical context to the phenomenon, provide some biographical information about Claparède, and provide a translation into English of Claparède's paper.

Gottfried Wilhelm Osann (1796-1866) was a German scientist most renowned for his work in chemistry and physics. However, inspired by Goethe's work on color, he published a paper on simultaneous color contrast in 1833 using a method that is similar to that of later authors: reflection of an achromatic spot from an angled piece of glass. He wrote at least four more papers on color contrasts, in 1836 using essentially the same method as credited to others. We provide a description and translation of Osann's 1833 paper and the relevant part of his 1836 paper, say why these papers are interesting and important, give some biographical information about Osann, comment on the fate of Osann's papers, and describe Osann's other papers on color.

Predictive coding explains visual perception as the result of an interaction between bottom-up sensory input and top-down generative models at each level of the visual hierarchy. Evidence for this comes from the visual mismatch negativity (vMMN): a more negative ERP for rare, unpredictable visual stimuli-deviants, than for frequent, predictable visual stimuli-standards. Here, we show that the vMMN does not require conscious experience. We measured the vMMN from monocular luminance-decrement deviants that were either perceived or not during binocular rivalry dominance or suppression, respectively. We found that both sorts of deviants elicited the vMMN at about 250 ms after stimulus onset, with perceived deviants eliciting a bigger vMMN than not-perceived deviants. These results show that vMMN occurs in the absence of consciousness, and that consciousness enhances the processing underlying vMMN. We conclude that generative models of visual perception are tested, even when sensory input for those models is not perceived., (© 2017 Society for Psychophysiological Research.)

A recent study has found that viewing one image with one eye and a lower-contrast image with the other eye as briefly as a few minutes alters the subsequent balance of activity between the two eyes revealing adult neuroplasticity., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

In 1847, Domenico Ragona-Scinà (1820-1892) published a method of optically superimposing images using an angled piece of colored glass. He showed that if one looks at a black, filled circle through the colored glass and superimposes on it the reflection from the glass of something white, the filled circle looks tinted with the complementary color of the background: simultaneous color contrast or contrast color. Although Ragona-Scinà's method and his observation have been cited into the 21st century, the former for its simplicity and the latter for its challenges to early theories of color vision, some errors have crept in and the phenomenon still lacks an agreed-on explanation. We provide some biographical information about Ragona-Scinà, set the method and the observation into their historical and theoretical contexts, and give a translation into English of Ragona-Scinà's Italian-language paper.

The visual mismatch negativity (vMMN) is a negative component of event-related potentials (ERPs). It occurs when an infrequent visual stimulus, a deviant, is randomly and unpredictably presented in a sequence of frequent visual stimuli, the standards, and is thought to reflect prediction and prediction error of visual input. We investigated the sensitivity of vMMN to eye of origin (utrocular) information as well as to orientation information. We presented 80% of binocular rivalry standards (one grating to one eye and an identical, orthogonally oriented grating to the other eye), and 20% of deviants, either by swapping the gratings between the eyes to change the eye of origin of the gratings (an eye-swap deviant) or by rotating the gratings by 45° to change the orientation of the gratings (an orientation deviant). We found an orientation vMMN that was maximal at about 250 ms and an eye-swap vMMN that was maximal at about 380 ms. We also found deviance-related activity to both sorts of stimuli earlier than is traditionally defined as a vMMN. We used standardized low-resolution brain electromagnetic tomography (sLORETA) to localize each vMMN component and found similar sources for both vMMNs in occipital and frontal areas of the brain but differences in parietal and temporal areas. We conclude that eye of origin information can be used to elicit vMMN, that eye-swap vMMN is different to orientation vMMN, and that vMMN can be generated from information of which observers are unaware.

