Prior knowledge about display inversion in biological motion perception

Display inversion severely impedes veridical perception of point-light biological motion (Pavlova and Sokolov, 2000 Perception & Psychophysics 62 889-899; Sumi, 1984 Perception 13 283-286). Here, by using a spontaneous-recognition paradigm, we ask whether prior information about display orientation improves biological motion perception. Participants were shown a set of 180 degrees inverted point-light stimuli depicting a human walker and quadrupeds (dogs). In experiment 1, one group of observers was not aware of the orientation of stimuli, whereas the other group was told beforehand that stimuli will be presented upside down. In experiment 2, independent groups of participants informed about stimulus orientation saw the same set of stimuli, in each of which either a moving or a static background line was inserted. The findings indicate that information about display inversion is insufficient for reliable recognition of inverted point-light biological motion. Instead, prior information facilitates display recognition only when it is complemented by additional contextual elements. It appears that visual impressions from inverted point-light stimuli remain impenetrable with respect to one's knowledge about display orientation. The origins of orientation specificity in biological motion perception are discussed in relation to the recent neuroimaging data obtained with point-light stimuli and fragmented Mooney faces.     

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.     

What learning to see arbitrary motion tells us about biological motion perception

In separate studies, observers viewed upright biological motion, inverted biological motion, or arbitrary motion created from systematically randomizing the positions of point-light dots. Results showed that observers (a) could learn to detect the presence of arbitrary motion, (b) could not learn to discriminate the coherence of arbitrary motion, although they could do so for upright biological motion, (c) could apply a detection strategy to learn to detect the presence of inverted biological motion nearly as well as they detected upright biological motion, and (d) performed better discriminating the coherence of upright biological motion compared with inverted biological motion. These results suggest that learning and form information play an important role in perceiving biological motion, although this role may only be apparent in tasks that require processing information from multiple parts of the motion display.     

Orientation unbound: Dissociation of identity and orientation under rapid serial visual presentation

Participants were shown rapid sequences of three letters, flanked by digits, each rotated 0 degree, 90 degrees, 180 degrees, or 270 degrees clockwise from upright. In Experiment 1, the participants tried to report the letter that matched the orientation of an arrow, presented either before (before task) or after (after task) the sequence. A third task (total task) required them to report all of the letters. Accuracy for individual letters was significantly better in the total task than in the before task, and better in the before task than in the after task, suggesting particular difficulty in binding orientation to identity. In Experiment 2, the participants were given letter probes and were asked to indicate the orientation of the probed letter. Although report was above chance, there were frequent illusory conjunctions. Since perception of orientation must depend on prior establishment of identity, our results suggest that orientation and identity may become unbound during processing and are held in parallel storage systems.     

The perception of emotion from body movement in point-light displays of interpersonal dialogue

We examined whether it is possible to identify the emotional content of behaviour from point-light displays where pairs of actors are engaged in interpersonal communication. These actors displayed a series of emotions, which included sadness, anger, joy, disgust, fear, and romantic love. In experiment 1, subjects viewed brief clips of these point-light displays presented the right way up and upside down. In experiment 2, the importance of the interaction between the two figures in the recognition of emotion was examined. Subjects were shown upright versions of (i) the original pairs (dyads), (ii) a single actor (monad), and (iii) a dyad comprising a single actor and his/her mirror image (reflected dyad). In each experiment, the subjects rated the emotional content of the displays by moving a slider along a horizontal scale. All of the emotions received a rating for every clip. In experiment 1, when the displays were upright, the correct emotions were identified in each case except disgust; but, when the displays were inverted, performance was significantly diminished for some emotions. In experiment 2, the recognition of love and joy was impaired by the absence of the acting partner, and the recognition of sadness, joy, and fear was impaired in the non-veridical (mirror image) displays. These findings both support and extend previous research by showing that biological motion is sufficient for the perception of emotion, although inversion affects performance. Moreover, emotion perception from biological motion can be affected by the veridical or non-veridical social context within the displays.     

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.     

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.     

