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Biomedical subjects

S F te Pas

Publications and source records attributed to S F te Pas.

6 recordsLinked to original sources
PubMed2001

First-order structure induces the 3-D curvature contrast effect.

A 3-D curvature contrast effect has been reported in shading-and-texture-defined (Curran & Johnson (1996). Vision Research 36, 3641-3653) and in stereoscopically defined (te Pas, Rogers, & Ledgeway (2000). Current Psychology Letters: Brain, Behaviour and Cognition 1, 117-126) stimuli. Our experiments show that a clear 3-D curvature contrast effect also occurs in motion-defined stimuli. The magnitude of the effect is similar in motion-, stereo- and shading-and-texture defined stimuli, suggesting that the 3-D curvature contrast effect is shape-based. We find a distinct contrast effect that is similar in the case of inducers that contain second-order (curvature) information and in the case of inducers that contain only first-order (slant and tilt) information. The effect with inducers that contain only zeroth-order (depth) information is very small. We conclude that the first-order structure is sufficient to induce a 3-D contrast effect.

Depth Perception↗
PubMed1997

Detection of second-order structure in optical flow fields.

We investigate how well the direction of the expansion gradient and the direction of a spatial step in expansion can be extracted from a complex optical flow field. The expansion gradient is an invariant of the second-order structure under rotations of the image plane. Our stimuli consist of sparse random dot patterns in a circular window with a diameter of 20 degrees. We find that translational velocity masks the direction of both the expansion gradient and the step in expansion. Thresholds for both the expansion gradient and the step in expansion are found to be similar for a wide range of expansions. We introduce two simple models that use the thresholds for the step in expansion to predict thresholds for the expansion gradient. Results of these models are comparable with the results of our subjects, showing that we do not need a hypothetical separate mechanism for the detection of second-order structure to explain the data.

Female↗
PubMed1996

Simulating the detection of first-order optical flow components.

Thresholds for the detection of rotation and divergence in the presence of a translational component in sparse random dot patterns are determined for human observers and two computer algorithms. The algorithms only make use of local velocity directions and not of local velocity magnitude (speed). The results show that psychophysical performance in this task can be well described without the need of specialized mechanisms tuned to either rotation or divergence. Possibly, integration of information over more than two frames occurs for low velocities. For high velocities the correspondence problem seems to limit performance.

Algorithms↗
PubMed1996

Detection of the sign of expansion as a function of field size and eccentricity.

Small optical flow fields are involved in object and shape recognition. These tasks depend on high resolution and low speed of the flow-field elements. Large optical flow fields are involved in tasks such as orientation and navigation, which require flow-field elements to move at high speed. From the above, we would expect to find different behavior of subjects for different parameter ranges of field size and speed of the elements. In this paper, we address the question of whether such different behavior exists for a task that is associated neither with object recognition nor with navigation. We obtained detection thresholds for expansion in the presence of translation for a wide range of field sizes. The same paradigm was used to investigate whether subjects made efficient use of peripheral information. We found viewing-distance invariance, meaning that subjects' performance scaled with stimulus size. Subjects performed very similarly with and without foveal information.

Adult↗
PubMed1994

Haptic discrimination of doubly curved surfaces.

Active haptic discrimination of mathematically well-defined surfaces was investigated. The quadric surfaces were defined in terms of 'shape index'--a quantity describing shape--and 'curvedness'--a quantity describing overall curvature. In these experiments shape index was varied and curvedness kept constant. The influence of laterality (unilateral versus bilateral discrimination) was tested in separate sessions. No influence of shape on the discrimination of solid objects was found. From the results an estimate can be made of the just-noticeable difference in terms of shape-index units. There was a significant effect of laterality on discrimination: performance with unilateral (successive) examination (both with left and with right hand) was better than with bilateral (simultaneous) examination.

Form Perception↗