Biomedical subjects

Karl Verfaillie

Publications and source records attributed to Karl Verfaillie.

7 recordsLinked to original sources

PubMed2002

Detection of intrasaccadic displacements and depth rotations of moving objects.

In a display with a stationary and a moving object, subjects saccaded towards one of the objects and had to detect intrasaccadic changes in position or orientation of either the saccade target or the saccade flanker. Compared to performance for stationary objects, displacement detection for translating objects was better and unaffected by saccadic status of the changed object. This pattern proved to be specific to position changes in translating objects and did not generalize to other types of motion (i.e., rotation) or to other types of intrasaccadic changes (i.e., orientation shifts). Superior transsaccadic coding of the position of a translating object was also observed in control experiments with only a single object present on each trial. Possible accounts in terms of selective attention to moving objects and perceptual relevance of object position are pitted against the data, suggesting qualitative differences in the transsaccadic representation of translating and stationary objects.

Depth Perception↗

PubMed2002

Transsaccadic perception of translating objects: effects of landmark objects and visual field position.

Previously Gysen, De Graef, and Verfaillie [Vision Research 42 (2002) 379] showed that, with stimulus displays presenting one stationary and one translating object, sensitivity for intrasaccadic displacements was higher for translating than for stationary objects. In the present paper the importance of the relative encoding of the path of the translating object towards the stationary object is investigated. In three experiments we compared detection of intrasaccadic displacements of translating objects in relative motion (moving towards the landmark object) and translating objects moving in isolation. No 'facilitatory' effect of relative motion was found. However a visual field effect was present. Performance was always better for the translating object presented in the lower part in comparison to the upper part of the visual field. A fourth experiment investigated the sensitivity for intrasaccadic displacements of stationary and translating objects presented in the upper as well as in the lower visual field. A lower visual field advantage was observed. The superior performance for translating objects, as was found previously, was confirmed in the lower and upper visual field.

Adult↗

PubMed2002

The effect of stimulus blanking on the detection of intrasaccadic displacements of translating objects.

In a display with a stationary and a translating object, subjects made a saccade towards one of the objects and had to detect intrasaccadic changes in the position of either the saccade target or the saccade flanker. Sensitivity for displacements of the stationary and moving objects was measured in conditions with (60 and 220 ms) and without blanking. In the conditions without blanking, displacement detection for translating objects was better than detection for stationary objects, which confirmed previous results (Vis. Res. 42 (2002) 379). This pattern was reversed in the blanking conditions: Sensitivity for intrasaccadic displacements of the translating object decreased drastically in comparison to conditions without a blank and was even lower than sensitivity for the stationary object. The results suggest differences in the transsaccadic spatial representation of translating and stationary objects. While a change in the spatial position of a stationary object can be detected after a blank period of 60 and 220 ms, this seems impossible for a translating object, indicating timing differences in postsaccadic spatial localization processes. Accounts in terms of a fast and accurate motion processing mechanism that possibly makes use of gain control are discussed.

Analysis of Variance↗

PubMed2002

Transsaccadic memory for visual object detail.

When we move our eyes around in real-world scenes, we typically have several peripheral previews of an object before we direct our eyes straight at the object. Numerous studies on transsaccadic memory have investigated whether there is any evidence for the integration of peripheral object information acquired presaccadically with foveal object information acquired postsaccadically. We review this evidence to illustrate the currently dominant view that transsaccadic object memory is sparse and contains little visual object detail. However, based on some recent studies of the role of postsaccadic stimulus blanking in transsaccadic change detection, we hypothesize that transsaccadic object memory involves the automatic emergence of a visual analog: a high-capacity, non-selective, internal representation of visual object detail. This hypothesis is tested by examining cued detection of intrasaccadic changes in the in-depth orientation of objects in scenes. The data provide preliminary support for the presence of the visual analog, but also show that its functionality is strictly limited by attentional and temporal constraints on the process of reading out information from the visual analog.

Cues↗

PubMed2002

Creating stimuli for the study of biological-motion perception.

In the perception of biological motion, the stimulus information is confined to a small number of lights attached to the major joints of a moving person. Despite this drastic degradation of the stimulus information, the human visual apparatus organizes the swarm of moving dots into a vivid percept of a moving biological creature. Several techniques have been proposed to create point-light stimuli: placing dots at strategic locations on photographs or films, video recording a person with markers attached to the body, computer animation based on artificial synthesis, and computer animation based on motion-capture data. A description is given of the technique we are currently using in our laboratory to produce animated point-light figures. The technique is based on a combination of motion capture and three-dimensional animation software (Character Studio, Autodesk, Inc., 1998). Some of the advantages of our approach are that the same actions can be shown from any viewpoint, that point-light versions, as well as versions with a full-fleshed character, can be created of the same actions, and that point lights can indicate the center of a joint (thereby eliminating several disadvantages associated with other techniques).

Biology↗

PubMed2002

Transsaccadic perception of saccade target and flanker objects.

To account for location-dependent and location-independent preview benefits in transsaccadic object perception, J. M. Henderson (1994) and J. M. Henderson and M. D. Anes (1994) proposed a dual-route model in which both episodic object representations and long-term memory representations store information across a saccade. Four experiments are reported in which the dual-route model was assessed. Preview benefits for saccade target objects were found to be location independent, whereas preview benefits for flanker objects were location dependent. These results support a single-route, 2-stage model of transsaccadic object perception. First, preattentive object files are set up to parse a set of attentional and/or saccade targets from peripheral vision, causing location-dependent preview benefits. Second, 1 object is attentionally selected for further processing, activating long-term memory representations and resulting in location-independent preview benefits.

Attention↗

PubMed1991

Representational momentum and event course anticipation in the perception of implied periodical motions.

After viewing an object in an implied rotation, subjects' short-term visual memory for the object's position is distorted in the direction of rotation. Previous accounts of this representational momentum effect have emphasized the analogy to physical momentum. This study provides a more general perspective: Position memory is influenced by anticipatory processes related to the future event course. In Experiment 1, subjects are presented with an implied periodical event in which a rectangle rotates back and forth. When a direction change in the implied rotation can be anticipated, memory distortion size drops back to zero. Experiment 2 rejects an alternative explanation for the findings of Experiment 1 in terms of enhanced position memory caused by repeated presentations of the memory pattern orientation within the same trial. In Experiment 3, the periods of the implied event are marked by changes in velocity rather than direction. The anticipation of a sudden velocity increase leads to a larger memory shift. We conclude that the perceptual system anticipates the event course on the basis of a representation of the higher order event structure rather than the local motion characteristics.

Adult↗