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Distributions of alternation rates in various forms of bistable perception.

Studying the temporal dynamics of bistable perception can be useful for understanding neural mechanisms underlying the phenomenon. We take a closer look at those temporal dynamics, using data from four different ambiguous stimuli. We focus our analyses on two recurrent themes in bistable perception literature. First, we address the question whether percept durations follow a gamma distribution, as is commonly assumed. We conclude that this assumption is not justified by the gamma distribution's approximate resemblance to distributions of percept durations. We instead present two straightforward distributions of reciprocal percept durations (i.e., rates) that both easily surpass the classic gamma distribution in terms of resemblance to empirical data. Second, we compare the distributions arising from binocular rivalry with those from other forms of bistable perception. Parallels in temporal dynamics between those classes of stimuli are often mentioned as an indication of a similar neural basis, but have never been studied in detail. Our results demonstrate that the distributions arising from binocular rivalry and other forms of bistable perception are indeed similar up to a high level of detail.

Attention↗

Temporal dynamics in bistable perception.

Bistable perception is fundamentally a dynamic process: Our perceptual experience continuously alternates when an ambiguous or rivalrous stimulus is observed. Here we present a method to analyze instantaneous measures of dominance and transition between percepts. The analysis extracts three time-varying probabilities. First, the transient preference represents the probability of perceiving one interpretation at one instant. Second, the reversal probability is the probability that the current percept will change at the next evaluation. Finally, the survival probabilities are the probability that at one instant the current percept will not switch to the alternative interpretation. We derive the relationships between these probabilities and offer a test of independence between consecutive percepts. We also introduce a simple technique to sample the observer's perception at regular intervals. The analyzing method is illustrated with the example of binocular rivalry. We demonstrate Levelt's second proposition with the survival probability measure and show that the consecutive rivalrous percepts are not independent.

Dominance, Ocular↗

Depth of interocular suppression associated with continuous flash suppression, flash suppression, and binocular rivalry.

When conflicting images are presented to the corresponding regions of the two eyes, only one image may be consciously perceived. In binocular rivalry (BR), two images alternate in phenomenal visibility; even a salient image is eventually suppressed by an image of low saliency. Recently, N. Tsuchiya and C. Koch (2005) reported a technique called continuous flash suppression (CFS), extending the suppression duration more than 10-fold. Here, we investigated the depth of this prolonged form of interocular suppression as well as conventional BR and flash suppression (FS) using a probe detection task. Compared to monocular viewing condition, CFS elevated detection thresholds more than 20-fold, whereas BR did so by 3-fold. In subsequent experiments, we dissected CFS into several components. By manipulating the number and timing of flashes with respect to the probe, we found that the stronger suppression in CFS is not due to summation between BR and FS but is caused by the summation of the suppression due to multiple flashes. Our results support the view that CFS is not a stronger version of BR but is due to the accumulated suppressive effects of multiple flashes.

Humans↗

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.

Awareness↗

Human cone light adaptation: from behavioral measurements to molecular mechanisms.

The ability of the cone visual system to regulate its sensitivity from twilight to bright sunlight is an extraordinary feat of biology. Here, we investigate the changes in visual processing that accompany cone light adaptation over a 5 log10 unit intensity range by combining measures of temporal sensitivity made in one eye with measures of the temporal delay between the two eyes in different states of adaptation. This combination of techniques, which provides more complete information than has been available before, leads to a simple model of steady-state light adaptation. At high light levels, visual sensitivity is maintained mainly by photopigment bleaching. At low-to-moderate light levels, it is maintained by trading unwanted sensitivity for speed and by an additional process that paradoxically increases the overall sensitivity as the light level rises. Each stage of the model can be linked to molecular mechanisms within the photoreceptor: The speeding up can be linked to faster rates of decay of activated molecules; the paradoxical sensitivity increases can be linked to faster rates of molecular resynthesis and to changes in channel sensitivity; and the sensitivity decreases can be linked to bleaching. Together, these mechanisms act to maintain the cone visual system in an optimal operating range and to protect it from overload.

Adaptation, Ocular↗

The time course of binocular rivalry reveals a fundamental role of noise.

