Biomedical subjects

Karl Verfaillie

Publications and source records attributed to Karl Verfaillie.

12 recordsLinked to original sources

PubMed2006

Perceiving depth in point-light actions.

The present study investigates how observers assign depth in point-light figures, by manipulating spatiotemporal characteristics of the stimuli. Previous research on the perception of point-light walkers revealed bistability (i.e., that a point-light walker is perceived as either facing the viewer or facing away from the viewer) and the presence of a perceptual bias (i.e., a tendency to perceive the figure as facing the viewer). Here, we study the generality of these phenomena by having observers indicate the global depth orientation of different ambiguous point-light actions. Results demonstrate bistability for all actions, but the presence of a preferred interpretation depends strongly on the performed action, showing that the process of depth assignment takes into account the movements the point-light figure performs. Two additional experiments, using unfamiliar movement patterns without strong semantic correlates, show that purely kinematic aspects of a naction also strongly affect d epth assignment. Together, the results reveal the perception of depth in point-light figures to be a flexible processinvolving both bottom-up and top-down components.

Depth Perception↗

PubMed2005

The relationship of visual extinction to luminance-contrast imbalances between left and right hemifield stimuli.

Visual extinction was investigated in six right brain-damaged patients with left visual neglect, using a psychophysical paradigm. Orientation discrimination thresholds were determined for both left and right hemifield gratings presented either in isolation or simultaneously with a contralateral distractor grating. To minimize the influence of possible sensory-perceptual deficits, the luminances of both target and distractor gratings were chosen to be 20 times the luminances necessary to discriminate between horizontal and vertical grating orientations. When the visibility of target and distractor gratings was subjectively equalized in this way, neglect patients still showed a significant extinction effect, i.e. a significant interference of the right hemifield distractor with left hemifield orientation sensitivity. By manipulating the luminances of left and right hemifield gratings during bilateral simultaneous stimulus presentation, we demonstrated the role of luminance-contrast imbalances in eliciting visual extinction. Both decreasing the right distractor luminance and increasing the left target stimulus luminance resulted in an elimination of the observed extinction effects. These results show that not the absolute salience of one of two simultaneously presented stimuli, but the relative salience of both stimuli, is the crucial factor for inducing extinction.

Attention↗

PubMed2005

A psychophysical study of visual extinction: ipsilesional distractor interference with contralesional orientation thresholds in visual hemineglect patients.

Visual extinction was investigated in left (n=15) and right (n=25) brain-damaged patients with or without visual neglect, and in normal control subjects (n=14), using a psychophysical paradigm. Orientation discrimination thresholds were determined for both left and right hemifield gratings presented either in isolation or simultaneously with a contralateral distractor grating. To minimize the influence of possible sensory-perceptual deficits, the luminances of both target and distractor gratings were chosen to be 20 times the luminances necessary to discriminate between horizontal and vertical grating orientations. The location of the target grating was always cued, making the distractor grating task irrelevant. Even after equalizing the visibility of left and right hemifield stimuli, neglect patients still displayed an increased interference effect from an ipsilesional distractor (and no interference from a contralesional distractor). Left or right brain-damaged controls did not show this asymmetric interference of irrelevant distractors, even the patients who demonstrated extinction on standard extinction testing. This suggests that visual extinction is a critical component of the visual neglect syndrome and that it involves an attentional deficit.

Adult↗

PubMed2004

Bistability and biasing effects in the perception of ambiguous point-light walkers.

The perceptually bistable character of point-light walkers has been examined in three experiments. A point-light figure without explicit depth cues constitutes a perfectly ambiguous stimulus: from all viewpoints, multiple interpretations are possible concerning the depth orientation of the figure. In the first experiment, it is shown that non-lateral views of the walker are indeed interpreted in two orientations, either as facing towards the viewer or as facing away from the viewer, but that the interpretation in which the walker is oriented towards the viewer is reported more frequently. In the second experiment the point-light figure was walking backwards, making the global orientation of the point-light figure opposite to the direction of global motion. The interpretation in which the walker was facing the viewer was again reported more frequently. The robustness of these findings was examined in the final experiment, in which the effects of disambiguating the stimulus by introducing a local depth cue (occlusion) or a more global depth cue (applying perspective projection) were explored.

Adult↗

PubMed2004

Perception of biological motion: a stimulus set of human point-light actions.

We present a set of stimuli representing human actions under point-light conditions, as seen from different viewpoints. The set contains 22 fairly short, well-delineated, and visually "loopable" actions. For each action, we provide movie files from five different viewpoints as well as a text file with the three spatial coordinates of the point lights, allowing researchers to construct customized versions. The full set of stimuli may be downloaded from www.psychonomic.org/archive/.

Biology↗

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↗