Karl Verfaillie

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/.

Watching Subtitled Television Automatic Reading Behavior

When foreign movies are subtitled in the local language, reading subtitles is more or less obligatory. Our previous studies have shown that knowledge of the foreign language or switching off the sound track does not affect the total time spent in the subtitled area. Long-standing familiarity with subtitled movies and processing efficiency have been suggested as explanations. Their effects were tested by comparing American and Dutch-speaking subjects who differ in terms of subtitling familiarity. In Experiment 1, American subjects watched an American movie with English subtitles. Despite their lack of familiarity with subtitles, they spent considerable time in the subtitled area. Accordingly, subtitle reading cannot be due to habit formation from long-term experience. In Experiment 2, a movie in Dutch with Dutch subtitles was shown to Dutch-speaking subjects. They also looked extensively at the subtitles, suggesting that reading subtitles is preferred because of efficiency in following and understanding the movie. However, the same findings can also be explained by the more dominant processing of the visual modality. The proportion of time spent reading subtitles is consistently larger with two-line subtitles than with one-line subtitles. Two explanations are provided for the differences in watching one- and two-line subtitles: (a) the length expectation effect on switching attention between picture and text and (b) the presence of lateral interference within two lines of text.

Representing and anticipating human actions in vision

Visual perception of human actions was investigated in a long-term priming experiment. On each trial in the priming phase, participants were presented with a pair of short motion animations showing a person performing a particular whole body movement and had to decide whether the actions were the same or different. In the testing block, the same observers had to determine whether static pictures of a human model showed a possible or an impossible body pose. For each test posture, a subject either (1) had seen a priming animation in which the actor would have reached the test posture if the animation had lasted longer, (2) had seen an animation in which the actor would have been in the test posture if the animation had started earlier, or (3) had not seen a related priming animation. Reliable facilitatory priming effects were obtained only when the test posture was preceded by a motion sequence that would have resulted in the test posture if the human model had continued the action. The findings support the hypothesis that, when confronted with a human action, observers anticipate the future posture of the actor and that this anticipation facilitates later identification of the posture.

Transsaccadic integration of biological motion.

In a transsaccadic integration paradigm, Ss had to detect saccade-contingent changes in a moving point-light walker. First, the nature of the object representation surviving a saccade was examined. The low detection of changes in the image-plane position of the figure and the high detection of changes in the upright walkers in-depth orientation indicated that transsaccadic object representations are position invariant but orientation dependent. Implications for object recognition are highlighted. The second issue concerned transsaccadic anticipation of the future event course. Ss anticipated the postsaccadic relative positions of the walkers body parts. In contrast, there was no anticipation of the postsaccadic absolute position of a translating figure; instead, Ss relied on memory of the figures presaccadic position. The anticipated in-depth orientation of a rotating walker seemed to be distorted in the direction of canonical views.

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.

Viewpoint-dependent priming effects in the perception of human actions and body postures

The identification of human actions and body postures viewed from different viewpoints was examined in four long-term priming experiments with static pictures of a human model. In Experiments 1 and 2 participants had to name or describe the pictures, and in Experiments 3 and 4 participants had to decide whether the pictures showed a possible or impossible body pose. Reliable priming effects were obtained only when priming and primed action or pose shared the same in-depth orientation (Experiments 1 and 4) and left-right reflection (Experiments 2 and 3). Having seen the same action or pose in a different orientation did not reliably facilitate identification performance later on. Also, there was no priming for poses that are impossible to perform with a human body, not even when an identical same-view prime was used. These findings suggest that the stored representations that mediate the identification of human actions and postures are viewpoint specific.

Orientation-dependent priming effects in the perception of biological motion

In a serial 2-choice reaction time task, subjects discriminated between a biological motion walker and a similar distractor. The point-light walker appeared in 1 of 2 possible in-depth orientations: The figure was walking either to the right or to the left in the sagittal plane. Reliable priming effects were established in consecutive trials but only when priming and primed walkers had the same in-depth orientation. This orientation-dependent priming effect was not tempered when priming and primed figures had different directions of articulatory motion (Experiments 1 to 6), different starting positions in the step cycle (Experiment 2), and different point-light localizations (Experiment 3) or when the figures were translating (Experiments 4 to 6). The data converge with neurophysiological findings that suggest that object recognition is accomplished by accessing high-level, orientation-dependent representations.

Correlated changes in perceptions of the gender and orientation of ambiguous biological motion figures

Summary The sensitivity of the mammalian visual system to biological motion cues has been shown to be general and acute [1–3]. Human observers, in particular, can deduce higher-order information, such as the orientation of a figure (which way it is facing), its gender, emotional state, and even personality traits, on the basis only of sparse motion cues. Even when the stimulus information is confined to point lights attached to the major joints of an actor (so-called point-light figures), observers can use information about the way the actor is moving to tell what they are doing, whether they are a male or female, and how they are feeling [4–6]. Here we report the novel finding that stimulus manipulations that made such walkers appear more female also had the effect of making the walkers appear more often as if they were walking away from rather than towards observers. Using frontal-view (or rear-view) point-light displays of human walkers, we asked observers to judge whether they seemed to be walking towards or away from the viewing position. Independent of their own gender, observers reliably reported those figures they perceived to be male as looking like they were approaching (as reported in [7]), but those they perceived to be female as walking away. Furthermore, figures perceived to be gender-neutral also appeared more often, although not exclusively, to be walking towards observers.

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).