Biomedical subjects

Raymond van Ee

Publications and source records attributed to Raymond van Ee.

At least 19 recordsLinked to original sources

PubMed2006

Stimulus flicker alters interocular grouping during binocular rivalry.

When the two eyes are presented with sufficiently different stimuli, the stimuli will engage in binocular rivalry. During binocular rivalry, a subject's perceptual state alternates between awareness of the stimulus presented to the right eye and that presented to the left eye. There are instances in which competition is not eye-based, but instead takes place between stimulus features, as is the case in flicker and switch rivalry (F&S). Here we investigate another such instance, interocular grouping, using a Diaz-Caneja type stimulus in conjunction with synchronous stimulus flicker. Our results indicate that stimulus flicker increases the total duration of interocularly bound percepts, and that this effect occurs for a range of temporal flicker frequencies. Furthermore, the use of contrast-inversion flicker causes a decrease of total dominance duration of the interocularly bound percepts. We argue that different flickering regimes can be used to differentially stimulate lower and higher levels of visual processing involved in binocular rivalry. We propose that the amount of interocularly combined pattern-completed percept can be regarded as a measure of the level at which binocular rivalry is resolved.

Dominance, Ocular↗

PubMed2006

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↗

PubMed2006

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↗

PubMed2006

Slant perception, and its voluntary control, do not govern the slant aftereffect: multiple slant signals adapt independently.

Although it is known that high-level spatial attention affects adaptation for a variety of stimulus features (including binocular disparity), the influence of voluntary attentional control-and the associated awareness-on adaptation has remained unexplored. We developed an ambiguous surface slant adaptation stimulus with conflicting monocular and binocular slant signals that instigated two mutually exclusive surface percepts with opposite slants. Using intermittent stimulus removal, subjects were able to voluntarily select one of the two rivaling slant percepts for extended adaptation periods, enabling us to dissociate slant adaptation due to awareness from stimulus-induced slant adaptation. We found that slant aftereffects (SAE) for monocular and binocular test patterns had opposite signs when measured simultaneously. There was no significant influence of voluntarily controlled perceptual state during adaptation on SAEs of monocular or binocular signals. In addition, the magnitude of the binocular SAE did not correlate with the magnitude of perceived slant. Using adaptation to one slant cue, and testing with the other cue, we demonstrated that multiple slant signals adapt independently. We conclude that slant adaptation occurs before the level of slant awareness. Our findings place the site of stereoscopic slant adaptation after disparity and eye posture are interpreted for slant [as demonstrated by Berends et al. (Berends, E. M., Liu, B., & Schor, C. M. (2005). Stereo-slant adaptation is high level and does not involve disparity coding. Journal of Vision 5 (1), 71-80), using that disparity scales with distance], but before other slant signals are integrated for the resulting awareness of the presented slant stimulus.

Adaptation, Physiological↗

PubMed2006

Endogenous influences on perceptual bistability depend on exogenous stimulus characteristics.

We investigated the influence of changing physical parameters and task on bistable perception of an ambiguously rotating sphere (SFM). Increasing dot-density and velocity decreased the duration of perceptual phases during both passive viewing and voluntary control exertion. Our main finding is that voluntary control of perception depends on the physical parameters constituting the stimulus. This dependency places important constraints on the mechanisms mediating voluntary control as these mechanisms cannot operate independently of stimulus characteristics. In addition, local asymmetries in dot-densities can trigger alternations towards the most salient direction, which is not necessarily associated with largest number of dots: competition between perceptual interpretations during SFM appears to occur between surface-based representations rather than between individual elements. Finally, we show that voluntary control remains effective, even when attentive tracking of individual stimulus elements is no longer possible.

Attention↗

PubMed2006

A single system explains human speed perception.

