Binocular rivalry: suppression depends on orientation and spatial frequency

Modulating the rate and rhythmicity of perceptual rivalry alternations with the mixed 5-HT2A and 5-HT1A agonist psilocybin

Binocular rivalry occurs when different images are presented simultaneously to corresponding points within the left and right eyes. Under these conditions, the observer's perception will alternate between the two perceptual alternatives. Motivated by the reported link between the rate of perceptual alternations, symptoms of psychosis and an incidental observation that the rhythmicity of perceptual alternations during binocular rivalry was greatly increased 10 h after the consumption of LSD, this study aimed to investigate the pharmacology underlying binocular rivalry and to explore the connection between the timing of perceptual switching and psychosis. Psilocybin (4-phosphoryloxy-N,N-dimethyltryptamine, PY) was chosen for the study because, like LSD, it is known to act as an agonist at serotonin (5-HT)1A and 5-HT2A receptors and to produce an altered state sometimes marked by psychosis-like symptoms. A total of 12 healthy human volunteers were tested under placebo, low-dose (115 microg/kg) and high-dose (250 microg/kg) PY conditions. In line with predictions, under both low- and high-dose conditions, the results show that at 90 min postadministration (the peak of drug action), rate and rhythmicity of perceptual alternations were significantly reduced from placebo levels. Following the 90 min testing period, the perceptual switch rate successively increased, with some individuals showing increases well beyond pretest levels at the final testing, 360 min postadministration. However, as some subjects had still not returned to pretest levels by this time, the mean phase duration at 360 min was not found to differ significantly from placebo. Reflecting the drug-induced changes in rivalry phase durations, subjects showed clear changes in psychological state as indexed by the 5D-ASC (altered states of consciousness) rating scales. This study suggests the involvement of serotonergic pathways in binocular rivalry and supports the previously proposed role of a brainstem oscillator in perceptual rivalry alternations and symptoms of psychosis.

A common oscillator for perceptual rivalries?

Perceptual rivalry is an oscillation of conscious experience that takes place despite univarying. if ambiguous, sensory input. Much current interest is focused on the controversy over the neural site of binocular rivalry, a variety of perceptual rivalry for which a number of different cortical regions have been implicated. Debate continues over the relative role of higher levels of processing compared with primary visual cortex and the suggestion that different forms of rivalry involve different cortical areas. Here we show that the temporal pattern of disappearance and reappearance in motion-induced blindness (MIB) (Bonneh et al, 2001 Nature 411 798-801) is highly correlated with the pattern of oscillation reported during binocular rivalry in the same individual. This correlation holds over a wide range of inter-individual variation. Temporal similarity in the two phenomena was strikingly confirmed by the effects of the hallucinogen LSD, which produced the same, extraordinary, pattern of increased rhythmicity in both kinds of perceptual oscillation. Furthermore. MIB demonstrates the two properties previously considered characteristic of binocular rivalry. Namely the distribution of dominance periods can be approximated by a gamma distribution and, in line with Levelt's second proposition of binocular rivalry, predominance of one perceptual phase can be increased through a reduction in the predominance time of the opposing phase. We conclude that (i) MIB is a form of perceptual rivalry, and (ii) there may be a common oscillator responsible for timing aspects of all forms of perceptual rivalry.

Visual competition

Binocular rivalry--the alternations in perception that occur when different images are presented to the two eyes--has been the subject of intensive investigation for more than 160 years. The psychophysical properties of binocular rivalry have been well described, but newer imaging and electrophysiological techniques have not resolved the issue of where in the brain rivalry occurs. The most recent evidence supports a view of rivalry as a series of processes, each of which is implemented by neural mechanisms at different levels of the visual hierarchy. Although unanswered questions remain, this view of rivalry might allow us to resolve some of the controversies and apparent contradictions that have emerged from its study.

Psychological factors and sex differences in acceptance of monovision

49 participants, adapted to using contact lenses, completed a battery of psychological tests to examine differences between persons who were successful in wearing monovision lenses for presbyopia and those who were unsuccessful. Although visual performance and requirements strongly affect participants' satisfaction with monovision lenses, personality characteristics and visual processing styles appear to in fluence the percentage of full-time wear. Of the participants, 41% (20 of 49) ultimately rejected monovision for a variety of both visual and psychological reasons. Sex differences were evidenced in the psychological variables correlated to monovision lens wear. Analysis indicated that men scoring as introverts rejected monovision while women with difficulty perceiving a visual pattern against visual background noise had difficulty adapting to monovision and exhibited shorter wearing times.