Levels in the identifying of orientation by preschool children

A new approach to understanding the perception of orientation proposes that the first level in the processing of orientation information results in perceiving whether a shape is upright or nonupright; nonupright orientations are not distinguished from each other. As predicted, children of 3 and 4 years discriminated upright from nonupright pictures more readily than they discriminated the nonuprights (upside down and sideways) from each other. The angular differences between the pairs of discriminanda (whether 90 or 180°) had no effect on performance. the difficulty of distinguishing between nonuprights could not be attributed simply to a lack of attention to the dimension of orientation. The theory appears to have considerable generality, and provides a basis for understanding (1) perception of the orientation of both realistic and geometric shapes, and (2) similarities in orientation perception observed between children and adults.     

Infant sensitivity to figural coherence in biomechanical motions

Two experiments assessed infant sensitivity to figural coherence in point-light displays moving as if attached to the major joints of a walking person. Experiment 1 tested whether 3- and 5-month-old infants could discriminate between upright and inverted versions of the walker in both moving and static displays. Using an infant-control habituation paradigm, it was found that both ages discriminated the moving but not the static displays. Experiment 2 was designed to clarify whether or not structural invariants were extracted from these displays. The results revealed that (1) moving point-light displays with equivalent motions but different topographic relations were discriminated while (2) static versions were not, and (3) arrays that varied in the amount of motion present in different portions of the display were also not discriminated. These results are interpreted as indicating that young infants are sensitive to figural coherence in displays of biomechanical motion.     

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.     

Turning objects on their heads: the influence of the stored axis on object individuation

We used repetition blindness (RB) as a measure of object recognition and compared the pattern of RB obtained for objects with a well-established upright orientation (mono-oriented objects) and those without a usual upright orientation (polyoriented objects), when the critical objects were either in identical orientations or differed by 30 degrees, 60 degrees, 90 degrees, or 180 degrees. Overall, we found robust RB despite differences in orientation, consistent with the idea that object recognition, as indexed by RB, is largely independent of orientation. However, whereas for polyoriented objects RB was obtained in all orientation conditions, for mono-oriented objects there was no RB between upright and upside-down versions of the stimuli. These findings suggest that the usual orientation of an object-when it exists-is stored in memory and can facilitate orientation processing when the principal axis of a viewed object is aligned with the stored axis orientation. This, in turn, allows for more rapid and successful construction of distinct episodic representations of an object, thus alleviating RB.     

Orientation congruency effects on the identification of disoriented shapes

Effects of orientation on identification can be attenuated when other patterns at the same (or a similar) orientation are identified in close temporal contiguity. In Experiments 1 and 2, letters were presented simultaneously in brief masked displays. Identification accuracy was much higher when the letters had consistent orientations than when the letters had different orientations within a display. In Experiment 3, two letters were presented sequentially. Identification accuracy was higher with congruent than with incongruent orientations. The results are unexpected if one assumes that the patterns themselves are rotated until upright prior to their identification, unless pattern rotation processes can be primed, and that priming requires orientation congruence between the priming and primed stimulus. The results are expected if the orientation of a frame of reference can be adjusted to the orientation of the patterns during the identification process.     

On the depth reversibility of point-light actions

We investigate the occurrence of perspective reversals for a depth-ambiguous point-light figure. In addition, we exploit the phenomenon of reversibility to search for stimulus features relevant in the process of depth assignment. Experiment 1 shows that perceptual switches indeed occur during prolonged viewing, although the switches occur infrequently. The reversibility is confirmed in Experiment 2, in which the perceptual ambiguity of the point-light action is manipulated as well as observers’ intention to perceive a particular alternative. In addition, the pattern of eye movements reveals local stimulus features specifically associated with the perception of the different alternatives. In Experiment 3, the importance of these features as determining factors of the initial interpretation is investigated by manipulating the location of the first fixation on the stimulus. Implications for a better understanding of biological motion perception are discussed.     