Background: Health care depends, in part, on the ability of a practitioner to see signs of disease and to see how to treat it. Visual illusions, therefore, could affect health care. Yet there is very little prospective evidence that illusions can influence treatment. We sought such evidence., Methods and Results: We simulated treatment using dentistry as a model system. We supplied eight, practicing, specialist dentists, endodontists, with at least 21 isolated teeth each, randomly sampled from a much larger sample of teeth they were likely to encounter. Teeth contained holes and we asked the endodontists to cut cavities in preparation for filling. Each tooth presented a more or less potent version of a visual illusion of size, the Delboeuf illusion, that made the holes appear smaller than they were. Endodontists and the persons measuring the cavities were blind to the parameters of the illusion. We found that the size of cavity endodontists made was linearly related to the potency of the Delboeuf illusion (p<.01) with an effect size (Cohen's d) of 1.41. When the illusion made the holes appear smaller, the endodontists made cavities larger than needed., Conclusions: The visual context in which treatment takes place can influence the treatment. Undesirable effects of visual illusions could be counteracted by a health practitioner's being aware of them and by using measurement.

Purpose: We sought brain activity that predicts visual consciousness., Methods: We used electroencephalography (EEG) to measure brain activity to a 1000-ms display of sine-wave gratings, oriented vertically in one eye and horizontally in the other. This display yields binocular rivalry: irregular alternations in visual consciousness between the images viewed by the eyes. We replaced both gratings with 200 ms of darkness, the gap, before showing a second display of the same rival gratings for another 1000 ms. We followed this by a 1000-ms mask then a 2000-ms inter-trial interval (ITI). Eleven participants pressed keys after the second display in numerous trials to say whether the orientation of the visible grating changed from before to after the gap or not. Each participant also responded to numerous non-rivalry trials in which the gratings had identical orientations for the two eyes and for which the orientation of both either changed physically after the gap or did not., Results: We found that greater activity from lateral occipital-parietal-temporal areas about 180 ms after initial onset of rival stimuli predicted a change in visual consciousness more than 1000 ms later, on re-presentation of the rival stimuli. We also found that less activity from parietal, central, and frontal electrodes about 400 ms after initial onset of rival stimuli predicted a change in visual consciousness about 800 ms later, on re-presentation of the rival stimuli. There was no such predictive activity when the change in visual consciousness occurred because the stimuli changed physically., Conclusion: We found early EEG activity that predicted later visual consciousness. Predictive activity 180 ms after onset of the first display may reflect adaption of the neurons mediating visual consciousness in our displays. Predictive activity 400 ms after onset of the first display may reflect a less-reliable brain state mediating visual consciousness.

Driver (1996) reported that the ventriloquist illusion can enhance selective listening of speech. Participants in his study listened to target and distractor words from a single loudspeaker while watching lip movements of the target words on a video monitor either above the loudspeaker or displaced to the left or right. He found that participants were more accurate in repeating the target words when the video was displaced from the loudspeaker than when the video was directly above the loudspeaker. Driver proposed that the ventriloquist illusion dragged the target sounds toward the location of the lip movements, freeing them from interference from the distractor words. However, successful attempts at replicating this finding are rare (we know of only three successful replications from 19 attempts). In five experiments, we found a weak advantage for selective listening from displaced lip movements only when there was a convincing ventriloquist illusion. We conclude that the ventriloquist illusion is necessary to confer the advantage for selective listening from displaced lip movements but that the phenomenon is a fleeting one at best., (PsycINFO Database Record (c) 2013 APA, all rights reserved.)

The visual mismatch negativity (vMMN) is a negative deflection in an event-related potential (ERP) between 200 and 400 ms after onset of an infrequent stimulus in a sequence of frequent stimuli. Binocular rivalry occurs when one image is presented to one eye and a different image is presented to the other. Although the images in the two eyes are unchanging, perception alternates unpredictably between the two images for as long as one cares to look. Binocular rivalry, therefore, provides a useful test of whether the vMMN is produced by low levels of the visual system at which the images are processed, or by higher levels at which perception is mediated. To investigate whether a vMMN can be evoked during binocular rivalry, we showed 80% standards comprising a vertical grating to one eye and a horizontal grating to the other and 20% deviants, in which the gratings either swapped between the eyes (eye-swap deviants) or changed their orientations by 45° (oblique deviants). Fourteen participants observed the stimuli in 16, 4-min blocks. In eight consecutive blocks, participants recorded their experiences of rivalry by pressing keys-we call this the attend-to-rivalry condition. In the remaining eight consecutive blocks, participants performed a demanding task at fixation (a 2-back task), also by pressing keys-we call this the reduced-attention condition. We found deviance-related negativity from about 140 ms to about 220 ms after onset of a deviant. There were two noticeable troughs that we call an early vMMN (140-160 ms) and a late vMMN (200-220 ms). These were essentially similar for oblique deviants and eye-swap deviants. They were also essentially similar in the attend-to-rivalry conditions and the reduced-attention conditions. We also found a late, deviance-related negativity from about 270 to about 290 ms in the attend-to-rivalry conditions. We conclude that the vMMN can be evoked during the ever-changing perceptual changes of binocular rivalry and that it is sensitive to the eye of origin of binocular-rivalry stimuli. This is consistent with the vMMN's being produced by low levels of the visual system.

