Biomedical subjects

Casper J Erkelens

Publications and source records attributed to Casper J Erkelens.

14 recordsLinked to original sources

PubMed2006

A single motion system suffices for global-motion perception.

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), 1573-1580; Khuu, S. K., & Badcock, D. R. (2002). Global speed processing: evidence for local averaging within, but not across two speed ranges. Vision Research, 42 (28), 3031-3042.] showed that thresholds were elevated when signal and noise speeds were similar, but not when they were different. The regions of increased threshold values for low and high signal speeds showed little overlap. On the basis of this evidence two independent speed-tuned systems were proposed: one for slow and one for fast-motion. However, in those studies only two signal speeds were used. We expanded the results by measuring threshold-curves for four different signal speeds. Considerable overlap of the threshold-curves was found between conditions. These results speak against a bipartite global-motion system. Model simulations indicate that present and previous experimental results can be produced by a single motion system providing that the mechanisms within it are speed-tuned.

Humans↗

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↗

PubMed2006

A quantitative measure for the uniformity of fixation density: The Voronoi method.

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 discussed. Both show good correlation with subjective visual evaluations of the uniformity of fixation densities. Not only are these measures objective and quantitative, they also have a simple intuitive meaning: They may be thought of as reflecting the clustering of fixations.

Eye Movements↗

PubMed2005

Coordination of smooth pursuit and saccades.

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 and pursuit are different and reflect independent processes. We discuss several limitations of the used targets. In this paper, we study latencies of saccades and smooth pursuit in response to a moving target that overlaps in time with a pursued moving target. We find that saccades and pursuit changes are synchronized. Furthermore, pursuit changes are made fast. Directional changes occur almost entirely within the accompanying saccade. To explain the results we hypothesize a two-stage mechanism for the coordinated generation of saccades and pursuit.

Adult↗

PubMed2005

Differences in perceived depth for temporally correlated and uncorrelated dynamic random-dot stereograms.

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 having one of two disparity values. Our results show that disparity-defined depth is judged differently in temporally correlated and temporally uncorrelated DRS above a temporal frequency of about 3 Hz. The results and simulations indicate that (i) above about 20 Hz, the complete absence of stereomotion is caused by temporal integration of luminance, (ii) the difference in perceived depth in temporally correlated and temporally uncorrelated DRS for temporal frequencies between 20 and 3 Hz, is caused by temporal integration of disparity.

Convergence, Ocular↗

PubMed2005

Temporal properties of disparity processing revealed by dynamic random-dot stereograms.

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 integration mechanisms and none has revealed which mechanism is responsible for depth perception in dynamic random-dot stereograms. We address these questions in the present study. Across five experiments, we investigated the temporal properties of stereopsis by varying interocular correlation as a function of time in controlled ways. We presented different types of dynamic random-dot stereograms, each consisting of two pairs of alternating random-dot patterns. Our experimental results demonstrate that (i) disparities from simultaneous monocular inputs dominate those from interocular delayed inputs; (ii) stereopsis is limited by temporal properties of monocular luminance mechanisms; and (iii) depth perception in dynamic random-dot stereograms results from cross-correlation-like operation on two simultaneous monocular inputs that represent the retinal images after having been subjected to a process of monocular temporal integration of luminance.

Adult↗

PubMed2004

Properties of 3D rotations and their relation to eye movement control.

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 dictate the relationship between muscle pulling direction and 3D eye orientation. We then show how this relation can be used to test the validity of oculomotor control hypotheses. We test the common modeling assumption that the extraocular muscle pairs can be treated as single bidirectional muscles. Finally, we investigate the consequences of assuming fixed muscle pulley locations when modeling the control of eye movements.

Animals↗

PubMed2004

Spatial-scale interaction in human stereoscopic vision in response to sustained and transient stimuli.

