Casper J Erkelens

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Verified

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• PhD
• Professor Emeritus at Utrecht University

Luneburg's model has been the reference for experimental studies of visual space for almost seventy years. His claim for a curved visual space has been a source of inspiration for visual scientists as well as philosophers. The conclusion of many experimental studies has been that Luneburg's model does not describe visual space in various tasks and...

In the literature, perspective space has been introduced as a model of visual space. Perspective space is grounded on the perspective nature of visual space during both binocular and monocular vision. A single parameter, that is, the distance of the vanishing point, transforms the geometry of physical space into that of perspective space. The persp...

A picture is a powerful and convenient medium for inducing the illusion that one perceives a three-dimensional scene. The relative invariance of picture perception across viewing positions has aroused the interest of painters, photographers, and visual scientists. This study explores variables that may underlie the invariance. It presents a computa...

Perspective plays an important role in the creation and appreciation of depth on paper and canvas. Paintings of extant scenes are interesting objects for studying perspective, because such paintings provide insight into how painters apply different aspects of perspective in creating highly admired paintings. In this regard the paintings of the Piaz...

Perspective space has been introduced as a computational model of visual space. The model is based on geometric features of visual space. The model has proven to describe a range of phenomena related to the visual perception of distance and size. Until now, the model lacks a mathematical description that holds for complete 3D space. Starting from a...

A picture is a powerful and convenient medium for inducing the illusion that one perceives a three-dimensional scene. The relative invariance of picture perception across viewing positions has aroused the interest of visual scientists for decades. This study explores variables that may underlie the invariance. To that end, sizes and distances of ob...

There are two types of perspective relevant to human vision, photographic and visual perspective. Photographic perspective results from projection of the three-dimensional world on two-dimensional surfaces such as canvases and retinae. Photographic perspective is widely applied in drawings and paintings to create an illusion of depth. Visual perspe...

A picture is a powerful and convenient medium for inducing the illusion that one perceives a real three-dimensional scene. The relative invariance of picture perception across viewing positions has aroused the interest of painters, photographers and visual scientists. Many studies have been devoted to perceptual invariance when pictures are viewed...

ECVP: Human vision is extremely sensitive to equidistance of spatial intervals. Thresholds for spatial equidistance have been extensively measured in bisecting tasks. In spite of the vast number of studies, the informational basis for equidistance perception is unknown. There are three possible sources of information for spatial equidistance in pic...

Human vision is extremely sensitive to equidistance of spatial intervals in the frontal plane. Thresholds for spatial equidistance have been extensively measured in bisecting tasks. Despite the vast number of studies, the informational basis for equidistance perception is unknown. There are three possible sources of information for spatial equidist...

Unlabelled: Ambiguous visual stimuli elicit different perceptual interpretations over time, creating the illusion that a constant stimulus is changing. We investigate whether such spontaneous changes in visual perception involve occipital brain regions specialized for processing visual information, despite the absence of concomitant changes in sti...

We perceive perspective angles, that is, angles that have an orientation in depth, differently from what they are in physical space. Extreme examples are angles between rails of a railway line or between lane dividers of a long and straight road. In this study, subjects judged perspective angles between bars lying on the floor of the laboratory. Pe...

Retinal images are perspective projections of the visual environment. Despite this, it is not self-evident that visual space is a perspective representation of physical space. Analysis of underlying spatial transformations shows that visual space is perspective only if its depth is finite. Three subjects judged perspective angles, i.e. angles perce...

The Necker cube is a striking example for perceptual dominance of 3D over 2D. Object symmetry and obliqueness of angles are co-varying cues that may underlie the perceived slant of Necker cubes. To investigate the power of the oblique-angle cue, slants were judged of extremely simple symmetrical shapes. Slant computations based on an assumption of...

Retinal images are perspective projections of the visual environment. Perspective projections do not explain why we perceive perspective in 3-D space. Analysis of underlying spatial transformations shows that visual space is a perspective transformation of physical space if parallel lines in physical space vanish at finite distance in visual space....

Depicted slant is defined as slant based on linear-perspective assumptions about lines imaged on a flat surface. Slants of triangles and trapezoids were computed as a function of depicted slant and slant of the obliquely viewed picture plane . Computations were based on assumptions of parallelism and orthogonality . Perceived slant was measured dur...

One of the striking features of vision is that we can experience depth in two-dimensional images. Since the Renaissance, artists have used linear perspective to create sensations of depth and slant. What is not known is how the brain measures linear perspective information from the retinal image. Here, an experimental technique and geometric comput...

