Mutuality Relations,Observation, and Intentional Constraints

This commentary focuses on the implications of Stoffregen's (target article, this issue) theory, as they apply to current research on human biological motion. We take up his suggestion that affordances, not events, are perceived and that data generated within event-perception research may reflect conversion of affordance-based perception to "event-based scales." Research on point-light walkers has been classed with event perception; however, results from our current research on perception of point-light sports displays suggest that accurate detection of humans and their actions in these displays may be controlled by complex relations better explained within an affordance-based account. We report the results of an experiment that controlled the presence and absence of relations between biological motion and a discrete environmental object. Detection was best when these affordance-relevant relations were available. Finally, we consider the utility of Stoffregen's ontological distinction as it may inform our understanding of past, current, and future research on perception of point-light walker displays.     

Perception of elliptic biological motion

We tested the ability of the mature visual system for discrimination between types of elliptic biological motion on the basis of event kinematics. Healthy adult volunteers were presented with point-light displays depicting elliptic motion when only a single dot, a moving point-light arm, or a whole point-light human figure was visible. The displays were created in accordance with the two-thirds power kinematic law (natural motion), whereas the control displays violated this principle (unnatural motion). On each trial, participants judged whether the display represented natural or unnatural motion. The findings indicate that adults are highly sensitive to violation of the two-thirds power kinematic law. Notably, participants can easily discriminate between natural and unnatural motions without recognising the stimuli, which suggests that people implicitly use kinematic information. Most intriguing, event recognition seems to diminish the capacity to judge whether event kinematics is unnatural. We discuss possible ways for a cross-talk between perception and production of biological movement, and the brain mechanisms involved in biological motion processing.     

Biological motion shown backwards: the apparent-facing effect

We examined how showing a film backwards (reverse transformation) affects the visual perception of biological motion. Adults and 6-year-old children saw first a point-light quadruped moving normally as if on a treadmill, and then saw the same display in reverse transformation. For other groups the order of presentation was the opposite. Irrespective of the presentation mode (normal or reverse) and of the facing of the point-light figure (rightward or leftward), a pronounced apparent-facing effect was observed: the perceptual identification of a display was mainly determined by the apparent direction of locomotion. The findings suggest that in interpreting impoverished point-light biological-motion stimuli the visual system may neglect distortions caused by showing a film backwards. This property appears to be robust across perceptual development. Possible explanations of the apparent-facing effect are discussed.     

Recognition of point-light biological motion displays by young children

We tested the ability of children 3-5 years of age to recognise biological motion displays. Children and adults were presented with moving point-light configurations depicting a walking person, four-legged animals (dogs), and a bird. Participants were able to reliably recognise displays with biological motion, but failed in the identification of a static (four consecutive frames taken from each sequence) version. The results indicate that, irrespective of the highly reduced and unusual structural information available in point-light displays, biological motion is sufficient for reliable recognition of human and non-human forms at an age as early as 3 years. Moreover, 5-year-olds exhibit the ceiling level of recognition. The findings are discussed in the context of the neuropsychological and brain mechanisms involved in biological motion perception.     

The effect of object and event orientation on perception of biological motion

Detection and recognition of point-light walking is reduced when the display is inverted, or turned upside down. This indicates that past experience influences biological motion perception. The effect could be the result of either presenting the human form in a novel orientation or presenting the event of walking in a novel orientation, as the two are confounded in the case of walking on feet. This study teased apart the effects of object and event orientation by examining detection accuracy for upright and inverted displays of a point-light figure walking on his hands. Detection of this walker was greater in the upright display, which had a familiar event orientation and an unfamiliar object orientation, than in the inverted display, which had a familiar object orientation and an unfamiliar event orientation. This finding supports accounts of event perception and recognition that are based on spatiotemporal patterns of motion associated with the dynamics of an event.     