Single units and conscious vision

Figures that can be seen in more than one way are invaluable tools for the study of the neural basis of visual awareness, because such stimuli permit the dissociation of the neural responses that underlie what we perceive at any given time from those forming the sensory representation of a visual pattern. To study the former type of responses, monkeys were subjected to binocular rivalry, and the response of neurons in a number of different visual areas was studied while the animals reported their alternating percepts by pulling levers. Perception-related modulations of neural activity were found to occur to different extents in different cortical visual areas. The cells that were affected by suppression were almost exclusively binocular, and their proportion was found to increase in the higher processing stages of the visual system. The strongest correlations between neural activity and perception were observed in the visual areas of the temporal lobe. A strikingly large number of neurons in the early visual areas remained active during the perceptual suppression of the stimulus, a finding suggesting that conscious visual perception might be mediated by only a subset of the cells exhibiting stimulus selective responses. These physiological findings, together with a number of recent psychophysical studies, offer a new explanation of the phenomenon of binocular rivalry. Indeed, rivalry has long been considered to be closely linked with binocular fusion and stereopsis, and the sequences of dominance and suppression have been viewed as the result of competition between the two monocular channels. The physiological data presented here are incompatible with this interpretation. Rather than reflecting interocular competition, the rivalry is most probably between the two different central neural representations generated by the dichoptically presented stimuli. The mechanisms of rivalry are probably the same as, or very similar to, those underlying multistable perception in general, and further physiological studies might reveal much about the neural mechanisms of our perceptual organization.

The role of temporal cortical areas in perceptual organization

The visual areas of the temporal lobe of the primate are thought to be essential for the representation of visual objects. To examine the role of these areas in the visual awareness of a stimulus, we recorded the activity of single neurons in monkeys trained to report their percepts when viewing ambiguous stimuli. Visual ambiguity was induced by presenting incongruent images to the two eyes, a stimulation condition known to instigate binocular rivalry, during which one image is seen at a given time while the other is perceptually suppressed. Previous recordings in areas V1, V2, V4, and MT of monkeys experiencing binocular rivalry showed that only a small proportion of striate and early extrastriate neurons discharge exclusively when the driving stimulus is seen. In contrast, the activity of almost all neurons in the inferior temporal cortex and the visual areas of the cortex of superior temporal sulcus was found to be contingent upon the perceptual dominance of an effective visual stimulus. These areas thus appear to represent a stage of processing beyond the resolution of ambiguities--and thus beyond the processes of perceptual grouping and image segmentation--where neural activity reflects the brain's internal view of objects, rather than the effects of the retinal stimulus on cells encoding simple visual features or shape primitives.

Spatial zones of binocular rivalry in central and peripheral 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.

Psychophysical measurement of eye drifts and tremor by dichoptic or monocular vernier acuity

If the two segments of a vernier target are presented to different eyes, i.e. dichoptically, thresholds are three to four times higher than with presentation to the same eye. This increase in thresholds is mainly due to uncorrelated movements of both eyes, such as tremor and drifts, that occur even under steady fixation. The psychophysically measured thresholds allow one to calculate an upper estimate for the amplitudes of uncorrelated eye movements during fixation. This estimate matches the best results from direct eye position recording, with the calculated mean amplitude of eye tremor corresponding to roughly one photoreceptor diameter. The combined amplitude of both correlated and uncorrelated eye movements was also measured by delaying one segment of the vernier relative to its partner under monocular or dichoptic conditions. Fixation proved to be relatively stable, and trained observers could sustain eye position within a few min arc.

An astable multivibrator model of binocular rivalry

The behavior of a neural network model for binocular rivalry is explored through the development of an analogy between it and an electronic astable multivibrator circuit. The model incorporates reciprocal feedback inhibition between signals from the left and the right eyes prior to binocular convergence. The strength of inhibitory coupling determines whether the system undergoes rivalrous oscillations or remains in stable fusion: strong coupling leads to oscillations, weak coupling to fusion. This implies that correlation between spatial patterns presented to the two eyes can affect the strength of binocular inhibition. Finally, computer simulations are presented which show that a reciprocal inhibition model can reproduce the stochastic behavior of rivalry. The model described is a counterexample to claims that reciprocal inhibition models as a class cannot exhibit many of the experimentally observed properties of rivalry.

