Jan Vanrie

Specificity of regions processing biological motion

Using functional magnetic resonance imaging and point light displays portraying six different human actions, we were able to show that several visual cortical regions, including human MT/V5 complex, posterior inferior temporal gyrus and superior temporal sulcus, are differentially active in the subtraction comparing biological motion to scrambled motion. Comparison of biological motion to three‐dimensional rotation (of a human figure), articulated motion and translation suggests that human superior temporal sulcus activity reflects the action portrayed in the biological motion stimuli, whereas posterior inferior temporal gyrus responds to the figure and hMT/V5+ to the complex motion pattern present in biological motion stimuli. These results were confirmed with implied action stimuli.

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 fromwww.psychonomic.org/archive/.

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.

Mental rotation versus invariant features in object perception from different viewpoints: an fMRI study

It has been proposed that object perception can proceed through different routes, which can be situated on a continuum ranging from complete viewpoint-dependency to complete viewpoint-independency, depending on the objects and the task at hand. Although these different routes have been extensively demonstrated on the behavioral level, the corresponding distinction in the underlying neural substrate has not received the same attention. Our goal was to disentangle, on the behavioral and the neurofunctional level, a process associated with extreme viewpoint-dependency, i.e. mental rotation, and a process associated with extreme viewpoint-independency, i.e. the use of viewpoint-invariant, diagnostic features. Two sets of 3-D block figures were created that either differed in handedness (original versus mirrored) or in the angles joining the block components (orthogonal versus skewed). Behavioral measures on a same-different judgment task were predicted to be dependent on viewpoint in the rotation condition (same versus mirrored), but not in the invariance condition (same angles versus different angles). Six subjects participated in an fMRI experiment while presented with both conditions in alternating blocks. Both reaction times and accuracy confirmed the predicted dissociation between the two conditions. Neurofunctional results indicate that all cortical areas activated in the invariance condition were also activated in the rotation condition. Parietal areas were more activated than occipito-temporal areas in the rotation condition, while this pattern was reversed in the invariance condition. Furthermore, some areas were activated uniquely by the rotation condition, probably reflecting the additional processes apparent in the behavioral response patterns.

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).

Multiple Routes to Object Matching from Different Viewpoints: Mental Rotation versus Invariant Features

Previous research has shown that object recognition from different viewpoints often yields strong effects of viewpoint. However, for some objects and experimental paradigms almost complete viewpoint invariance is obtained. This suggests the existence of multiple routes to object recognition. In this study we further strengthen this notion by designing two different conditions using the same experimental paradigm (simultaneous matching) and highly similar objects (multiblock figures). In the first condition (involving a handedness violation), strong effects of viewpoint were obtained. In the second condition (involving an invariance violation), the effects of viewpoint were negligible. This result illustrates that asking under what circumstances object recognition is viewpoint dependent or independent is more fruitful than attempting to show that object recognition is either viewpoint dependent or independent.

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 an action also strongly affect depth assignment. Together, the results reveal the perception of depth in point-light figures to be a flexible process involving both bottom-up and top-down components.

Additional road markings as an indication of speed limits: Results of a field experiment and a driving simulator study

Although speed limits are indicated by road signs, road users are not always aware, while driving, of the actual speed limit on a given road segment. The Roads and Traffic Agency developed additional road markings in order to support driver decisions on speed on 70 km/h roads in Flanders-Belgium. In this paper the results are presented of two evaluation studies, both a field study and a simulator study, on the effects of the additional road markings on speed behaviour. The results of the field study showed no substantial effect of the markings on speed behaviour. Neither did the simulator study, with slightly different stimuli. Nevertheless an effect on lateral position was noticed in the simulator study, showing at least some effect of the markings. The role of conspicuity of design elements and expectations towards traffic environments is discussed. Both studies illustrate well some strengths and weaknesses of observational field studies compared to experimental simulator studies.

Perceptual coupling of multiple point-light figures

When confronted with multiple bistable stimuli at the same time, the visual system tends to generate a common interpretation for all stimuli. We exploit this perceptual-coupling phenomenon to investigate the perception of depth in bistable point-light figures. Observers indicate the global depth orientation of simultaneously presented point-light figures while the similarity between the stimuli is manipulated. In a first experiment, a higher occurrence of coupled percepts is found for identical figures, but coupling breaks down when either the movement pattern or both the viewpoint and the phase-relation are changed. A second experiment confirms these results, but also demonstrates that different point-light actions can be subject to perceptual coupling as long as they share the same viewpoint and exhibit equivalent degrees of perceptual ambiguity. The data are consistent with an explanation in terms of differential contributions of stored, view-dependent object and action representations and of an interaction between stages processing local stimulus features. The consequences of these results are discussed in the framework of an explicit model for the perception of depth in biological motion.

Effects of spatial separation between stimuli in whole report from brief visual displays.

Direct measurements of the effects of spatial separation between stimuli in whole report from brief visual displays are reported. The stimuli were presented on the periphery of an imaginary circle centered on fixation. In Experiment 1, each display showed two capital letters (letter height ≅ 1.3°, width ≅ 0.9°, eccentricity ≅5.5°). The proportion of correctly reported letters was a strictly increasing, decelerating function of the spatial separation between the letters for center-to-center separations ranging from less than 2° to more than 10° of visual angle. Experiment 2 yielded similar results with triples of letters. Experiment 3 showed that accuracy increased with spatial separation for report of two short words, and Experiment 4 showed the same result for words presented upside down. The results are explained by a model of lateral masking (crowding) based on competitive interactions within receptive fields of cortical neurons.