Sociability modifies dogs’ sensitivity to biological motion of different social relevance

Preferential attention to living creatures is believed to be an intrinsic capacity of the visual system of several species, with perception of biological motion often studied and, in humans, it correlates with social cognitive performance. Although domestic dogs are exceptionally attentive to human social cues, it is unknown whether their sociability is associated with sensitivity to conspecific and heterospecific biological motion cues of different social relevance. We recorded video clips of point-light displays depicting a human or dog walking in either frontal or lateral view. In a preferential looking paradigm, dogs spontaneously viewed 16 paired point-light displays showing combinations of normal/inverted (control condition), human/dog and frontal/lateral views. Overall, dogs looked significantly longer at frontal human point-light display versus the inverted control, probably due to its clearer social/biological relevance. Dogs’ sociability, assessed through owner-completed questionnaires, further revealed that low-sociability dogs preferred the lateral point-light display view, whereas high-sociability dogs preferred the frontal view. Clearly, dogs can recognize biological motion, but their preference is influenced by their sociability and the stimulus salience, implying biological motion perception may reflect aspects of dogs’ social cognition.     

Essential kinematic information,athletic experience,and affordance perception for others

The present study investigated the role of different types of movement in affordance perception, as well as the influence of sports experience. Perception of another actor’s maximum vertical jumping height and horizontal long-jumping distance was evaluated for basketball players, soccer players, and nonplayer controls after viewing point-light representations of the actors’ movements. Perceptual reports were more accurate after jumping-related movements (walking and squatting) were viewed than after nonrelated movements (standing and twisting). Vertical jump reports were more accurate than horizontal jump reports. Basketball and soccer players demonstrated higher accuracy than did controls. This research establishes that point-light displays contain essential kinematic information sufficient to support accurate affordance perception, and athletes appear better attuned to kinematic information specifying affordances for others as a result of their sports experience.     

Infants perceive human point-light displays as solid forms

While five-month-old infants show orientation-specific sensitivity to changes in the motion and occlusion patterns of human point-light displays, it is not known whether infants are capable of binding a human representation to these displays. Furthermore, it has been suggested that infants do not encode the same physical properties for humans and material objects. To explore these issues we tested whether infants would selectively apply the principle of solidity to upright human displays. In the first experiment infants aged six and nine months were repeatedly shown a human point-light display walking across a computer screen up to 10 times or until habituated. Next, they were repeatedly shown the walking display passing behind an in-depth representation of a table, and finally they were shown the human display appearing to pass through the table top in violation of the solidity of the hidden human form. Both six- and nine-month-old infants showed significantly greater recovery of attention to this final phase. This suggests that infants are able to bind a solid vertical form to human motion. In two further control experiments we presented displays that contained similar patterns of motion but were not perceived by adults as human. Six- and nine-month-old infants did not show recovery of attention when a scrambled display or an inverted human display passed through the table. Thus, the binding of a solid human form to a display in only seems to occur for upright human motion. The paper considers the implications of these findings in relation to theories of infants' developing conceptions of objects, humans and animals.     

Human biological and nonbiological point-light movements: Creation and validation of the dataset

Human action perception is so powerful that people can identify movement efficiently in the absence of pictorial information, such as in point-light displays. Interest is growing in this type of stimulus for research in neuroscience. This interest stems from the advantage of separating the component of pure human action kinematics from other pictorial information, such as facial expression and muscle contraction. Although several groups have previously developed datasets of human point-light actions, due to the lack of datasets composed of daily actions with short durations, we developed 20 biological and 40 control (scrambled) point-light movements by using the technique of recording people wearing reflector patches. The videos are about 1 s long. Subsequently, we performed a judgment task in which 100 participants (50 male and 50 female) evaluated each video according to three categories: human action resemblance, performed action, and gender of actor. We present the mean scores of each evaluation for each video, and further propose a selection of the most suitable videos to be used as human point-light action displays and scrambled point-light displays for control. Finally, we discuss our findings on the gender attributions of the point-light displays.     

Recognition of biological motion from blurred natural scenes

Biological-motion perception can be regarded as a template-matching process. We are concerned with the visual cues in this template. Biological-motion perception is usually studied with point-light displays similar to the point-light displays invented by Johansson (1973 Perception and Psychophysics 14 201 - 211). These stimuli are in some ways abstract. In order to use more natural stimuli, we recorded movies of different actions in natural scenes. By blurring the scenes we modified the visual cues, particularly the local form and motion information. Observers were asked to identify the action portrayed. Our results demonstrate that templates for biological-motion recognition combine global form and motion cues. Reductions of local form and local motion information by blurring can be compensated by global form change and global motion. Local motion information is also used for segmentation.     