One of the striking features of vision is that we can experience depth in two-dimensional images. Since the Renaissance, artists have used linear perspective to create sensations of depth and slant. What is not known is how the brain measures linear perspective information from the retinal image. Here, an experimental technique and geometric comput...

Virtual slant is defined here as the slant of a surface based on the assumption of linear perspective. Virtual slants of obliquely viewed 2D figures consisting of skewed columnar grids were computed as a function of depicted slant and slant of the picture surface. Computations were based on an assumption of parallelism. Virtual slants were compared...

Virtual slants of obliquely viewed figures consisting of skewed grids, computed as a function of depicted slant and slant of the physical surface, were compared with perceived slants in both monocular and binocular viewing conditions. Computations were based on an assumption of parallelism. Perceived slant was well-correlated with virtual slant, ev...

When searching, people usually scan the visual field by making saccades. From the literature we know that saccade amplitude and fixation duration are related to the amount of information to be processed during fixations. Contrast borders may affect target detectability due to lateral masking. We hypothesize that the oculomotor system uses both targ...

The Necker cube is a famous demonstration of ambiguity in visual perception of 3D shape. Its bistability is attributed to indecisiveness because monocular cues do not allow the observer to infer one particular 3D shape from the 2D image. A remarkable but not appreciated observation is that Necker cubes are bistable during binocular viewing. One wou...

A fundamental question in vision is how 3D perception is inferred from 2D images. Many studies showed that monocular and binocular sources of information (cues) contribute to perceived depth and 3D shape. To test contributions of individual cues, several laboratories have measured perceived slants induced by single and combined, ie disparity and pe...

In monocular vision, the horizontal/vertical aspect ratio (shape) of a frontoparallel rectangle can be based on the comparison of the perceived directions of the rectangle’s edges. In binocular vision of a typical three-dimensional scene (when occlusions are present), this is not the case: Frontoparallel rectangles would be perceived in a distorted...

Pure vergence movements are the eye movements that we make when we change our binocular fixation between targets differing in distance but not in direction relative to the head. Pure vergence is slow and controlled by visual feedback. Saccades are the rapid eye movements that we make between targets differing in direction. Saccades are extremely fa...

Purpose: Recently, we showed that colour illusions in half-occlusions result from a feedback mechanism that reduces binocular colour differences and affects the colour appearances of neighbouring monocular objects. Here we examine whether this mechanism can fully explain the phenomenon of binocular colour mixing. Methods: Stereograms were displayed...

Form and motion are processed along parallel neural streams. Unified visual perception of form and motion requires interaction between these streams of information. How this interaction occurs is an unsolved problem, known as the binding problem. Here we present a stimulus that demonstrates that motion is not directly bound to form. The stimulus co...

Understanding conscious visual perception is one of the main challenges in vision science. Here we developed both an experimental paradigm and a model to study conscious selection of bi-stable percepts in stereoscopic vision. The paradigm exploits a visual stimulus in which we independently manipulated two signals that our brain uses to retrieve th...

We observed that two disparity — defined depth planes were perceived at a single depth when disparity alternated rapidly between two values. We investigated quantitatively how depth of the plane depended on the temporal frequency of disparity. We alternated the two disparity values between 9 Hz and 35 Hz, which is above the limit of stereomotion. W...

Where we look when we scan visual scenes is an old question that continues to inspire both fundamental and applied research. Recently, it has been reported that depth is an important variable in driving eye movements: the directions of spontaneous saccades tend to follow depth gradients, or, equivalently, surface tilts (L. Jansen, S. Onat, & P. Kön...

Monocular vision uses assumptions known as the Gestalt laws of perceptual organization. Additional assumptions about the eye’s point of view and about the layout of the 2D image lead to 3D perception that is surprisingly often veridical. Binocular vision is based on epipolar geometry. In the absence of monocularly recognizable patterns, the binocul...

Monocular vision uses assumptions known as the Gestalt laws of perceptual organization. Additional assumptions about the eye's point of view and about the layout of the 2D image lead to 3D perception that is surprisingly often veridical. Binocular vision is based on epipolar geometry. In the absence of monocularly recognizable patterns, the binocul...

We make eye movements continuously to obtain an understanding of the 3D world around us, but how does the visual system plan these scanning movements of the environment? Here we study what 3D visual information is used to plan saccades. Cue conflict studies have shown that the visual system combines cues in a statistically optimal way for perceptio...