Biological motion preference in humans at birth: role of dynamic and configural properties

The present study addresses the hypothesis that detection of biological motion is an intrinsic capacity of the visual system guided by a non-species-specific predisposition for the pattern of vertebrate movement and investigates the role of global vs. local information in biological motion detection. Two-day-old babies exposed to a biological motion point-light display (depicting a walking hen) and a non-biological motion display (a rotating rigid object) preferentially looked at the biological display (Experiment 1). A new group of newborns showed themselves to be capable of discriminating, following habituation, a biological motion display from a spatially scrambled version of it (Experiment 2). However, a third group of newborns, at their first exposure to such displays, did not show any preference between them (Experiment 3). Results confirm and extend previous comparative and developmental data, supporting an inborn predisposition to attend to biological motion in humans. This ability is presumably part of an evolutionarily ancient and non-species-specific system predisposing animals to preferentially attend to other animals.     

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.     

A Simon effect with stationary moving stimuli

To clarify whether motion information per se has a separable influence on action control, the authors investigated whether irrelevant direction of motion of stimuli whose overall position was constant over time would affect manual left-right responses (i.e., reveal a motion-based Simon effect). In Experiments 1 and 2, significant Simon effects were obtained for sine-wave gratings moving in a stationary Gaussian window. In Experiment 3, a direction-based Simon effect with random-dot patterns was replicated, except that the perceived direction of motion was based on the displacement of single elements. Experiments 4 and 5 studied motion-based Simon effects to point-light figures that walked in place--displays requiring high-level analysis of global shape and local motion. Motion-based Simon effects occurred when the displays could be interpreted as an upright human walker, showing that a high-level representation of motion direction mediated the effects. Thus, the present study establishes links between high-level motion perception and action.     

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.     

Perception of Mooney faces by young infants: the role of local feature visibility, contrast polarity, and motion

We examined the ability of young infants (3- and 4-month-olds) to detect faces in the two-tone images often referred to as Mooney faces. In Experiment 1, this performance was examined in conditions of high and low visibility of local features and with either the presence or absence of the outer head contour. We found that regardless of the presence of the outer head contour, infants preferred upright over inverted two-tone face images only when local features were highly visible (Experiment 1a). We showed that this upright preference disappeared when the contrast polarity of two-tone images was reversed (Experiment 1b), reflecting operation of face-specific mechanisms. In Experiment 2, we investigated whether motion affects infants' perception of faces in Mooney faces. We found that when the faces appeared to be rigidly moving, infants did show an upright preference in conditions of low visibility of local features (Experiment 2a). Again the preference disappeared when the contrast polarity of the image was reversed (Experiment 2b). Together, these results suggest that young infants have the ability to integrate fragmented image features to perceive faces from two-tone face images, especially if they are moving. This suggests that an interaction between motion and form rather than a purely motion-based process (e.g., structure from motion) facilitates infants' perception of faces in ambiguous two-tone images.     

The Thatcher illusion: rotating the viewer instead of the picture

Faces are difficult to recognise when presented upside down. This effect of face inversion was effectively demonstrated with the 'Thatcher illusion' by Thompson (1980 Perception 9 483-484). It has been tacitly assumed that this effect is due to inversion relative to retinal coordinates. Here we tested whether it is due to egocentric (i.e. retinal) inversion or whether the orientation of the body with respect to gravity also influences the face-inversion effect. A 3-D human turntable was used to test subjects in 5 different body-tilt (roll) orientations: 0 degree, 45 degrees, 90 degrees, 135 degrees, and 180 degrees. The stimuli consisted of 4 'normal' and 4 'thatcherised' faces and were presented in 8 different orientations in the picture plane. The subjects had to decide in a yes-no task whether the faces were 'normal' or 'thatcherised'. Analysis of the d' values revealed a significant effect of stimulus orientation and body tilt. The significant effect of body tilt was due to a drop in d' values in the 135 degrees orientation. This result is compared to findings of studies on the subjective visual vertical, where larger errors occurred in body-tilt orientations between 90 degrees and 180 degrees. The present findings suggest that the face-inversion effect relies mainly on retinal coordinates, but that in head-down body-tilt orientations around 135 degrees the gravitational reference frame has a major influence on the perception of faces.