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 levels of interocular correlation are above a certain threshold. Therefore, detection of correlation is an important step in stereopsis. Across different experiments, we varied the level of interocular correlation as a function of time by alternating correlated and un/anticorrelated random-dot stereograms. Also, we varied their spatial scale (coarse, fine and alternating between coarse and fine). The presence of stereopsis was examined using a forced-choice depth detection task. For both sustained and transient presentations of the stimuli, the results show that (i) stereopsis has similar temporal properties at coarse and fine spatial scales; (ii) interaction between spatial scales depends on their relative sizes. The results indicate a strong inhibitory influence of rivalry at a coarse scale on stereopsis at a fine scale, and just a weak inhibitory influence of rivalry at a fine scale on stereopsis at a coarse scale. This study provides experimental evidence for a hierarchical organisation of spatial scales in human stereoscopic vision based on neural interaction instead of vergence eye movements.

Adult↗

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↗

PubMed2002

The role of the cyclopean eye in vision: sometimes inappropriate, always irrelevant.

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 perceived directions from angular information alone, and do not require positional information supplied by a cyclopean eye. We show that experimental findings reported as evidence for the cyclopean concept can also be explained solely by angular information without the need for a cyclopean eye. Recent findings concerning binocular shape perception and the cyclopean illusion demonstrate that binocular perception is incompatible with vision from a single vantage point. The concept of the cyclopean eye is sometimes inappropriate and always irrelevant as far as vision is concerned.

Depth Perception↗

PubMed2002

Multi-coloured stereograms unveil two binocular colour mechanisms in human vision.

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 stereograms whose left and right images differ in colour. The build-up of the colours is slow and takes tens of seconds or even minutes in certain individuals. The induced colours remain visible during monocular viewing of the patch and decay gradually. The same colours are induced irrespective of whether the patches are placed in fusible or rivalrous stereograms. We show that these colour effects cannot be induced by monocular colour mechanisms, either alone or in combination with binocular colour mixing. We suggest that the colours are induced by a binocular feedback mechanism, which reduces colour differences between the colour appearances of two monocular images. Induced colours are not observed if the achromatic patches are binocular. However, induced colours are apparent if one switches to monocular viewing after prolonged binocular viewing of the binocular patches. This aftereffect suggests that binocular colour induction acts on the monocular representations of binocular images. We suggest that during binocular viewing the fast process of binocular colour mixing masks the changes in colour appearance produced by the much slower process of binocular colour induction.

Adult↗

PubMed2002

Cause of kinematic differences during centrifugal and centripetal saccades.

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 signals, oculomotor mechanics or a combination of the two. By using the result of muscle force measurements that were made in recent years (Orbit 1.8 Gaze mechanics simulation, Eidactics, San Francisco, 1999) we simulated the muscle forces during centrifugal and centripetal saccades. Based on these simulations we show that the cause of the kinematic differences between the centrifugal and centripetal saccades is the non-linear force-velocity relationship (i.e. muscle viscosity) of the muscles.

Biomechanical Phenomena↗

PubMed2002

Vertical disparity can alter perceived direction.

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 magnification to the image presented to one eye. We used a staircase paradigm to measure whether perceived straight-ahead depended on the amount of vertical magnification in the stimulus. Subjects judged whether a test dot was flashed to either the left or the right side of straight-ahead. We found that perceived straight-ahead did indeed depend on the amount of vertical magnification but only after subjects adapted (for 5 min) to vertical scale (and only in five out of nine subjects). We argue that vertical disparity is a factor in the calibration of the relationship between eye-position signals and perceived direction.

Adaptation, Physiological↗

PubMed2002

Bi-stability in perceived slant when binocular disparity and monocular perspective specify different slants.

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 were slanted about the vertical axis and observers estimated the slant relative to the frontal plane. We were particularly interested in the metrical aspects of perceived slant for a broad spectrum of possible combinations of disparity--and perspective-specified slants. We found that observers perceived only one grid orientation when the two specified orientations were similar. More interestingly, when the monocular--and binocular-specified orientations were rather different, observers experienced perceptual bi-stability (they were able to select either a perspective--or a disparity-dominated percept).

Depth Perception↗