Stereo-vision is generally considered to provide information about depth in a visual scene derived from disparities in the positions of an image on the two eyes; a new study has found evidence that retinal-image coding relative to the head is also important.

Stereopsis is 3-D vision that results from small differences (disparities) between the image patterns formed on the retinas of our eyes. In spite of having two physical vantage points, the two eyes, the general belief is that we judge direction from a hypothetical eye, called the cyclopean eye. In this concept, depth and direction are treated as in...

We investigated the influence of perceived depth on vergence eye movements while binocularly viewing a reverse perspective stimulus. In a reverse perspective stimulus, the perspective depth of a scene painted on a 3D surface strongly conflicts with the physical depth of the 3D surface. Subjects unfamiliar with the reverse perspective stimulus usual...

Binocular rivalry (BR) and dichoptic masking (DM) are often-used tools in the study of conscious perception. In BR two incompatible stimuli in the two eyes compete perceptually; in DM a stimulus to one eye becomes invisible when preceded by a stimulus in the other eye, the optimal delay being about 100ms. Spatial aspects of BR, like differences of...

Voluntary saccades are made to change fixation to another target. Such saccades require engagement of the saccadic system to the new target and disengagement from the old one. Here we study the occurrence of these changes by investigating smooth pursuit preceding and following voluntary saccades. The rationale is that smooth pursuit may reveal the...

In this study, we investigated the effect of changing size on vergence. Erkelens and Regan (1986) proposed that this cue to motion in depth affects vergence in a similar way as it affects perception. The measured effect on vergence was small and we wondered why the vergence system would use changing size as an additional cue to changing disparity....

We investigated the influence of perceived surface slant on saccade size using ambiguous slant rivalry stimuli. In a slant rivalry stimulus, the perspective cue specifies a surface slant oppositely oriented to the surface slant defined by the disparity cue. Observers of such a stimulus perceive one of two mutually exclusive surface slants with oppo...

Binocular rivalry may ensue between stimuli that are presented intermittently and asynchronously, as long as each of the rivalrous patterns are presented at R Q Hz. We call this limit the temporal limit to rivalry. We show that the 3Hz limit is not reached instantaneously. At the beginning of the trial stimuli need to be presented at RT Hz to induc...

When the two eyes are presented with conflicting stimuli, perception starts to fluctuate over time (i.e., binocular rivalry). A similar fluctuation occurs when two patterns are presented to a single eye (i.e., monocular rivalry), or when they are swapped rapidly and repeatedly between the eyes (i.e., stimulus rivalry). Although all these cases lead...

Presenting the eyes with spatially mismatched images causes a phenomenon known as binocular rivalry-a fluctuation of awareness whereby each eye's image alternately determines perception. Binocular rivalry is used to study interocular conflict resolution and the formation of conscious awareness from retinal images. Although the spatial determinants...

We studied the influence of perceived surface orientation on vergence accompanying a saccade while viewing an ambiguous stimulus. We used the slant rivalry stimulus, in which perspective foreshortening and disparity specified opposite surface orientations. This rivalrous configuration induces alternations of perceived surface orientation, while the...

A schematic representation of the motion-only conflict stimuli with continuous presentations. (0.05 MB MPG)

A schematic representation of the Motion and Form conflict stimuli with continuous presentations. (0.06 MB MPG)

Oculomotor behavior contributes importantly to visual search. Saccadic eye movements can direct the fovea to potentially interesting parts of the visual field. Ensuing stable fixations enables the visual system to analyze those parts. The visual system may use fixation duration and saccadic amplitude as optimizers for visual search performance. Her...

A central problem in perception is how to parse a continuous stream of information into meaningful events, resolve ambiguities, and shape awareness. We presented the eyes with temporally modulated incompatible images (gratings) which resulted in frequent perceptual alternations between the images in the two eyes: binocular rivalry. We show that riv...

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...

Human vision is highly sensitive to bilateral symmetry in 2-D images. It is, however, not clear yet whether this visual sensitivity relates to symmetry of 3-D objects or whether it relates to symmetry of the 2-D image itself. We used a stereoscopically presented stimulus and a 3-D bisection task that enable us to dissociate object symmetry from ima...

Global-motion perception is the perception of coherent motion in a noisy motion stimulus. Thresholds for coherent motion perception were measured for different combinations of signal and noise speeds. Previous research [Edwards, M., Badcock, D. R., & Smith, A. T. (1998). Independent speed-tuned global-motion systems. Vision Research, 38 (11), 1573-...

