Eye movements,afterimages and monocular rivalry

The eye-movement/afterimage theory of “monocular rivalry” (MR) between gratings was tested and strongly supported. In three experiments perceptual dominance of vertical or horizontal components of the pattern and fluctuations in perceived contrast of a single grating were shown to depend on the nature of the preceding shift in fixation position in the manner predicted by the theory. In a fourth experiment the angular selectivity of these fluctuations was eliminated, as predicted, when appropriate eye movements were made. Fixation-contingent fluctuations became equally strong for 15° and 90° angles. Taken together with data on afterimages, the results appear to resolve most of the problems recently raised against the theory.     

Binocular motion rivalry in macaque monkeys: eye dominance and tracking eye movements

When the two eyes are exposed to markedly different patterns, perception becomes unstable, falling into oscillations, so that the image of one eye is seen first and then that from the other. With large stimuli the alternation is piecemeal, whilst when small stimuli are used the whole pattern alternates in unison. The purpose of this study was to determine whether a reliable, objective indicator of the perceptual state during binocular rivalry could be developed in the nonhuman primate. Monkeys (Macaca mulatta) were trained to discriminate direction of motion when presented with vertically drifting gratings moving in opposite directions in the two eyes. A high correlation was found between the direction of the slow phase of the optokinetic nystagmus (OKN) elicited by the drifting gratings during rivalry and the direction of motion reported by the monkey even though the gain of the OKN was reduced during rivalry, and the latency was longer. Behavioral eye dominance during rivalry varied significantly over time, between individuals and as a function of interocular contrast differences. Since the direction of tracking eye movements can be used to reliably monitor perceptual state during binocular motion rivalry, the opportunity exists in nonhuman primates to study the neurophysiological mechanisms underlying motion perception during the perceptually ambiguous condition of binocular rivalry.     

The role of eye movements in motion detection

The roles of small eye movements of fixation, and of different kinds of background in motion detection were studied. Minimum detectable displacement for a luminous line oscillating either in a blank field or in the presence of three types of background was measured under two viewing conditions: normal, when eye movements generate normal movements of the image on the retina, and stabilized when these retinal image movements were nearly eliminated. It was demonstrated that eye movements enhance motion detection for a sinusoidally moving target when the target is superimposed on a patterned background; they are detrimental when there is no background. In addition, it was found that the function relating threshold amplitude to frequency of movement is band-pass when the image is stabilized or when the bar moves on a blank field, and is more low-pass when both the background and the test target are subject to the effects of eye movements.     

Development of rivalry and dichoptic masking in human infants

PURPOSE: To examine the development of rivalry, dichoptic masking, and binocular interactions in infants more than 5 months of age using the visual evoked potential (VEP). METHODS: VEPs were recorded in 35 infants between 5 and 15 months of age and 23 adults between 13 and 59 years of age. Counterphasing, sinusoidal, 1 cycle/deg gratings were presented dichoptically. Responses from each eye were isolated by "tagging" each half-image with a different temporal frequency (5 or 7.5 Hz). Observers were presented with fixed 80% contrast gratings in each eye in experiment 1. Rivalry was detected on the basis of a negative correlation between the simultaneously measured response amplitudes at the second harmonics of the two eye-tagging frequencies. In a second analysis of the same data, response amplitudes recorded under dichoptic viewing conditions were compared to those obtained in a monocular control condition (dichoptic masking). In experiment 2, a 40% fixed-contrast grating was presented to one eye, whereas the other eye viewed a grating that was swept in contrast from 1% to 67%. Dichoptic masking was measured as the reduction in the fixed-grating response caused by the variable contrast grating. RESULTS: Experiment 1: although adults showed evidence of VEP amplitude alternations between the eyes for cross-oriented half-images (physiological rivalry), infants did not. This immature response to rivalrous stimuli occurred despite the presence of responses at nonlinear combination frequencies recorded with gratings of the same orientation in each eye, a definitive indication of binocular interaction. In addition, both iso- and cross-oriented half-images produced less dichoptic masking in infants than in adults in this experiment. Experiment 2: dichoptic masking in the infants was equivalent to that seen in adults with parallel gratings in the two eyes; however, masking with cross-oriented configurations was approximately five times weaker in the infants relative to the adults. CONCLUSIONS: The authors have identified a set of stimulus conditions under which infants between 5 and 15 months of age fail to demonstrate physiological rivalry despite the presence of binocular interactions. The observed lack of binocular rivalry may be the result of a specific immaturity in dichoptic, cross-orientation suppression.     