During binocular rivalry, perception alternates between two different images presented one to each eye. At any moment, one image is visible, dominant, while the other is invisible, suppressed. Alternations in perception during rivalry could involve competition between eyes, eye-rivalry, or between images, image-rivalry, or both. We measured response criteria, sensitivities, and thresholds to brief contrast increments to one of the rival stimuli in conventional rivalry displays and in a display in which the rival stimuli swapped between the eyes every 333 ms-swap rivalry-that necessarily involves image rivalry. We compared the sensitivity and threshold measures in dominance and suppression to assess the strength of suppression. We found that response criteria are essentially the same during dominance and suppression for the two sorts of rivalry. Critically, we found that swap-rivalry suppression is weak after a swap and strengthens throughout the swap interval. We propose that image rivalry is responsible for weak initial suppression immediately after a swap and that eye rivalry is responsible for the stronger suppression that comes later.

During binocular rivalry visual consciousness fluctuates between two dissimilar monocular images. We investigated the role of attention in this phenomenon by comparing event-related potentials (ERPs) when binocular-rivalry stimuli were attended with when they were unattended. Stimuli were dichoptic, orthogonal gratings that yielded binocular rivalry and dioptic, identically oriented gratings that yielded binocular fusion. Events were all possible orthogonal changes in orientation of one or both gratings. We had two attention conditions: In the attend-to-grating condition, participants had to report changes in perceived orientation, focussing their attention on the gratings. In the attend-to-fixation condition participants had to report changes in a central fixation target, taking attention away from the gratings. We found, surprisingly, that attending to rival gratings yielded a smaller ERP component (the N1, from 160-210 ms) than attending to the fixation target. To explain this paradoxical effect of attention, we propose that rivalry occurs in the attend-to-fixation condition (we found an ERP signature of rivalry in the form of a sustained negativity from 210-300 ms) but that the mechanism processing the stimulus changes is more adapted in the attend-to-grating condition than in the attend-to-fixation condition. This is consistent with the theory that adaptation gives rise to changes of visual consciousness during binocular rivalry.

When viewing a different stimulus with each eye, we experience the remarkable phenomenon of binocular rivalry: alternations in consciousness between the stimuli [1, 2]. According to a popular theory first proposed in 1901, neurons encoding the two stimuli engage in reciprocal inhibition [3-8] so that those processing one stimulus inhibit those processing the other, yielding consciousness of one dominant stimulus at any moment and suppressing the other. Also according to the theory, neurons encoding the dominant stimulus adapt, weakening their activity and the inhibition they can exert, whereas neurons encoding the suppressed stimulus recover from adaptation until the balance of activity reverses, triggering an alternation in consciousness. Despite its popularity, this theory has one glaring inconsistency with data: during an episode of suppression, visual sensitivity to brief probe stimuli in the dominant eye should decrease over time and should increase in the suppressed eye, yet sensitivity appears to be constant [9, 10]. Using more appropriate probe stimuli (experiment 1) in conjunction with a new method (experiment 2), we found that sensitivities in dominance and suppression do show the predicted complementary changes., (Copyright (c) 2010 Elsevier Ltd. All rights reserved.)