Motion is fully described by a direction and a speed. The processing of direction information by the visual system has been extensively studied; much less is known, however, about the processing of speed. Although it is generally accepted that the direction of motion is processed by a single motion system, no such consensus exists for speed. Psychophysical data from humans suggest two separate systems processing luminance-based fast and slow speeds, whereas neurophysiological recordings in monkeys generally show continuous speed representation, hinting at a single system. Although the neurophysiological findings hint at a single system, they remain inconclusive as only a limited amount of cells can be measured per study and, possibly, the putative different motion systems are anatomically separate. In three psychophysical motion adaptation experiments, we show that predictions on the basis of the two-motion system hypothesis are not met. Instead, concurrent modeling showed that both here-presented and previous data are consistent with a single system subserving human speed perception. These findings have important implications for computational models of motion processing and the low-level organization of the process.

Adaptation, Physiological↗

PubMed2005

The role of saccades in exerting voluntary control in perceptual and binocular rivalry.

We have investigated the role of saccades and fixation positions in two perceptual rivalry paradigms (slant rivalry and Necker cube) and in two binocular rivalry paradigms (grating and house-face rivalry), and we compared results obtained from two different voluntary control conditions (natural viewing and hold percept). We found that for binocular rivalry, rather than for perceptual rivalry, there is a marked positive temporal correlation between saccades and perceptual flips at about the moment of the flip. Across different voluntary control conditions the pattern of temporal correlation did not change (although the amount of correlation did frequently, but not always, change), indicating that subjects do not use different temporal eye movement schemes to exert voluntary control. Analysis of the fixation positions at about the moment of the flips indicates that the fixation position by itself does not determine the percept but that subjects prefer to fixate at different positions when asked to hold either of the different percepts.

Depth Perception↗

PubMed2005

Activation in visual cortex correlates with the awareness of stereoscopic depth.

Using event-related functional magnetic resonance imaging, we studied the activation correlating with the awareness of stereoscopic depth using a bistable slanted surface (slant rivalry). Bistability resulted from incongruence between two slant-defining cues: binocular disparity and monocular perspective. The stimulus was perceived as alternating between the perspective-dominated percept (monocular depth) and the disparity-dominated percept (stereopsis), while sensory input remained constant, enabling us to study changes in awareness of depth associated with either cue. Transient activation relating to perceptual alternations was found bilaterally in the caudal part of the intraparietal sulcus, in the right-hemispheric anterior intraparietal sulcus, within visual area V4d-topo, and inferior to area MT+. Transient activation correlating specifically with alternations toward the disparity-dominated percept was found in a number of visual areas, including dorsal visual areas V3A, V7, and V4d-topo and visual areas MT+ and lateral occipital complex. No activation was found for alternations toward the perspective-dominated percept. Our results show that of all visual areas responsive to disparity-defined depth, V4d-topo shows the most robust signal changes correlating with the instigation of stereoscopic depth awareness (stereopsis).

Awareness↗

PubMed2005

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↗

PubMed2005

Voluntarily controlled bi-stable slant perception of real and photographed surfaces.

We have quantified voluntarily selected perceived slant of real trapezoidal surfaces (a 'reverse-perspective' scene) and their photographed counterparts (pictorial space). The surfaces were slanted about the vertical axis and observers estimated slant relative to the frontal plane. We were particularly interested in those cases in which binocular disparity and monocular perspective provided conflicting slant information. We varied the monocularly and binocularly specified surface slants independently across stimulus presentations. To eliminate texture and shading cues we used sand-blasted aluminium trapezoidal surfaces illuminated from all directions. When disparity-specified slant and perspective-specified slant were conflicting, observers were able to perceive the surfaces in two ways: they perceived either a trapezoid or a rectangle. Our main finding is twofold. First, when subjects chose to perceive the trapezoid, the slant estimates followed the disparity-predicted slant with only a slight underestimation, as if they selected a pure binocular representation of slant governed only by disparity. Second, when subjects chose to perceive the rectangle their estimates for real surfaces were similar to those for photographed surfaces, as if they selected a representation of slant governed by perspective foreshortening.

Humans↗

PubMed2005

Dynamics of perceptual bi-stability for stereoscopic slant rivalry and a comparison with grating, house-face, and Necker cube rivalry.