When our two eyes view incongruent images, we experience binocular rivalry: An ongoing cycle of dominance periods of either image and transition periods when both are visible. Two key forces underlying this process are adaptation of and inhibition between the images' neural representations. Models based on these factors meet the constraints posed by data on dominance periods, but these are not very stringent. We extensively studied contrast dependence of dominance and transition durations and that of the occurrence of return transitions: Occasions when an eye loses and regains dominance without intervening dominance of the other eye. We found that dominance durations and the incidence of return transitions depend similarly on contrast; transition durations show a different dependence. Regarding dominance durations, we show that the widely accepted rule known as Levelt's second proposition is only valid in a limited contrast range; outside this range, the opposite of the proposition is true. Our data refute current models, based solely on adaptation and inhibition, as these cannot explain the long and reversible transitions that we find. These features indicate that noise is a crucial force in rivalry, frequently dominating the deterministic forces.

Adaptation, Physiological↗

Human stereopsis.

This paper reviews much of the basic literature on stereopsis for the purpose of providing information about the ability of humans to utilize stereoscopic information under operational conditions. This review is organized around five functional topics that may be important for the design of many stereoscopic display systems: geometry of stereoscopic depth perception, visual persistence, perceptual interaction among stereoscopic stimuli, neurophysiology of stereopsis, and theoretical considerations. The paper concludes with the presentation of several basic ideas related to the design of stereoscopic displays.

Animals↗

Performance under dichoptic versus binocular viewing conditions: effects of attention and task requirements.

Three experiments investigated subjects' ability to allocate attention and cope with task requirements under dichoptic versus binocular viewing conditions. Experiments 1 and 2 employed a target detection task in compound and noncompound stimuli, and Experiment 3 employed a relative-proximity judgment task. The tasks were performed in a focused attention condition in which subjects had to attend to the stimulus presented to one eye or field (under dichoptic and binocular viewing conditions, respectively) while ignoring the stimulus presented to the other eye or field, and in a divided attention condition in which subjects had to attend the stimuli presented to both eyes or fields. Subjects' performance was affected by the interaction of attention conditions with task requirements, but it was generally the same under dichoptic and binocular viewing conditions. The more dependent the task was on finer discrimination, the more performance was impaired by divided attention. These results suggest that at least with discrete tasks and relatively short exposure durations, performance when each eye is presented with a separate stimulus is the same as when the entire field of stimulation is viewed by both eyes.

Adult↗

fMRI/EEG in paroxysmal activity elicited by elimination of central vision and fixation.

Using functional MRI (fMRI) with concurrent EEG monitoring the authors studied cortical activation associated with epileptiform discharges in three patients with fixation-off sensitivity. The EEG paroxysmal activity elicited by eliminating central vision correlated significantly with an increased blood oxygen level-dependent signal in the extrastriate cortex (Brodmann areas 19 and 37). fMRI provides a unique opportunity for localizing precisely the cortical areas generating paroxysmal activity in patients with fixation-off sensitivity.

Adult↗

Binocular processes in vernier acuity.

We investigate the role of binocular mechanisms in vernier acuity, using dichoptic variants of spatial-frequency masking and flank-line interference paradigms. The finding that grating masks and flanking lines presented to one eye elevate (worsen) thresholds for detecting vernier offsets presented to the other eye suggests that neural mechanisms mediating vernier acuity receive binocular inputs, thus placing the loci of these mechanisms at postreceptoral sites. The observation that these threshold elevation effects are orientation dependent is consistent with a contribution to vernier acuity from oriented cortical filters.

Humans↗

Visual assessment of variable-resolution imagery.

A technique is described for producing variable-resolution images whose spatial detail decreases as a function of distance from their centers. These images can be matched in some sense to the normal spatial inhomogeneities of the human visual system, as well as to various abnormalities in spatial discrimination. A set of images was generated with a series of linear distortion functions whose low-pass characteristics differed at both the center and the periphery of the image as well as across the image. A forced-choice procedure was used to determine which test images were indistinguishable from unprocessed versions of themselves. Certain of the threshold distortion functions are compared with eccentricity scaling functions that have been used by others to characterize various aspects of peripheral vision. Finally, the concept of locally band-limited spaces is discussed, and an efficient sampling technique based on the concept is described. This technique can be used to generate an image that, under certain conditions, is visually equivalent to an otherwise identical image containing significantly more information.