Spatial phase dependence and the role of motion detection in monocular and dichoptic forward masking

Contrast thresholds for briefly flashed gratings were measured by the QUEST procedure, under conditions of forward masking by gratings of the same spatial frequency (usually 1 c/deg). Low-contrast masks reduced threshold at short onset asynchronies (0 to 50 msec), while higher contrasts raised threshold over a broader temporal range (0 to 100-140 msec). Both effects depended on the spatial phase relation, but in different ways. Threshold reduction at 0- +/- 90 deg phases probably arises from spatio-temporal filtering by direction-selective mechanisms. This conclusion was supported by computer simulation of a motion detector model. The direction-selective stage of motion analysis may be entirely monocular, since facilitation at 90 deg was abolished by dichoptic presentation. Threshold elevation was phase-dependent at short SOA's (20-50 msec), with a minimum at +/- 90 deg, but was not phase-dependent at longer SOA's (70-140 msec). In-phase masking (0 deg) was about equally strong monocularly and dichoptically, but dichoptic threshold elevation showed no phase-dependence at any SOA. Threshold elevation at longer SOA's, and with dichoptic presentation, may reflect a purely suppressive binocular masking effect, unselective for spatial phase, and its basis may be the same as contrast adaptation. At short SOA's, monocular and binocular masking data apparently reflect a mixture of this phase-independent suppression and phase-selective facilitation.     

Both monocular and binocular signals contribute to motion rivalry

There is an ongoing debate on whether binocular rivalry involves competition among monocular cells or binocular cells. We investigated this issue psychophysically with two specially designed test stimuli. One test stimulus contained monocular motion signals but greatly reduced binocular motion signals, while the other contained binocular motion signals but no monocular motion signals. For comparison, we also employed a normal rivalrous control containing both monocular and binocular motion signals, and a non-rivalrous flicker-noise control with neither monocular nor binocular motion signals. We found that binocular rivalry for the two test stimuli was significantly reduced compared with the normal rivalrous control, but not completely eliminated compared with the non-rivalrous control. Therefore, both monocular and binocular motion signals appear to contribute to motion rivalry, suggesting that motion rivalry must involve competition among both monocular and binocular cells.     

Angular selectivity of monocular rivalry: Experiment and computer simulation

The angular selectivity of monocular rivalry (MR) was re-examined with achromatic gratings (4 c/deg, 10% contrast). Various measures of rivalry rate and duration all gave the same result: MR increased linearly for angles up to 60° and was constant thereafter, but the duration of a bout of rivalry was invariant with angle. Absolute orientation (vertical vs oblique) had no effect. It is proposed that MR occurs when eye movements produce partial cancellations between the stimulus pattern and its negative after-image. Computer simulation shows that this model can account for almost all the results in the literature and therefore suggests that MR does not reveal the nature of visual pattern-analyzing mechanisms.     

Binocular rivalry and multi-stable perception: independence and monocular channels

When discrepant images are shown to the two eyes, each can intermittently disappear. This is known as binocular rivalry (BR). The causes of BR are debated. One view is that BR is driven by a low-level visual process, characterized by competition between monocular channels. Another is that BR is driven by higher level processes involved in interpreting ambiguous input. This would link BR to other phenomena, wherein perception changes without input changes. We reasoned that if this were true, the timing of BR changes might be related to the timing of changes in other multi-stable stimuli. We tested this using combinations of simple (orthogonal gratings) and complex (pictures of houses and faces) stimuli. We also presented simple stimuli in conjunction with a stimulus that induced an ambiguous direction of rotation. We found that the timing of simple BR changes was unrelated to the timing of either complex BR changes or to direction changes within an ambiguous rotation. However, the timings of changes within proximate BR stimuli, both simple and complex, were related, but only when similar images were encoded in the same monocular channels. These observations emphasize the importance of monocular channel interactions in determining the timing of binocular rivalry changes.     

On the relation between dichoptic masking and binocular rivalry

When our two eyes view incompatible images, the brain invokes suppressive processes to inhibit one image, and favor the other. Two phenomena are typically observed: dichoptic masking (reduced sensitivity to one image) for brief presentations, and binocular rivalry (alternation between the two images), over longer exposures. However, it is not clear if these two phenomena arise from a common suppressive process. We investigated this by measuring both threshold elevation in simultaneous dichoptic masking and mean percept durations in rivalry, whilst varying relative stimulus orientation. Masking and rivalry showed significant correlations, such that strong masking was associated with long dominance durations. A second experiment suggested that individual differences across both measures are also correlated. These findings are consistent with varying the magnitude of interocular suppression in computational models of both rivalry and masking, and imply the existence of a common suppressive process. Since dichoptic masking has been localised to the monocular neurons of V1, this is a plausible first stage of binocular rivalry.     

