Inferring intentions from biological motion: A stimulus set of point-light communicative interactions

We present the first database of communicative interactions reproduced through point-light displays (Communicative Interaction Database). The database contains 20 communicative interactions performed by male and by female couples. For each action, we provide movie files from four different viewpoints, as well as text files with the 3-D spatial coordinates of the point lights, allowing researchers to construct customized versions. By including various types of actions performed with different social motives, the database contains a diverse sample of nonconventional communicative gestures. Normative data collected to assess the recognizability of the stimuli suggest that, for most action stimuli, information in point-light displays is sufficient for clear recognition of the action as communicative, as well as for identification of the specific communicative gesture performed by the actor. The full set of stimuli may be downloaded from http://brm.psychonomic-journals.org/ content/supplemental and from http://ppw.kuleuven.be/labexppsy/lepSite/resources/CID.rar.     

Young children can extend motion verbs to point-light displays

In the first study using point-light displays (lights corresponding to the joints of the human body) to examine children's understanding of verbs, 3-year-olds were tested to see if they could perceive familiar actions that corresponded to motion verbs (e.g., walking). Experiment 1 showed that children could extend familiar motion verbs (e.g., walking and dancing) to videotaped point-light actions shown in the intermodal preferential looking paradigm. Children watched the action that matched the requested verb significantly more than they watched the action that did not match the verb. In Experiment 2, the findings of Experiment 1 were validated by having children spontaneously produce verbs for these actions. The use of point-light displays may illuminate the factors that contribute to verb learning.     

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.     

Cues for self-recognition in point-light displays of actions performed in synchrony with music

Self–other discrimination was investigated with point-light displays in which actions were presented with or without additional auditory information. Participants first executed different actions (dancing, walking and clapping) in time with music. In two subsequent experiments, they watched point-light displays of their own or another participant’s recorded actions, and were asked to identify the agent (self vs. other). Manipulations were applied to the visual information (actions differing in complexity, and degradation from 15 to 2 point-lights within the same clapping action) and to the auditory information (self-generated vs. externally-generated vs. none). Results indicate that self-recognition was better than chance in all conditions and was highest when observing relatively unconstrained patterns of movement. Auditory information did not increase accuracy even with the most ambiguous visual displays, suggesting that judgments of agent identity depend much more on motor cues than on auditory (action-generated) or audiovisual (synchronization) information.     

Expertise and attunement to kinematic constraints

Three experiments were undertaken to ascertain the extent to which expertise in natural anticipatory tasks is characterised by superior attunement to the biomechanical (kinematic) constraints of the movement pattern being observed. Twelve world-class and twelve non-expert badminton players were required to predict the depth of an opponent's stroke from either video displays or point-light displays of the opposing player's hitting action. The information available within the displays was manipulated through temporal and/or spatial occlusion. Consistent with predictions that can be derived from the constraint-attunement hypothesis (Vicente and Wang, 1998 Psychological Review 105 33-57), experts showed: (i) an unchanged pattern of information pick-up when the display was reduced from video to point-light and only kinematic information was available; (ii) superior information pick-up from kinematic features that non-experts could use; and (iii) attunement to early kinematic information from the lower body to which non-experts were not sensitive. Consistent with predictions that can be derived from a common-coding perspective (Prinz, 1997 European Journal of Cognitive Psychology 9 129-154), the anticipation of stroke depth was facilitated more for experts than non-experts when the perceptual display provided linked segment information reminiscent of the cross-segmental torque transfers that occur during expert movement production.     

Observing or producing a motor action improves later perception of biological motion: Evidence for a gender effect

Two experiments are presented addressing the issue of whether observing (visual priming) or producing (motor priming) a running activity during a very short period (30 s) facilitates the perception of the direction of a point-light runner embedded in a dense dynamical mask. Experiment 1 showed that perceptual judgements improved and response time increased in the visual priming compared to the neutral priming condition (video of a moving car) in which judgements were at random. Because this effect was observed for male participants only, we performed a second experiment with the aim of evaluating the role of gender congruency in the visual priming condition. Results confirmed the facilitation effect and demonstrated that this effect was strictly dependent on the gender congruency between the perceiver and the priming information. Moreover, we found that actually producing a motor activity similar to the one presented in the video sequence improved to the same extent participants’ judgement of the direction of the point-light runner, without any gender effect. As a whole, these findings argue in favour of common representation for the perception and the production of human movement and showed that the perception of biological motion can be improved by prior motor activity either performed or observed. However, the gender-dependent effect of visual priming suggested that motor repertoire differed in males and females.     

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.     

Perceiving minimal distinctions in ASL under normal and point-light display conditions

Perceptual confusions within 36 formationally minimal pairs of signs were assessed for native signers under two conditions of video presentation: (1) normally lighted black-and-white displays and (2) point-light displays constructed by affixing 26 points of retroreflective tape on the fingertips, back knuckles, and wrists and 1 point on the nose of the signer. Nine pairs were selected for each of the formational parameters of handshape, location, movement, and orientation. For each minimal pair, a native signer constructed and signed an ASL sentence that was syntactically and semantically appropriate for both members of the pair. Fourteen highly fluent ASL users responded by selecting a picture appropriate to the viewed sentence, thus avoiding contamination by English. For both viewing conditions, subjects discriminated the minimal pairs significantly better than chance. Performance was better (1) when location or orientation differentiated the signs than when movement or handshape did and (2) in normal lighting than in the point-light displays. With the point/light displays, discrimination of location, movement, and orientation was poorer and handshape discrimination was at chance. The discussion considers (1) the confusion of signs in the absence of linguistic redundancy, (2) the effectivenese of the point-light configurations as “minimal cues” for the contrasts, and (3) the efficacy of using these point-light configurations to reduce the information in ASL to a narrow bandwidth.     

