Biomedical subjects

W J van Damme

Publications and source records attributed to W J van Damme.

6 recordsLinked to original sources

PubMed1999

Perceived distance, shape and size.

If distance, shape and size are judged independently from the retinal and extra-retinal information at hand, different kinds of information can be expected to dominate each judgement, so that errors in one judgement need not be consistent with errors in other judgements. In order to evaluate how independent these three judgments are, we examined how adding information that improves one judgement influences the others. Subjects adjusted the size and the global shape of a computer-simulated ellipsoid to match a tennis ball. They then indicated manually where they judged the simulated ball to be. Adding information about distance improved the three judgements in a consistent manner, demonstrating that a considerable part of the errors in all three judgements were due to misestimating the distance. Adding information about shape that is independent of distance improved subjects' judgements of shape, but did not influence the set size or the manually indicated distance. Thus, subjects ignored conflicts between the cues when judging the shape, rather than using the conflicts to improve their estimate of the ellipsoid's distance. We conclude that the judgements are quite independent, in the sense that no attempt is made to attain consistency, but that they do rely on some common measures, such as that of distance.

Cues↗

PubMed1998

Judging distance from ocular convergence.

Subjects misjudge distances considerably when forced to rely on extra-retinal information. Nevertheless, they can reproducibly set a target to the same distance as a reference, or to double or half that distance, even when they have to look back and forth between them because they are prevented from seeing one when looking at the other. Our explanation for this apparent discrepancy is that people have access to reasonably accurate extra-retinal information on changes in ocular convergence, but can only use this information to judge distances if they had reliable information about the orientation of the eyes before the convergence changed.

Convergence, Ocular↗

PubMed1997

Holding an object one is looking at: kinesthetic information on the object's distance does not improve visual judgments of its size.

Visual judgements of distance are often inaccurate. Nevertheless, information on distance must be procured if retinal image size is to be used to judge an object's dimensions. In the present study, we examined whether kinesthetic information about an object's distance--based on the posture of the arm and hand when holding it--influences the object's perceived size. Subjects were presented with a computer simulation of a cube. This cube's position was coupled to that of a rod in the subject's hand. Its size was varied between presentations. Subjects had to judge whether the cube they saw was larger than, smaller than, or the same size as a reference. On some presentations, a small difference was introduced between the positions of the rod and of the simulated cube. When the simulated cube was slightly closer than the rod, subjects judged the cube to be larger. When it was farther away, they judged it to be smaller. We show that these changes in perceived size are due to alterations in the cube's distance from the subject rather than to kinesthetic information.

Distance Perception↗

PubMed1996

Non-visual information in structure-from-motion.

We examined whether non-visual signals improve visual perception of three-dimensional structure-from-motion. Observers discriminated curvature in quadratic surfaces defined by random dot cinematograms with limited lifetime. They either explored visually a static surface by making head movements that were fed back to the display (HM condition) or they viewed statically the same surface which now rotated (NHM condition). Both conditions showed a clear build-up of performance as lifetime increases, but with different time constants for the HM and NHM condition. A second experiment showed that these differences could not be caused by differences in motion detection for the HM and NHM conditions. We suggest that non-visual information is combined with visual information at a high stage of visual processing, and that it does not mainly serve as input for a retinal stabilization process.

Depth Perception↗

PubMed1994

Discrimination of 3-D shape and 3-D curvature from motion in active vision.

We examined the ability of human observers to discriminate between different 3-D quadratic surfaces defined by motion, and with head position fed back to the stimulus to provide an up-to-date dynamical perspective view. We tested whether 3-D shape or 3-D curvature would affect discrimination performance. It appeared that discrimination of 3-D quadratic shape clearly depended on shape but not on the amount of curvature. Even when the amount of curvature was randomized, subjects' performance was not altered. On the other hand, the discrimination of 3-D curvature clearly depended linearly on curvature with Weber fractions of 20% on the average and, to a small degree, on 3-D shape. The experiment shows that observers can easily separate 3-D shape and 3-D curvature, and that Koenderink's shape index and curvedness provide a convenient way to specify shape. These results warn us against using just any arbitrary 3-D shape in 3-D shape perception tasks and indicate, for example, that emphasizing 3-D shape in computer displays by exaggerating curvature does not have any effect.

Discrimination Learning↗

PubMed1993

Active vision and the identification of three-dimensional shape.

We examined the performance of human observers to identify three-dimensional (3D) shape from motion induced by exploratory head movements. Subjects categorized 3D quadratic surfaces with randomly chosen shape but with a fixed amount of curvature in one of eight shape categories. Human observers had their best performance with convex parabolic and concave parabolic shapes, but had more difficulties in identifying the hyperbolic shapes. The identification of 3D shape was not significantly influenced by the amount of curvature. This means that our description of 3D shape closely matches the intuitive notion of shape, and that its use in 3D shape perception tasks is justified.

Form Perception↗