The apparent size of objects situated in front of or behind the horopter

Objects in the visual world that are located outside the horopter project upon non-corresponding parts of both retinae. Hence, most environments will generate--within a portion of the binocular field of view--retinal images that contain clearly dissimilar pattern elements at corresponding retinal points. Fusion is impossible there and the brain has to decide locally from which eye conflicting information is to be chosen. The binocular field of view thus consists of parts where information from both eyes contributes to perception and others where only one eye is 'switched on' at a given time. Two easily performed perception experiments demonstrate the advantage of having two eyes, even if fusion of parts of the images they receive is impossible: the apparent size of large objects outside the fixation plane (i.e., visual obstacles) is reduced. Quantitative measurements agree well with the predictions derived from a simple geometrical model. It is also shown that only a very limited number of objects can elicit 'true' (physiological) double images.

Perceptual "blankout" of monocular homogeneous fields (Ganzfelder) is prevented with binocular viewing

The loss of visual perception or "blankout" which occurs when a homogeneous field (Ganzfeld) is presented monocularly is prevented when the same field is viewed binocularly. Thus, blankout cannot be retinal; and contours or transients in time and space are unnecessary for the continuous maintenance of visual perception. Experiments are reported in which blankout ensues only if the two eyes receive luminance disparities ca 0.75 log I. Furthermore, blankout is only marginally affected by stimulus intensity, nor is it dependent on stimulus hue. However, equally luminant but disparate hues presented to the two eyes produce perceptions reminiscent of blankout, with the darkness of blankout replaced with that of color. It is hypothesized that the underlying mechanisms have a commonality in the phenomena of blankout and binocular rivalry but several noncongruent features require explanation.

Reversing ocular dominance and suppression in a single flash

An observer views a long (1-2 sec) monocular stimulus. Following stimulus offset there is a dark interstimulus interval (ISI). After the ISI, brief test flashes are presented to both eyes simultaneously. For ISIs less than 100-200 msec, only the test flash contralateral to the stimulated eye is seen. The test flash in the ipsilateral eye is not seen. For ISIs of 200-1000 msec, the contralateral test flash is seen more clearly and completely than the ipsilateral flash (Experiments 1 and 2). The initial and ipsilateral test flash need not be spatially similar. The initial stimulus can be dim but must be longer than 150 msec (Experiment 3). Experiment 3 precludes forward masking or light adaptation as explanation. The effect is relatively independent of the length of the test flash (Experiment 4). The results can be explained in terms of the known properties of binocular rivalry and reinforce the hypothesis (Fox and Check, 1968) that rivalry acts nonselectively, independent of factors such as spatial frequency, luminance, or orientation.

Non-fusable stimuli and the role of binocular inhibition in normal and pathologic vision, especially strabismus

Stimuli on corresponding points of both retinae that cannot be fused may cause binocular rivalry: the stimuli suppress each other alternately. This effect was used to study the influence of image sharpness upon binocular inhibition. Blurring an image means decreasing its contrast and attenuating its high spatial frequencies. Both factors diminish the time that a stimulus is perceived during rivalry. This fact has implications both for normal vision--as objects off the horopter are normally blurred--and for disturbed vision when the image of one or both eyes is (locally) deteriorated. In both cases, the binocular field of view can be combined from the 'good' parts of both eyes. Hence, the field of view may consist, in a piece-meal fashion, of parts stemming from the right or the left eye exclusively and others where both images are superimposed. We present evidence for the hypothesis that there is a common neural mechanism causing both binocular rivalry and functional amblyopia in anisometropia and strabismus. Consequences of the results on rivalry suppression for the pathophysiology and therapy of strabismic amblyopia are discussed.

Influence of spatial frequency, luminance, and duration on binocular rivalry and abnormal fusion of briefly presented dichoptic stimuli

Orthogonal gratings, presented dichoptically, do not fuse into a single percept. Parts of each are seen while other parts are suppressed in an unstable perception (binocular rivalry). However, it has been previously noted that, if the gratings are briefly flashed, they will appear to fuse into a plaid or checkerboard pattern. Three experiments are reported which have been designed to define more clearly the spatial and temporal parameters of this effect in the hope that this would lead to better understanding of the normal mechanisms of dominance and suppression. Stimuli appear fused if flashed for less than 150 ms. The effect is independent of substantial changes in spatial frequency and luminance. Single flashes that appear fused when presented in isolation produce rivalry if separated by intervals less than about 150 ms. Intervals greater than 150 ms produce continued abnormal fusion. Possible mechanisms are discussed.