To understand the brain areas associated with visual awareness and their anatomical interconnections, we studied binocular rivalry with functional magnetic resonance imaging (fMRI) and diffusion tensor imaging (DTI). Binocular rivalry occurs when one image is viewed by one eye and a different image by the other; it is experienced as perceptual alternations between the two images. Our first experiment addressed problems with a popular comparison condition, namely permanent suppression, by comparing rivalry with binocular fusion instead. We found an increased fMRI signal in right frontal, parietal, and occipital regions during rivalry viewing. The pattern of neural activity differed from findings of permanent suppression comparisons, except for adjacent activity in the right superior parietal lobule. This location was near fMRI signal changes related to reported rivalry alternations in our second experiment, indicating that neighbouring areas in the right parietal cortex may be involved in different components of rivalry. In our second experiment, we used probabilistic tractography to detect white matter fibres between right-hemispheric areas that showed event-related fMRI signal changes time-locked to reported perceptual alternations during rivalry viewing. Most of these functionally defined areas were linked by probabilistic fibre tracts, some of which followed long-distance connections such as the inferior occipitofrontal fasciculus. Corresponding anatomical pathways might mediate communication within the functional network associated with changes in conscious perception during binocular rivalry.

Binocular rivalry occurs when different images are presented one to each eye: the images are visible only alternately. Monocular rivalry occurs when different images are presented both to the same eye: the clarity of the images fluctuates alternately. Could both sorts of rivalry reflect the operation of a general visual mechanism for dealing with perceptual ambiguity? We report four experiments showing similarities between the two phenomena. First, we show that monocular rivalry can occur with complex images, as with binocular rivalry, and that the two phenomena are affected similarly by the size (Experiment 1) and colour (Experiment 2) of the images. Second, we show that the distribution of dominance periods during monocular rivalry has a gamma shape and is stochastic (Experiment 3). Third, we show that during periods of monocular-rivalry suppression, the threshold to detect a probe (a contrast pulse to the suppressed stimulus) is raised compared with during periods of dominance (Experiment 4). The threshold elevation is much weaker than during binocular rivalry, consistent with monocular rivalry's weak appearance. We discuss other similarities between monocular and binocular rivalry, and also some differences, concluding that part of the processing underlying both phenomena is a general visual mechanism for dealing with perceptual ambiguity.

Binocular rivalry occurs when dissimilar images are presented to corresponding retinal regions of the two eyes: visibility alternates irregularly between the two images, interspersed by brief transitions when parts of both may be visible. We measured event-related potentials (ERPs) following binocular rivalry by changing the stimulus viewed by one eye to be identical to that in the other eye, eliciting binocular fusion. Because of the rivalry, observers either saw the change, when it happened to the visible stimulus, or did not see the change, when it happened to the invisible stimulus. The earliest ERP differences between visible and invisible changes occurred after about 100 ms (P1) when the rivalry was between stimuli differing in orientation, and after about 200 ms (N1) when the rivalry was between stimuli differing in colour. These differences originated from ventro-lateral temporal and prefrontal areas. We conclude that the rivalling stimulus property influences the timing of modulation of correlates of visual awareness in a property-independent cortical network.

During binocular rivalry, one stimulus is visible (dominant), while the other stimulus is invisible (suppressed); after a few seconds, perception reverses. To determine whether these alternations involve competition between the eyes or between the images, we measured suppression depth to monocular probes. We did so in conventional rival stimuli and in rival stimuli swapping between the eyes at 1.5 Hz (both sorts of rivalry were shown either with or without 18-Hz flicker). The conventional conditions cause rivalry that could involve either competition between the eyes or between the images or both. The eye-swapping conditions cause rivalry that could involve competition between the images. Probes were either a small spot or a contrast increment to one of the rival stimuli. Using both yes-no and forced-choice procedures, we found that conventional conditions yielded large suppression depth and that eye-swapping conditions yielded small suppression depth. Weak suppression during image rivalry is consistent with conventional rivalry's involving competition between eyes and between images and flicker-and-swap rivalry's involving little, if any competition between eyes and mainly competition between images.

When something appears, how soon is the first neural correlate of awareness of it, and where is that activity in the brain? To answer these questions, we measured the electroencephalogram under conditions in which visual stimuli changed identically but in which awareness differed. We manipulated awareness by using binocular rivalry between orthogonal gratings viewed one to each eye. Then we changed the orientation of the grating to one eye to be the same as that to the other eye. Because of the rivalry, sometimes this happened to the visible grating, producing a clear change in perceived orientation, and other times it happened to the invisible grating, producing no change in perceived orientation. This procedure allowed us to analyze time-locked topographic scalp and tomographic primary current densities of the event-related potentials to physically identical events differing in their perceptual consequences. When the change in orientation reached awareness, neural responses began at about 100 ms, spreading mainly from dorsal occipital areas. When the change in orientation did not reach awareness, neural responses also began at about 100 ms, but they were attenuated, particularly in the right fusiform gyrus. We place the earliest correlate of visual awareness following binocular rivalry in the ventrolateral occipitotemporal cortex.