Adult↗

Achromatizing the human eye: the problem of chromatic parallax.

Attempts to correct the chromatic difference of focus of the human eye will introduce unwanted chromatic parallax if the eye is misaligned with the optical axis of the achromatizing system. Using geometrical optics, we show that the amount of parallax is approximately proportional to the amount of misalignment of the eye, with the constant of proportionality equal to the eye's chromatic difference of refractive error. This prediction was confirmed by the experimental determination of chromatic parallax for two commercially available achromatizing lenses. On the basis of these results, we calculated that anticipated improvements in the polychromatic modulation transfer function of the eye offered by achromatizing lenses will be canceled by approximately 0.4 mm of the misalignment between the lens and the eye. Our prediction that further misalignment would severely reduce image quality of the achromatized eye was verified by psychophysical measurements of contrast sensitivity.

Color Perception↗

Critical flicker frequency under monocular and binocular conditions.

Critical flicker frequency of 40 undergraduate students, 20 men and 20 women whose mean age was 19.3 yr., was measured under monocular and binocular viewing conditions, using the Lafayette Visual Perception Control with Display Unit. Half of the subjects received monocular treatment first and binocular treatment second, and the other half received the treatments in the reversed order. A 2-min. rest was allowed between these treatments. Analysis shows that mean CFF under the monocular condition was significantly lower than that under binocular conditions. The findings were discussed in terms of visual constraint and eye strain and fatigue under the monocular viewing condition.

Adult↗

Connection between synchronization of oscillatory activities at early stages and a final stage in the visual system.

Singer's group and we found synchronized oscillatory responses at early stages and at a final stage in visual processing, respectively. However, the former occurs on a millisecond time scale and the latter on a second time scale. We have considered that this results from a difference in scale of something which takes part in the oscillations. From this viewpoint, here we first find suppression effects on both figure and ground of a pattern and on only the ground in a binocular-rivalry situation and then examine, using the effects of two steps, what oscillates and synchronizes at the last stage for form. Finally, we conclude that the synchronous neuronal oscillations depend on firing patterns of neurons at early stages in the visual system and the psychophysical ones on configurations of the spread effect of strength of pattern involved closely in whole processing of the visual system and the latter originates in the former.

Field Dependence-Independence↗

Pattern of cortical activity, degree of synchronization, binocular fusion, and binocular rivalry.

The synchronization hypothesis is the likely idea for the binding problem in the brain. Here we tested whether the theory is applicable for the occurrence of either binocular fusion or binocular rivalry. We first showed patterns of activated patches in V1 with proceeding from fusion to rivalry on the basis of Hubel and Wiesel's 1979 illustration. We then assumed that the strength of synchrony between the patches in the left-eye and right-eye ocular dominance columns is a crucial determinant for the divergence between fusion and rivalry. By using the strength of fusion between the paired images as a measure of degree of synchrony, we confirm the assumption about interocular vision and the synchronization hypothesis as well.

Form Perception↗

Emotion space as a predictor of binocular rivalry.

In our 1999 report, we examined robustness of a two-dimensional structure of facial expressions of emotion under the condition of some perceptual ambiguity, using a stereo-scope. The current study aimed to replicate and extend the previous work by adding facial photographs of different persons and by measuring participants' perception of stereoscopically presented faces. Multidimensional scaling provided a two-dimensional configuration of facial expressions comparable with the previous studies. Although binocular rivalry was a less frequent phenomenon, it was suggested that the distances between facial expressions in the derived space were a contributing factor in eliciting binocular rivalry.

Emotions↗

Recovery of fusional convergence after systematic practice.

Fusional convergence was trained systematically with orthoptic devices in three patients with a severe deficit in convergent fusion resulting from vascular (one case) or traumatic (two cases) brain damage. Within a single-subject baseline design two of the three patients showed a significant, and the third patient a moderate, recovery of fusional range. All patients showed a significant improvement in visual acuity for objects close by, improved reading performance and an increase in stereo acuity (two cases). The results suggest a considerable potential for recovery of oculomotor functions in brain-damaged patients when appropriate treatment methods are applied.

Accommodation, Ocular↗