Memory for people and their actions: further evidence for an age-related associative deficit

The associative deficit hypothesis (M. Naveh-Benjamin, 2000) attributes age-related memory deficits to the inability to encode and retrieve bound units of information. The present experiment extended this deficit to a new form of stimuli, dynamic displays of people and their performance of everyday actions. Older and younger adults viewed a series of brief video clips, each showing a different person performing a different action, and were tested over memory for individual people, individual actions, and the person-action combinations. Older adults did exhibit an associative deficit, and this was related to an increased proportion of false alarms on the associative test.     

Techniques for the production of point-light and fully illuminated video displays from identical recordings

Illumination of only a few key points on a moving human body or face is enough to convey a compelling perception of human motion. A full understanding of the perception ofbiological motion from point-light displays requires accurate comparison with the perception of motion in normal, fully illuminated versions of the same images. Traditionally, these two types of stimuli (point-light and fully illuminated) have been filmed separately, allowing the introduction of uncontrolled variation across recordings. This is undesirable for accurate comparison of perceptual performance across the two types of display. This article describes simple techniques, using proprietary software, that allow production of point-light and fully illuminated video displays from identical recordings. These techniques are potentially useful for many studies of motion perception, by permitting precise comparison of perceptual performances across point-light displays and their fully illuminated counterparts with accuracy and comparative ease.     

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.     

Baby do–baby see!: How action production influences action perception in infants

With a series of four experiments we show that self-produced actions influence infants’ perception of actions performed by others. After having played with an object, 7–11-month-olds simultaneously watched two videos presenting adults who act on either the same object or a different one. The 9- and 11-month-old preferred to watch the same-object video indicating an influence of action production on action perception at this age. Follow-up studies showed that this influence was restricted to object-related actions. Agentive experience enhanced interest in actions with objects, but not in watching objects or persons per se. These findings indicate that infants are not only interested in acting on objects themselves, but that this experience increases their interest in the actions of other people with the same object.     

Behavioral speed contagion: automatic modulation of movement timing by observation of body movements

To coordinate our actions with those of others, it is crucial to not only choose an appropriate category of action but also to execute it at an appropriate timing. It is widely documented that people tend to unconsciously mimic others' behavior. The present study show that people also tend to modify their movement timing according to others' movements even when the observed and the to-be-executed movements are unrelated. Observers viewed either point-light biological motion, scrambled biological motion, or solid object motion. The stimulus sequence was presented at three different (half, normal, and double) rates. After a 300-2400-ms blank period, the observers performed a simple choice reaction-time task that was unrelated to the presented stimulus sequence. The observation of the biological motion produced a negative correlation between reaction time and stimulus speed, whereas no such trend was observed with the scrambled or solid object motion. Furthermore, speed-dependent modulation occurred only when the task was imposed within approximately 1s after the offset of the biological motion. These results suggest that behavioral tempo may be contagious; the speed of others' movements may automatically influence the timing of movement execution by the observer.     

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.     

The role of appearance and motion in action prediction

We used a novel stimulus set of human and robot actions to explore the role of humanlike appearance and motion in action prediction. Participants viewed videos of familiar actions performed by three agents: human, android and robot, the former two sharing human appearance, the latter two nonhuman motion. In each trial, the video was occluded for 400?ms. Participants were asked to determine whether the action continued coherently (in-time) after occlusion. The timing at which the action continued was early, late, or in-time (100, 700 or 400?ms after the start of occlusion). Task performance interacted with the observed agent. For early continuations, accuracy was highest for human, lowest for robot actions. For late continuations, the pattern was reversed. Both android and human conditions differed significantly from the robot condition. Given the robot and android conditions had the same kinematics, the visual form of the actor appears to affect action prediction. We suggest that the selection of the internal sensorimotor model used for action prediction is influenced by the observed agent's appearance.     

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.     

Recognizing people from their movement

Human observers demonstrate impressive visual sensitivity to human movement. What defines this sensitivity? If motor experience influences the visual analysis of action, then observers should be most sensitive to their own movements. If view-dependent visual experience determines visual sensitivity to human movement, then observers should be most sensitive to the movements of their friends. To test these predictions, participants viewed sagittal displays of point-light depictions of themselves, their friends, and strangers performing various actions. In actor identification and discrimination tasks, sensitivity to one's own motion was highest. Visual sensitivity to friends', but not strangers', actions was above chance. Performance was action dependent. Control studies yielded chance performance with inverted and static displays, suggesting that form and low-motion cues did not define performance. These results suggest that both motor and visual experience define visual sensitivity to human action.     

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/.