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. Psycho...

Human vision is highly sensitive to bilateral symmetry in 2-D images. It is not clear yet whether our visual sensitivity relates to image symmetry itself or that image symmetry is regarded as a special case of object symmetry. 2-D images are not appropriate stimuli to address this question. We used a bisection task during the viewing of stereograms...

In order to characterize the uniformity of fixation density, we propose aquantitative measure based on Voronoi diagrams, in which cells are defined around fixation locations. We examined how normalized cell size distributions are related to homogeneous and inhomogeneous fixation densities. Two possible measures for use with the Voronoi method are d...

Smooth pursuit and saccades are two components of tracking eye movements. Their coordination has usually been studied by investigating latencies of pursuit onset in response to a moving target appearing simultaneously with the disappearance of the stationary fixation target. The general finding from such studies has been that latencies of saccades...

Bilateral symmetry in binocular vision C J Erkelens, R van Ee (Helmholtz Institute, Utrecht University, Princetonplein 5, NL 3584 CC Utrecht, The Netherlands; e-mail: c.j.erkelens@uu.nl) Human vision is highly sensitive to bilateral symmetry in 2-D images. It is not clear yet whether our visual sensitivity relates to image symmetry itself or that i...

We investigated the influence of temporal frequency on binocular depth perception in dynamic random-dot stereograms (DRS). We used (i) temporally correlated DRS in which a single pair of images alternated between two disparity values, and (ii) temporally uncorrelated DRS consisting of the repeated alternation of two uncorrelated image pairs each ha...

In studies of the temporal flexibility of the stereoscopic system, it has been suggested that two different processes of binocular depth perception could be responsible for the flexibility: tolerance for interocular delays and temporal integration of correlation. None has investigated the relationship between tolerance for delays and temporal integ...

Switching fixation from one target to another requires that eye movement control systems are disengaged from the current target and engaged to the new one. The current opinion is that disengagement precedes engagement and that both neural states are mediated by the process of visual attention. We studied the smooth pursuit eye movements made before...

Rotations of the eye are generated by the torques that the eye muscles apply to the eye. The relationship between eye orientation and the direction of the torques generated by the extraocular muscles is therefore central to any understanding of the control of three-dimensional eye movements of any type. We review the geometrical properties that dic...

We investigated temporal properties of stereopsis at different spatial scales in dynamic random-dot stereograms (DRS) consisting of (i) the repeated presentation of two image pairs (i.e. sustained presentation) and (ii) single presentations of two image pairs (transient presentation). In dense stereograms perception of depth is possible if the leve...

In this paper, the authors investigate whether the idea of independent control of version and vergence eye movements is compatible with the mechanics of the eye plant. By computing the change in the axes of action of the eye muscles as a function of ocular vergence, they prove that, regardless of the muscle pulley locations, the required muscle act...

Search performance can be improved by restricting search to regions relevant to the task: no time is lost then to search in areas that certainly do not contain targets. Cognitive or physical borders can define relevant regions. We examined how the shape of search areas affected saccadic amplitudes and directions. We recorded eye movements during se...

Apparent motion enables us to see motion in movies and on electronic displays such as televisions and computer monitors. The alleged explanation for apparent motion is that neurons early in the visual cortex act as spatiotemporal filters that smooth out discrete displacements, rendering apparent motion indistinguishable from natural motion. Recentl...

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...

Abstract of oral presentation at ECVP 2003 Averaging of motion across short-interval displacements C J Erkelens (Helmholtz Institute, Utrecht University, PO Box 80 000, NL 3508 TA Utrecht, The Netherlands; e-mail: c.j.erkelens@uu.nl) Apparent motion enables us to see motion in movies and on electronic displays such as televisions and computer mon...

When the level of correlation between the images presented to the left and right eye is sufficiently high, we perceive depth during the viewing of random-dot stereograms. When the level of correlation is low, we experience rivalry between the images and no depth. To investigate the interaction between processes that underlie stereopsis and rivalry,...

Measurements of eye movements have shown that centrifugal movements (i.e. away from the primary position) have a lower maximum velocity and a longer duration than centripetal movements (i.e. toward the primary position) of the same size. In 1988 Pelisson proposed that these kinematic differences might be caused by differences in the neural command...