The relative influences of movement kinematics and extrinsic object characteristics on the perception of lifted weight

Humans are able to perceive unique types of biological motion presented as point-light displays (PLDs). Thirty years ago, Runeson and Frykholm (Human Perception and Performance, 7(4), 733, 1981, Journal of Experimental Psychology: General, 112(4), 585, 1983) studied observers’ perceptions of weights lifted by actors and identified that the kinematic information in a PLD is sufficient for an observer to form an accurate perception of the object weight. However, research has also shown that extrinsic object size characteristics also influence the perception of object weight (Gordon, Forssberg, Johansson, & Westling in Experimental Brain Research, 83(3), 477–482, 1991). This study addresses the relative contributions of these two types of visual information to observers’ perceptions of lifted weight, through an experiment in which participants viewed an actor lifting boxes of various sizes (small, medium, or large) and weights (25, 50, or 75 lb) under four PLD conditions—box-at-rest, moving-box, actor-only, and actor-and-box—and one full-vision video condition, and then provided a weight estimate for each box lifted. The results indicated that lift kinematics and box size contributed independently to weight perception. Interestingly, the most robust weight differentiations were elicited in the conditions in which both types of information were presented concurrently, despite their converse natures. Furthermore, full-vision video presentation, which contained visual information beyond kinematics and object information, elicited the best estimates.     

The efficiency of biological motion perception

Humans can readily perceive biological motion from point-light (PL) animations, which create an image of a moving human figure by tracing the trajectories of a small number of light points affixed to a moving human body. We have applied ideal observer analysis to a standard biological motion discrimination task involving either full-figure or PL displays. Contrary to current dogma, we find that PL animations can be rich inpotential stimulus information but that human observers are remarkably inefficient at exploiting this information. Although our findings do not discount the utility of PL animation, they do provide a realistic measure of the computational challenge posed by biological motion perception.     

Concurrent motion in infant event perception

Infant sensitivity to motion relationships specifying certain complex events, such as a person walking, has recently been demonstrated, but the perceptual principles underlying early event perception are not well understood. Retinal motion toward a common point (concurrent motion) specifies translation in depth to adult perceivers in the absence of conflicting information (Börjesson & von Hofsten, 1973). We tested this principle of event perception with 28 16-week-old infants. One group was habituated in a dark room to a concurrent motion: three points of light moving in a frontoparallel plane toward and away from a central point (not seen). After habituation, the room was illuminated, and looking time was tested to alternate presentations of two displays. In one display (depth motion), three lights were attached to a triangle actually moving in depth; in the other display (surface motion), the three lights moved visibly along the surface of a fronto-parallel stationary triangle. If concurrent motion, in the absence of conflicting information, specifies motion in depth to infants, they were expected to look longer after habituation at the surface motion display. A control group tested infants' relative interest in the two test displays with no prior habituation period.Control-group infants marginally preferred the depth movement display. The habituation group responded three times as much to the surface motion display, suggesting that motion in depth had been perceived during habituation. Specification of motion in depth by concurrency of relative proximal stimulus motions seems to be an operative principle in infants' perception; moreover, at least some principles of early event perception are unrelated to person perception or biological motion. The relation of these results to recent findings in infant object perception is discussed.     

Active versus passive processing of biological motion

Johansson's point-light walker figures remain one of the most powerful and convincing examples of the role that motion can play in the perception of form (Johansson, 1973 Perception & Psychophysics 14 201 - 211; 1975 Scientific American 232(6) 76 - 88). In the current work, we use a dual-task paradigm to explore the role of attention in the processing of such stimuli. In two experiments we find striking differences in the degree to which direction-discrimination performance in point-light walker displays appears to rely on attention. Specifically, we find that performance in displays thought to involve top-down processing, either in time (experiment 1) or space (experiment 2) is adversely affected by dividing attention. In contrast, dividing attention has little effect on performance in displays that allow low-level, bottom-up computations to be carried out. We interpret these results using the active/passive motion distinction introduced by Cavanagh (1991 Spatial Vision 5 303-309).     