Aims: To examine the relationship between athlete drinking motives and hazardous drinking across differing levels of sporting participation (club vs elite-provincial vs elite-international)., Methods: Data from 1214 New Zealand sportspeople was collected. We assessed hazardous drinking with the WHO's AUDIT questionnaire and sportspeople's psychosocial reasons for drinking with the ADS. Level of sporting participation (club/social, provincial/state, or international/olympic level) was also assessed., Results: Hazardous drinking behaviours differed across levels of sporting participation, with elite-provincial sportspeople showing the highest level of hazardous drinking, club/social sportspeople the next highest and elite-international sportspeople the lowest. Sportspeople who placed a greater emphasis on drinking as a reward for participating in their sports tended to display more hazardous drinking behaviours, but other ADS motives differed over level of sporting participation. Elite-provincial sportspeople and elite-international sportspeople placed more emphasis on drinking as a way to cope with the stresses of participating in their sports. A relationship between team/group motives and AUDIT scores was fully mediated by positive reinforcement motives, and partially mediated by stress-related coping motives., Conclusions: These findings have implications for alcohol education programs targeted at sportspeople and sport administration, and may help improve the efficacy and focus of intervention programs.

When one looks up a hill from below, its peak appears lower than it is; when one looks at a hill across a valley from another peak, the peak of that hill appears higher than it is. These illusions have sometimes been explained by assuming that the subjective horizontal is assimilated to the nearby slope: when looking up a slope, the subjective horizontal is raised, diminishing the height of the peak above the subjective horizontal, and making the peak appear lower than it is. When looking down a slope towards another hill, the subjective horizontal is lowered, increasing the height of that hill above the subjective horizontal, and making its peak appear higher than it is. To determine subjective horizontals we measured visually perceived eye levels (VPELs) in 21 real-world scenes on a range of slopes. We found that VPEL indeed assimilates by about 40% to slopes between 7 degrees downhill and 7 degrees uphill. For larger uphill slopes up to 23 degrees, VPEL asymptotes at about 4.5 degrees. For larger downhill slopes, the assimilation of VPEL diminishes, and at 23 degrees is raised by about 1 degree. These results are consistent with the assimilation explanation of the illusions if we assume that steep downhill slopes lose their effectiveness by being out of view. We also found that VPEL was raised when viewing from a height, in comparison with ground-level views, perhaps because the perceived slope increases with viewing height.

We studied the effects of visual grouping on binocular rivalry in the left and right hemispheres of a split-brain observer, JW. In Experiments 1 and 2, we compared responses to traditional rivalry stimuli (e.g., a red vertical grating presented to the left eye and a green horizontal grating presented to the right eye) with responses to Diaz-Caneja stimuli (i.e., half of each grating was presented to one eye and the other half to the other eye). As found for intact-brain observers, JW reported episodes of exclusive visibility of coherent stimuli (e.g., of a red vertical grating alternating with a green horizontal grating) with Diaz-Caneja stimuli that were fewer and briefer than with traditional stimuli. This occurred in both hemispheres, demonstrating that during binocular rivalry, contours from one eye can be grouped with those of the opposite eye to create a coherent percept, even in the isolated hemispheres of the split-brain observer. In Experiment 3, we studied the tendency of rivalry in adjacent patches to synchronize. When both patches were in one of JW's hemifields, rivalry synchronized for similarly oriented stimuli, the same as happened for intact-brain observers. When the patches were in JW's opposite hemifields, there was no synchronizing of rivalry, unlike what happened for intact-brain observers. This suggests that rivalry processed in JW's two hemispheres is independent. We conclude that rivalry is processed fully within each hemisphere.

Some visual stimuli are bistable, with perception alternating irregularly between two alternatives. Recent work suggests that the neural processing of these alternations must occur at low levels of the visual system.