Dichoptic masking and binocular rivalry share common perceptual dynamics

Two of the strongest tools to manipulate visual awareness of potentially salient stimuli are binocular rivalry and dichoptic masking. Binocular rivalry is induced by presenting incompatible images to the two eyes over prolonged periods of time, leading to an alternating perception of the two images. Dichoptic masking is induced when two images are presented once in rapid succession, leading to the perception of just one of the images. Although these phenomena share some key characteristics, most notably the ability to erase from awareness potentially very salient stimuli, their relationship is poorly understood. We investigated the perceptual dynamics during long-lasting dynamic stimulation leading to binocular rivalry or dichoptic masking. We show that the perceptual dynamics during dichoptic masking conditions meet the classifiers used to classify a process as binocular rivalry; that is, (1) Levelt's 2nd proposition is obeyed; (2) perceptual dominance durations follow a gamma distribution; and (3) dominance durations are sequentially independent. We suggest that binocular rivalry and dichoptic masking may be mediated by the same inhibitory mechanisms.     

Effects of attention on visual experience during monocular rivalry

There is a long-running debate over the extent to which volitional attention can modulate the appearance of visual stimuli. Here we use monocular rivalry between afterimages to explore the effects of attention on the contents of visual experience. In three experiments, we demonstrate that attended afterimages are seen for longer periods, on average, than unattended afterimages. This occurs both when a feature of the afterimage is attended directly and when a frame surrounding the afterimage is attended. The results of these experiments show that volitional attention can dramatically influence the contents of visual experience.     

The role of eye movements in decision making and the prospect of exposure effects

The aim of the current study was to follow on from previous findings that eye movements can have a causal influence on preference formation. Shimojo et al. (2003) previously found that faces that were presented for a longer duration in a two alternative forced choice task were more likely to be judged as more attractive. This effect only occurred when an eye movement was made towards the faces (with no effect when faces were centrally presented). The current study replicated Shimojo et al.’s (2003) design, whilst controlling for potential inter-stimuli interference in central presentations. As per previous findings, when eye movements were made towards the stimuli, faces that were presented for longer durations were preferred. However, faces that were centrally presented (thus not requiring an eye movement) were also preferred in the current study. The presence of an exposure duration effect for centrally presented faces casts doubt on the necessity of the eye movement in this decision making process and has implications for decision theories that place an emphasis on the role of eye movements in decision making.     

Retinal image shifts, but not eye movements per se, cause alternations in awareness during binocular rivalry

Particularly promising studies on visual awareness exploit a generally used perceptual bistability phenomenon, "binocular rivalry"--in which the two eyes' images alternately dominate--because it can dissociate the visual input from the perceptual output. To successfully study awareness, it is crucial to know the extent to which eye movements alter the input. Although there is convincing evidence that perceptual alternations can occur without eye movements, the literature on their exact role is mixed. Moreover, recent work has demonstrated that eye movements, first, correlate positively with perceptual alternations in binocular rivalry, and second, often accompany covert attention shifts (that were previously thought to be purely mental). Here, we asked whether eye movements cause perceptual alternations, and if so, whether it is either the execution of the eye movement or the resulting retinal image change that causes the alternation. Subjects viewed repetitive line patterns, enabling a distinction of saccades that did produce foveal image changes from those that did not. Subjects reported binocular rivalry alternations. We found that, although a saccade is not essential to initiate percept changes, the foveal image change resulting from a (micro)saccade is a deciding factor for percept dominance. We conclude that the foveal image must change to have a saccade cause a change in awareness. This sheds new light on the interaction between spatial attention shifts and perceptual alternations.     

Version and vergence eye movements in humans: open-loop dynamics determined by monocular rather than binocular image speed

We examined the velocity dependence of the vergence and version eye movements elicited by motion stimuli that were symmetric or asymmetric at the two eyes. Movements of both eyes were recorded with the scleral search coil technique. Vergence was computed as the difference in the positions of the two eyes (left-right) and version was computed as the average position of the two eyes ((left+right)/2). Subjects faced a large tangent screen onto which two identical random-dot patterns were back-projected. Each pattern was viewed by one eye only using crossed-polarizers and its position was controlled by X/Y mirror galvanometers. Viewing was always binocular and horizontal velocity steps (range, 5-240 deg/s) were applied to one (asymmetric stimulus) or both (symmetric stimulus) patterns approximately 50 ms after a centering saccade. With the symmetric stimulus, the motion at the two eyes could be either in the opposite direction (eliciting vergence responses) or in the same direction (eliciting version responses). The asymmetric stimuli elicited both vergence and version. In all cases, minimum response latencies were very short (<90 ms). Velocity tuning curves (based on the changes in vergence and version over the time period, 90-140 ms) were all sigmoidal and peaked when the monocular (i.e., retinal) image velocities were 30-60 deg/s. The vergence (version) responses to symmetric stimuli were linearly related to the vergence (version) responses to asymmetric stimuli when expressed in terms of the monocular rather than the binocular image velocities. We conclude that the dynamical limits for both vergence and version are imposed in the monocular visual pathways, before the inputs from the two eyes are combined.