Detection of the relevant type of locomotion in infancy: crawlers versus walkers

Human infants show a preference for individuals who are similar to them. Using point-light displays of human walkers and crawlers as stimuli, we examined whether infants' preference for the motions of crawling and walking changes between, before, and after the onset of bipedal walking. The results show that crawling and walking infants prefer the types of locomotion that are similar to their own, respectively. These indicate that the infants detect the similarities between the motions they performed and they observed, which provides the behavioral evidence that the production of a particular motion is connected to its perception in infancy.     

Evidence for distinct contributions of form and motion information to the recognition of emotions from body gestures

The importance of kinematics in emotion perception from body movement has been widely demonstrated. Evidence also suggests that the perception of biological motion relies to some extent on information about spatial and spatiotemporal form, yet the contribution of such form-related cues to emotion perception remains unclear. This study reports, for the first time, the relative effects on emotion recognition of inverting and motion-reversing patch-light compared to fully illuminated displays of whole-body emotion gestures. Inverting the gesture movies or playing them backwards significantly impaired emotion classification accuracy, but did so more for patch-light displays than for identical but fully illuminated movement sequences. This result suggests that inversion impairs the processing of form information related to the configuration of body parts, and reversal impairs the sequencing of form changes, more than these manipulations impair the processing of kinematic cues. This effect was strongest for inversion, suggesting an important role for configural information in emotion recognition. Nevertheless, even in combination these stimulus manipulations did not abolish above chance recognition of any of the emotions, suggesting that kinematics help distinguish emotions expressed by body gestures. Disproportionate impairments in recognition accuracy were observed for fear and disgust under inversion, and for fear under motion reversal, suggesting a greater role for form-related cues in the perception of these emotions.     

The influence of motor expertise and motor experience on action and actor recognition

Two experiments examined whether different levels of motor and visual experience influence action perception and whether this effect depends on the type of perceptual task. Within an action recognition task (Experiment 1), professional basketball players and novice college students were asked to identify basketball dribbles from point-light displays. Results showed faster reaction times and greater accuracy in experts, but no advantage when observing either own or teammates’ actions compared with unknown expert players. Within an actor recognition task (Experiment 2), the same expert players were asked to identify the model actors. Results showed poor discrimination between teammates and players from another team, but a more accurate assignment of own actions to the own team. When asked to name the actor, experts recognised themselves slightly better than teammates. Results support the hypothesis that motor experience influences action recognition. They also show that the influence of motor experience on the perception of own actions depends on the type of perceptual task.     

Comparing solid-body with point-light animations

The movement of faces provides useful information for a variety of tasks and is now an active area of research. We compare here two ways of presenting face motion in experiments: as solid-body animations and as point-light displays. In the first experiment solid-body and point-light animations, based on the same motion-captured marker data, produced similar levels of performance on a sex-judgment task. The trend was for an advantage for the point-light displays, probably in part because of residual spatial cues available in such stimuli. In the second experiment we compared spatially normalised point-light displays of marker data with solid-body animations and pseudorandom point-light animations. Performance with solid-body animations and normalised point-light displays was similar and above chance, while performance with the pseudorandom point-light stimuli was not above chance. We conclude that both relatively few well-placed points and solid-body animations provide useful information about facial motion, but that a greater number of randomly placed points does not support above-chance performance. Solid-body animations have the methodological advantages of reducing the importance of marker placement and are more effective in isolating motion information, even if they are subsequently rendered as point-light displays.     

Perception of American sign language in dynamic point-light displays

Perception of dynamic events of American Sign Language (ASL) was studied by isolating information about motion in the language from information about form. Four experiments utilized Johansson's technique for presenting biological motion as moving points of light. In the first, deaf signers were highly accurate in matching movements of lexical signs presented in point-light displays to those normally presented. Both discrimination accuracy and the pattern of errors were similar in this matching task to that obtained in a control condition in which the same signs were always represented normally. The second experiment showed that these results held for discrimination of morphological operations presented in point-light displays as well. In the third experiment, signers were able to accurately identify signs of a constant handshape and morphological operations acting on signs presented in point-light displays. Finally, in Experiment 4, we evaluated what aspects of the motion patterns carried most of the information for sign identifiability. We presented signs in point-light displays with certain lights removed and found that the movement of the fingertips, but not of any other pair of points, is necessary for sign identification and that, in general, the more distal the joint, the more information its movement carries.