During binocular rivalry, visual perception switches between a stimulus viewed by one eye and a different stimulus viewed by the other. We studied rivalry in split-brain observers to test two explanations. Rivalry could reflect switching of activity between the cerebral hemispheres, or switching by a structure in the right frontoparietal cortex. From these two theories, we predict no rivalry when stimuli are presented to a split-brain observer's left hemisphere. Yet we found similar rivalry from the left and right hemispheres of the split-brain observers, consistent with switchings being mediated by low-level processes within each hemisphere.

Diaz-Caneja (1928) made some prescient observations about binocular rivalry. Being in French, however, his paper remained largely unknown to the broader research community. His findings are similar to those reported very recently by contemporary researchers who had independently observed similar phenomena. Using concentric circles and parallel lines as stimuli, Diaz-Caneja presented half of each form to opposite eyes to provoke binocular rivalry. He observed periods in the ensuing binocular alternations in which rivalry occurred between the good Gestalt forms, despite the fact that they were distributed between the eyes. He proposed that each half of a good form generates synchronised oscillations in the visual system, and that this synchronisation enables the dichoptically viewed halves of the one form to be perceived as a whole.

In a previous study we found that blurred edges presented in peripheral vision look sharper than when they are looked at directly, a phenomenon we have called peripheral sharpness overconstancy (Galvin et al. (1997). Vision Research, 37, 2035-2039). In the current study we show that when visibility of the stimulus edges is compromised by very brief presentations, we can demonstrate sharpness overconstancy for static, foveal viewing. We also test whether the degree of sharpening is a function of the current visual context, but find no difference between the peripheral sharpness overconstancy (at 24 degrees eccentricity) of edges measured in a blurred context and that measured in a sharp context. We conclude that if the visual system does carry a template for sharp edges which contributes to edge appearance when visibility is poor, then that template is resistant to changes in context.

Broerse and O'Shea [(1995) Vision Research, 35, 207-226] proposed that the subjective colours in McCollough effects (MEs) consist of two components: edge colours appearing along the edges of contours, and spread colours radiating from edge colours into adjacent uncontoured regions of test patterns. This proposal was examined in five experiments. First, we demonstrated that fine coloured lines located immediately adjacent to the edges of otherwise achromatic square-wave gratings (i.e. colour-fringed gratings) are sufficient to induce MEs comparable in strength to MEs induced with desaturated versions of traditional uniformly-coloured gratings (Experiments 1 & 2). We then quantified edge and spread colours while varying light/dark duty cycles (white-bar width) in gratings with colour-fringed edges (Experiment 3), uniformly-coloured gratings (Experiment 4), and in achromatic gratings tinged with ME colours after adaptation to colour-fringed gratings (Experiment 5). Whereas the perceived magnitude of edge colours remained constant in all cases, spread colours remained constant only for uniformly-coloured gratings. For both MEs and gratings with colour-fringed edges, spread colours decreased as a function of increasing duty cycle, confirming that conventional MEs may be simulated by gratings with colour-fringed edges. We propose that edge colours arise as a consequence of neural operations correcting for the eye's chromatic aberration, while spread colours reveal a neural filling-in process operating to achieve colour constancy. In seeking to implement these suggestions, we present a putative framework based on the receptive-field properties of single cells described in contemporary neurophysiological investigations of colour.

Although much has been learned about the spatial sampling and filtering properties of peripheral vision, little attention has been paid to the remarkably clear appearance of the peripheral visual field. To study the apparent sharpness of stimuli presented in the periphery, we presented Gaussian blurred horizontal edges at 8.3, 16.6, 24, 32, and 40 deg eccentricity. Observers adjusted the sharpness of a similar edge, viewed foveally, to match the appearance of the peripheral stimulus. All observers matched blurred peripheral stimuli with sharper foveal stimuli. We have called this effect "sharpness overconstancy". For field sizes of 4 deg, there was greater overconstancy at larger eccentricities. Scaling the field size of the peripheral stimuli by a cortical magnification factor produced sharpness overconstancy which was independent of eccentricity. In both cases, there was a slight sharpness underconstancy for peripherally presented edges blurred only slightly. We consider various explanations of peripheral sharpness overconstancy.