A way to study conscious perception is to expose the visual system to an ambiguous stimulus that instigates bi-stable perception. This provides the opportunity to study neural underpinnings related to the percepts rather than to the stimulus. We have recently developed a slant-rivalry paradigm that has beneficial metrical (quantitative) aspects and that exhibits temporal aspects of perceptual reversals that seemed to be under considerable voluntary control of the observer. Here we examined a range of different aspects of the temporal dynamics of the perceptual reversals of slant rivalry and we compared these with the dynamics of orthogonal grating rivalry, house-face rivalry, and Necker cube rivalry. We found that slant rivalry exhibits a qualitatively similar pattern of dynamics. The drift of the perceptual reversal rate, both across successive experimental repetitions, and across successive 35-s portions of data were similar. The sequential dependence of the durations of perceptual phases, too, revealed very similar patterns. The main quantitative difference, which could make slant rivalry a useful stimulus for future neurophysiological studies, is that the percept durations are relatively long compared to the other rivalry stimuli. In the paper that accompanies this paper [van Ee, R., van Dam, L. C. J., Brouwer, G. J. (2005). Voluntary control and the dynamics of perceptual bi-stability. Vision Research,] we focused on the role of voluntary control in the dynamics of perceptual reversals.

Humans↗

PubMed2005

The role of (micro)saccades and blinks in perceptual bi-stability from slant rivalry.

We exposed the visual system to an ambiguous 3D slant rivalry stimulus consisting of a grid for which monocular (perspective) and binocular (disparity) cues independently specified a slant about a horizontal axis. When those cues specified similar slants, observers perceived a single slant. When the difference between the specified slants was large, observers alternatively perceived a perspective- or a disparity-dominated slant. Eye movement measurements revealed that there was no positive correlation between a perceptual flip and both saccades (microsaccades as well as larger saccades) and blinks that occurred prior to a perceptual flip. We also found that changes in horizontal vergence were not responsible for perceptual flips. Thus, eye movements were not essential to flip from one percept to the other. After the moment of a perceptual flip the occurrence probabilities of both saccades and blinks were reduced. The reduced probability of saccades mainly occurred for larger voluntary saccades, rather than for involuntary microsaccades. We suggest that the reduced probability of voluntary saccades reflects a reset of saccade planning.

Blinking↗

PubMed2004

Stereoscopic matching and the aperture problem.

In order to perceive stereoscopic depth, the visual system must define binocular disparities. Consider an oblique line seen through an aperture formed by flanking occluders. Because the line is perceived behind the aperture, the line must have disparity relative to the aperture. What is the assigned disparity of the line in this aperture problem? To answer this question five observers adjusted the horizontal disparity of a probe until it was perceived at the same depth as the disparate line behind the aperture. The results show that, when both the horizontal and the vertical disparities of the occluders are well-defined, the probe must have the same horizontal disparity as the horizontal separation between the line half-images. However, when the horizontal and vertical disparities of the occluders are ill-defined, the intersections of the line and the occluder borders can determine the matching direction. In the latter case, the matching direction varies with the aperture orientation and there is considerable variability across observers.

Depth Perception↗

PubMed2003

Perceived slant from Werner's illusion affects binocular saccadic eye movements.

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

Adult↗

PubMed2003

Attention-biased multi-stable surface perception in three-dimensional structure-from-motion.

Retinal velocity distributions can lead to a percept of three-dimensional (3D) structure (structure-from-motion [SFM]). SFM stimuli are intrinsically ambiguous with regard to depth ordering. A classic example is the orthographic projection of a revolving transparent cylinder, which can be perceived as a 3D cylinder that rotates clockwise and counterclockwise alternately. Prevailing models attribute such bistable percepts to inhibitory connections between neurons that are tuned to opposite motion directions at equal binocular disparities. Cylinder stimuli can yield not only two but as many as four different percepts. Besides the well-documented clockwise and counterclockwise spinning transparent cylinders, observers can also perceive two transparent half-cylinders, either convex or concave, one in front of the other. Observers are able to bias the time during which a percept is present by attending to one or the other percept. We examined this phenomenon quantitatively and found that in standard SFM stimuli, the percept of two convex transparent half-cylinders can occur just as often as the percept of (counter-) clockwise spinning cylinders. So far, however, all interpretations of experimental (neurophysiological) data and all proposed mechanisms for SFM perception have focused solely on the two classical cylinder percepts. Prevailing models cannot explain the existence of the other two percepts. We suggest an alternative model to explain attention-biased multi-stable perception.

