Allocation of attention to biological motion: Local motion dominates global shape

Life motion signals bias the perception of apparent motion direction

Walking direction conveyed by biological motion (BM) cues, which humans are highly sensitive to since birth, can elicit involuntary shifts of attention to enhance the detection of static targets. Here, we demonstrated that such intrinsic sensitivity to walking direction could also modulate the direction perception of simultaneously presented dynamic stimuli. We showed that the perceived direction of apparent motion was biased towards the walking direction even though observers had been informed in advance that the walking direction of BM did not predict the apparent motion direction. In particular, rightward BM cues had an advantage over leftward BM cues in altering the perception of motion direction. Intriguingly, this perceptual bias disappeared when BM cues were shown inverted, or when the critical biological characteristics were removed from the cues. Critically, both the perceptual direction bias and the rightward advantage persisted even when only local BM cues were presented without any global configuration. Furthermore, the rightward advantage was found to be specific to social cues (i.e., BM), as it vanished when non-social cues (i.e., arrows) were utilized. Taken together, these findings support the existence of a specific processing mechanism for life motion signals and shed new light on their influences in a dynamic environment.

The domestic chick as an animal model of autism spectrum disorder: building adaptive social perceptions through prenatally formed predispositions

Equipped with an early social predisposition immediately post-birth, humans typically form associations with mothers and other family members through exposure learning, canalized by a prenatally formed predisposition of visual preference to biological motion, face configuration, and other cues of animacy. If impaired, reduced preferences can lead to social interaction impairments such as autism spectrum disorder (ASD) via misguided canalization. Despite being taxonomically distant, domestic chicks could also follow a homologous developmental trajectory toward adaptive socialization through imprinting, which is guided via predisposed preferences similar to those of humans, thereby suggesting that chicks are a valid animal model of ASD. In addition to the phenotypic similarities in predisposition with human newborns, accumulating evidence on the responsible molecular mechanisms suggests the construct validity of the chick model. Considering the recent progress in the evo-devo studies in vertebrates, we reviewed the advantages and limitations of the chick model of developmental mental diseases in humans.

Naïve chicks do not prefer objects with stable body orientation, though they may prefer behavioural variability

Domestic chicks (Gallus gallus domesticus) have been widely used as a model to study the motion cues that allow visually naïve organisms to detect animate agents shortly after hatching/birth. Our previous work has shown that chicks prefer to approach agents whose main body axis and motion direction are aligned (a feature typical of creatures whose motion is constrained by a bilaterally symmetric body plan). However, it has never been investigated whether chicks are also sensitive to the fact that an agent maintains a stable front-back body orientation in motion (i.e. consistency in which end is leading and which trailing). This is another feature typical of bilateria, which is also associated with the detection of animate agents in humans. The aim of the present study was to fill this gap. Contrary to our initial expectations, after testing 300 chicks across 3 experimental conditions, we found a recurrent preference for the agent which did not maintain a stable front-back body orientation. Since this preference was limited to female chicks, the results are discussed also in relation to sex differences in the social behaviour of this model. Overall, we show for the first time that chicks can discriminate agents based on the stability of their front-back orientation. The unexpected direction of the effect could reflect a preference for agents' whose behaviour is less predictable. Chicks may prefer agents with greater behavioural variability, a trait which has been associated with animate agents, or have a tendency to explore agents performing "odd behaviours".

Life Detection From Biological Motion

Life motion, the active movements of people and other animals, contains a wealth of information that is potentially accessible to the visual system of an observer. Biological-motion point-light displays have been widely used to study both the information contained in life motion stimuli and the visual mechanisms that make use of it. Biological motion conveys motion-mediated dynamic shape, which in turn can be used for identification and recognition of the agent, but it also contains local visual invariants that humans and other animals use as a general detection system that signals the presence of other agents in the visual environment. Here, we review recent research on behavioral, neurophysiological, and genetic aspects of this life-detection system and discuss its functional significance in the light of earlier hypotheses.