We measured binocular rivalry between dichoptic, orthogonal, sinusoidal gratings both having spatial frequencies of 0.5, 1, 2, 4, 8 or 16 c deg-1 in fields ranging from 0.5 to 8 deg of visual angle in diameter. Total time that one or the other grating was exclusively visible had an inverted U-shaped relationship with spatial frequency, with the peak shifting to coarser spatial frequencies as the field size increased. We computed for each spatial frequency the maximum field size over which a criterion duration of exclusive visibility would spread. When expressed as areas, these sizes were inversely proportional to spatial frequency. This dependence of rivalry on spatial frequency is similar to those for stereopsis and fusion, consistent with the notion that all three binocular phenomena have a common mechanism.

Studies have shown that blur can act as a pictorial cue to depth perception. But blurring a stimulus reduces its contrast, and studies have also shown that contrast can act as a pictorial cue to depth perception. To determine whether blur and contrast have separate influences on depth perception, each variable was independently manipulated in two experiments. Observers reported depth alternations in a simple reversible figure. Both contrast and blur were found to influence depth perception, but blur had its greatest effect at moderate contrasts. When blurred and sharp stimuli were equated on either Michelson or RMS contrast, blur continued to affect depth perception. Hence blur can act as a depth cue independently of contrast. It is speculated that blur is effective as a pictorial cue because of its usual association with other depth cues, particularly in pictures and photographs.

To test whether the binocular contour rivalry mechanism is tritanopic, we presented isoluminant, rival stimuli visible only via the short-wavelength-sensitive (S) cones. We stimulated only the S cones with violet gratings superimposed on a bright yellow field that adapted the responses of the middle- and long-wavelength-sensitive (M and L) cones. We found that an S-cone grating presented to one eye rivalled with an orthogonal grating presented to the other. Rivalry persisted over a range of luminances and contrasts of the S-cone stimuli, and was greater than could be accounted for by nonrival fading. The spatial spread of rivalry from S-cone stimuli is similar to that for the same stimuli when visible also to the M and L cones (luminance stimuli). We found that an S-cone stimulus would rival with a luminance stimulus, and exploited this to determine the equivalent luminance contrast of S-cone stimuli by putting them in a rivalry competition with luminance stimuli. For rivalry, the equivalent luminance contrast of isoluminant, S-cone stimuli is much less than their S-cone contrast. The existence of rivalry with isoluminant stimuli, along with earlier evidence that such stimuli can support stereopsis, challenges the view that an achromatic channel alone drives certain higher level functions such as depth perception.

Lehmkuhle and Fox [(1976) Vision Research, 16, 428-430] reported that interocular transfer (IOT) of a translational motion aftereffect (MAE) was greater if the non-adapting eye viewed an equiluminant field than if it viewed a dark field. They recommended equiluminant occlusion of the non-adapted eye when measuring IOT of aftereffects. We tested this proposal in three experiments. First, we assessed IOT with equiluminant and dark occlusion for three different classes of aftereffects. Although transfer was greater with equiluminant occlusion for the translational MAE, there was no significant difference in the amount of transfer for the tilt aftereffect or the contrast threshold elevation effect. Second, we tested the hypothesis that spuriously large IOT could be the result of an aftereffect from tracking eye movements in the non-adapting eye. When potential tracking movements were reduced by using rotating spokes, a rotating spiral or contracting concentric circles, there was a corresponding reduction in the occlusion-dependent transfer. Third, we found that luminance shifts had no influence on the amount of transfer when all contours were eliminated from the non-adapting eye. We conclude that the type of occlusion used for measuring IOT of the translational MAE is important only when visible contours in the non-adapting eye contribute to the adapting process.