Two different colours, one presented to one eye and the other presented to the other eye, often create the impression of a third colour. This percept is known as binocular colour mixture. Here we use coloured stereograms to study binocular colour appearance. Vivid pastel colours are induced in monocular, achromatic patches, if these are placed in s...

During binocular fixation, the eyes usually point in different directions, and yet, each object is judged to lie in a single direction. It is commonly believed that a particular location in the head serves as the origin for such directional judgments. This location is known as the cyclopean eye. We argue here that observers can judge visually perce...

We examined how much depth we perceive when viewing a depiction of a slanted plane in which binocular disparity and monocular perspective provide different slant information. We exposed observers to a grid stimulus in which the monocular--and binocular-specified grid orientations were varied independently across stimulus presentations. The grids we...

It has been well established that vertical disparity is involved in perception of the three-dimensional layout of a visual scene. The goal of this paper was to examine whether vertical disparities can alter perceived direction. We dissociated the common relationship between vertical disparity and the stimulus direction by applying a vertical magnif...

Stereopsis: The binding of depth to visual directions rather than to patterns C. J. Erkelens, E. Gheorghiu and R. van Ee Helmholtz Institute, Utrecht University, The Netherlands. When the level of correlation between the left and right eye’s images is sufficiently high we perceive depth during the viewing of random-dot stereograms. When the level...

Over E.A.B., Hooge I.T.C. & Erkelens C.J. (2002) A quantitative measure for the spatial distribution of fixations. Perception, 31, S182.

In stereoscopic vision, gaze angles are used for perceiving both direction and depth. In depth perception, it is known that gaze angles are estimated from both eye-position signals and vertical disparities. Here, we investigated the role of vertical disparity in perceiving direction. We dissociated the common relationship between the vertical dispa...

There is a variety of stereoscopic phenomena in which perceived surface slant is not what one would predict from the presented binocular disparities. Our subjects estimated perceived slant produced by anaglyphically presented grids in which disparity-specified and perspective-specified slants were conflicting. We were specifically interested in con...

The purpose of the present study was to investigate whether adaptation can occur to disparity per se. The adapting stimuli were large random-dot patterns of which the two half-images were transformed such that the depth effects induced by the vertical transformations were nulled by horizontal transformations. Thus, the adapting stimuli were percept...

A novel type of dynamic random-dot stereogram (DRS) was used to study vergence movements and depth detection in response to temporal modulations of interocular correlation. Each DRS consisted of the repeated presentation of a pair of correlated images alternated by the presentation of a pair of uncorrelated images. The intervals of high (T(c)) and...

The goal of the present study is to compare the strengths of depth effects induced by different types of vertical disparity. We use a nulling task, in which the depth effects induced by vertical disparity are nulled by horizontal disparity. The advantage of this method is that it prevents cue conflicts from arising between disparity and other depth...

In monocular vision, the horizontal/vertical aspect ratio (shape) of a frontoparallel rectangle can be based on the comparison of the perceived directions of the rectangle's edges. In binocular vision of a typical three-dimensional scene (when occlusions are present), this is not the case: Frontoparallel rectangles would be perceived in a distorted...

Perceived visual directions are derived from combining retinal signals and oculomotor signals. Up to now the general belief is that the oculomotor signals of the two eyes are first pooled before they become available for perception of depth and direction. In this sense the eyes are believed to act together as one unit known as the cyclopean eye. Th...

By means of intramuscular electromyographic recordings, we studied the firing frequencies and recruitment/decruitment thresholds of individual motor units in two elbow flexors, the biarticular biceps brachii muscle and the monoarticular brachioradialis muscle. Subjects had to perform isometric contractions with increasing elbow flexion torque until...

Experiments were performed on human elbow flexor and extensor muscles and jaw-opening and -closing muscles to observe the effect on rhythmic movements of sudden loading. The load was provided by an electromagnetic device, which simulated the appearance of a smoothly increasing spring-like load. The responses to this loading were compared in jaw and...

Bi-manual movement tasks present us with an intriguing paradox: on one hand it is clear that we are capable of controlling our hands independently, yet at the same time we often notice that one hand influences the other. We investigated bi-manual pointing movement towards a common target and hypothesised that if (parts of) bi-manual movements are p...

The present study concerns the dynamics of multiple fixation search. We tried to gain insight into: (1) how the peripheral and foveal stimulus affect fixation duration; and (2) how fixation duration affects the peripheral target selection for saccades. We replicated the non-corroborating results of Luria and Strauss (1975) ('Eye movements during se...