Attention↗

PubMed2003

The influence of cyclovergence on unconstrained stereoscopic matching.

In order to perceive depth from binocular disparities the visual system has to identify matching features of the two retinal images. Normally, the assigned disparity is unambiguously determined by monocularly visible matching constraints. The assigned disparity is ambiguous when matching is unconstrained, such as when we view an isolated long oblique disparate line. Recently we found that in order to perceive a depth probe at the same depth as the oblique line, the probe needs to have the same horizontal disparity as the line (i.e. matching occurs along horizontal "search-zones" [Vis. Res. 40 (2000) 151]). Here we examined whether the depth probe disparity in unconstrained matching of long lines is influenced by cyclovergence, by cyclorotation between stereogram half-images, or by combinations of the two. We measured retinal rotation (>6 deg in cyclovergence conditions). We found that in those conditions in which the retinal images were the same (a condition with, say, both zero cyclovergence and zero cyclorotation between the half-images, creates the same retinal images as a condition with both 6 deg cyclovergence and 6 deg cyclorotation) assigned depth was the same too, i.e. independent of cyclovergence. Thus, the assigned depth of the test-line seems to be determined solely by the retinal test-line orientation, implying that the binocular matching algorithm does not seem to incorporate the eyes' cyclovergence when matching is unconstrained.

Depth Perception↗

PubMed2003

Correlation between stereoanomaly and perceived depth when disparity and motion interact in binocular matching.

The aim of this study was to find out to what extent binocular matching is facilitated by motion when stereoanomalous and normal subjects estimate the perceived depth of a 3-D stimulus containing excessive matching candidates. Thirty subjects viewed stimuli that consisted of bars uniformly distributed inside a volume. They judged the perceived depth-to-width ratio of the volume by adjusting the aspect ratio of an outline rectangle (a metrical 3-D task). Although there were large inter-subject differences in the depth perceived, the experimental results yielded a good correlation with stereoanomaly (the inability to distinguish disparities of different magnitudes and/or signs in part of the disparity spectrum). The results cannot be explained solely by depth-cue combination. Since up to 30% of the population is stereoanomalous, stereoscopic experiments would yield more informative results if subjects were first characterized with regard to their stereo capacities. Intriguingly, it was found that motion does not help to define disparities in subjects who are able to perceive depth-from-disparity in half of the disparity spectrum. These stereoanomalous subjects were found to rely completely on the motion signals. This suggests that the perception of volumetric depth in subjects with normal stereoscopic vision requires the joint processing of crossed and uncrossed disparities.

Depth Perception↗

PubMed2003

Bayesian modeling of cue interaction: bistability in stereoscopic slant perception.

Our two eyes receive different views of a visual scene, and the resulting binocular disparities enable us to reconstruct its three-dimensional layout. However, the visual environment is also rich in monocular depth cues. We examined the resulting percept when observers view a scene in which there are large conflicts between the surface slant signaled by binocular disparities and the slant signaled by monocular perspective. For a range of disparity-perspective cue conflicts, many observers experience bistability: They are able to perceive two distinct slants and to flip between the two percepts in a controlled way. We present a Bayesian model that describes the quantitative aspects of perceived slant on the basis of the likelihoods of both perspective and disparity slant information combined with prior assumptions about the shape and orientation of objects in the scene. Our Bayesian approach can be regarded as an overarching framework that allows researchers to study all cue integration aspects-including perceptual decisions--in a unified manner.

Bayes Theorem↗