Dodwell and O'Shea's [(1987) Vision Research, 27, 569-580] conclusions that contingent coloured aftereffects (CAEs) depend on gobal pattern organization were investigated in four experiments. In Expt 1, we replicated findings that CAEs can be induced with complex patterns (concentric circles; radial spokes) under conditions of systematic eye movements. Contrary to Dodwell and O'Shea's argument that eye movements should uniformly cancel local orientation-colour contingencies, leaving only global effects, we reduced CAE magnitude by halving the diameter of the test stimuli. This suggests that cancellation did not occur uniformly over whole patterns, and that CAEs observed on these patterns are the residuals of uncancelled local orientation-colour contingencies. In Expt 2 we used central-fixation induction procedures to demonstrate that it is possible to induce CAEs with randomly-organized and locally-orthogonal orientation components. These findings are inconsistent with Dodwell and O'Shea's failure to observe CAEs under these conditions, and with their conclusion that global organization is necessary for CAE induction. However, CAEs induced with randomly-organized components were significantly weaker than those induced with globally-organized components. We examined the contribution of global organization in two additional experiments. In Expt 3 we induced CAEs with randomly-organized components under conditions in which the need for central fixation was removed, and found that CAE strength was directly related to the organization as well as the density of local-orientation components. In Expt 4, we found that the global organization of local-orientation components enhanced CAE strength only in regions away from the edges of these components: pattern organization did not affect the strength of CAEs at edges. We interpret these findings as evidence that CAEs may involve separate edge- and spread-colour components, and conclude that such components may account for observations previously attributed to global pattern geometry.

One consequence of aerial perspective is that far objects have lower contrast than near objects. We tested the efficacy of contrast as a cue for depth perception by measuring the apparent, relative depth of two areas that differed in contrast with a background and in size. We tested monocularly and binocularly. Differences in contrast were achieved by making the areas different in luminance, than varying the luminance of the background. Subjects reported that the area having lower contrast with the background appeared farther than the area having higher contrast. Even when size opposed it, contrast continued to have a significant effect on depth perception. Monocular observation yielded larger apparent depth than binocular observation. Contrast is an effective depth cue in the absence of any other depth information. We suggest that contrast acts as a pictorial depth cue simulating the optical effects of aerial perspective.

A study is reported of human binocular rivalry and fusion over a range of luminances from scotopic to photopic. At scotopic light levels, rivalry alternations were very slow and complete. Suppression spread over much larger areas of the visual field than at photopic light levels. As luminances decreased from photopic to scotopic levels there was a rod-cone break for binocular rivalry. Mean suppression durations became abruptly greater as light levels dropped below those allowing the cones to be active. Horizontal disparities allowing fusion were 4 to 6 times greater at scotopic than at photopic light levels. Binocular vision at scotopic luminances was sluggish and of low resolution. It is as though connections to, and within, binocular vision are changed when light levels allow only rod input.

Purpose: To compare binocularity in central and peripheral vision of people with early-onset strabismus and people with normal binocular vision., Methods: Ten subjects with early-onset strabismus, and nine subjects with normal binocular vision were tested. To assess binocularity, interocular transfer (IOT) of a rotary movement aftereffect (MAE) was measured. The MAE stimuli were either confined to the central 2.8 degrees of the visual field or were presented 10 degrees into peripheral vision., Results: In peripheral vision, there was no significant difference in IOT for the two groups of subjects. In central vision, there was a significant decrease of IOT in subjects with early-onset strabismus. Their IOT was, however, significantly greater than zero., Conclusions: Early-onset strabismus appears to spare binocularity in peripheral vision but reduces it in central vision. It does not abolish binocularity assessed by IOT of MAE, suggesting that some binocular connections survive early-onset strabismus, even in central vision.

This paper presents results from psychophysical experiments on human binocular rivalry in central and peripheral vision. Results show that the incidence of periods of exclusive visibility of a given eye's rival target increased with decreasing target size, and for a given sized target exclusive visibility increased with retinal eccentricity. Control measures confirmed that these results were not attributable solely to reduced peripheral acuity, to Troxler's effect, or to spatial frequency. We computed the minimum-sized stimulus that would lead to a criterion level of exclusive visibility of one or the other eye; this we term the spatial zone of binocular rivalry. The change in estimated size of spatial zones of rivalry with eccentricity compares favorably with estimates of human cortical magnification. We propose a model that assumes concentrically organized zones of rivalry. These zones do not function independently, but instead exhibit a high degree of mutual excitatory cooperativity. The model has multiple solutions for the foveal zone size, but the best fits predict a diameter of 5.3 or 7.3 min of visual angle; these values dovetail nicely with our empirical estimates of the foveal zone size.