Using EEG movement tagging to isolate brain responses coupled to biological movements

Detecting biological motion is essential for adaptive social behavior. Previous research has revealed the brain processes underlying this ability. However, brain activity during biological motion perception captures a multitude of processes. As a result, it is often unclear which processes reflect movement processing and which processes reflect secondary processes that build on movement processing. To address this issue, we developed a new approach to measure brain responses directly coupled to observed movements. Specifically, we showed 30 male and female adults a point-light walker moving at a pace of 2.4 Hz and used EEG frequency tagging to measure the brain response coupled to that pace ('movement tagging'). The results revealed a reliable response at the walking frequency that was reduced by two manipulations known to disrupt biological motion perception: phase scrambling and inversion. Interestingly, we also identified a brain response at half the walking frequency (i.e., 1.2 Hz), corresponding to the rate at which the individual dots completed a cycle. In contrast to the 2.4 Hz response, the response at 1.2 Hz was increased for scrambled (vs. unscrambled) walkers. These results show that frequency tagging can be used to capture the visual processing of biological movements and can dissociate between global (2.4 Hz) and local (1.2 Hz) processes involved in biological motion perception, at different frequencies of the brain signal.

A two-stage framework for neural processing of biological motion

It remains to be understood how biological motion is hierarchically computed, from discrimination of local biological motion animacy to global dynamic body perception. Here, we addressed this functional separation of the correlates of the perception of local biological motion from perception of global motion of a body. We hypothesized that local biological motion processing can be isolated, by using a single dot motion perceptual decision paradigm featuring the biomechanical details of local realistic motion of a single joint. To ensure that we were indeed tackling processing of biological motion properties we used a discrimination instead of detection task. We discovered using representational similarity analysis that two key early dorsal and two ventral stream regions (visual motion selective hMT+ and V3A, extrastriate body area EBA and a region within fusiform gyrus FFG) showed robust and separable signals related to encoding of local biological motion and global motion-mediated shape. These signals reflected two independent processing stages, as revealed by representational similarity analysis and deconvolution of fMRI responses to each motion pattern. This study showed that higher level pSTS encodes both classes of biological motion in a similar way, revealing a higher-level integrative stage, reflecting scale independent biological motion perception. Our results reveal a two-stage framework for neural computation of biological motion, with an independent contribution of dorsal and ventral regions for the initial stage.

Perception of Dynamic Point Light Facial Expression

This study uses point light displays both to investigate the roles of global and local motion analyses in the perception of dynamic facial expressions and to measure the information threshold for reliable recognition of emotions. We videotaped the faces of actors wearing black makeup with white dots while they dynamically produced each of 6 basic Darwin/Ekman emotional expressions. The number of point lights was varied to systematically manipulate amount of information available. For all but one of the expressions, discriminability (d′) increased approximately linearly with number of point lights, with most remaining largely discriminable with as few as only 6 point lights. This finding supports reliance on global motion patterns produced by facial muscles. However, discriminability for the happy expression was notably higher and largely unaffected by number of point lights and thus appears to rely on characteristic local motion, probably the unique upward curvature of the mouth. The findings indicate that recognition of facial expression is not a unitary process and that different expressions may be conveyed by different perceptual information, but in general, basic facial emotional expressions typically remain largely discriminable with as few as 6 dynamic point lights.

Posterior brain sensorimotor recruitment for inhibition of delayed responses in children

Inhibitory control, the ability to suppress irrelevant thoughts or actions, is central to cognitive and social development. Protracted maturation of frontal brain networks has been reported as a major restraint for this ability, yet, young children, when motivated, successfully inhibit delayed responses. A better understanding of the age-dependent neural inhibitory mechanism operating during the awaiting-to-respond window in children may elucidate this conundrum. We recorded ERPs from children and parental adults to a visual-spatial working memory task with delayed responses. Cortical activation elicited during the first 1000 ms of the awaiting-to-respond window showed, as predicted by prior studies, early inhibitory effects in prefrontal ERPs (P200, 160-260 ms) associated with top-down attentional-biasing, and later effects in parietal/occipital ERPs (P300, 270-650 ms) associated with selective inhibition of task-irrelevant stimuli/responses and recurrent memory retrieval. Children successfully inhibited delayed responses and performed with a high level of accuracy (often over 90%), although, the prefrontal P200 displayed reduced amplitude and uniformly delayed peak latency, suggesting low efficacy of top-down attentional-biasing. P300, however, with no significant age-contrasts in latency was markedly elevated in children over the occipital/inferior parietal regions, with effects stronger in younger children. These results provide developmental evidence supporting the sensorimotor recruitment model of visual-spatial working memory relying on the occipital/parietal regions of the early maturing dorsal-visual network. The evidence is in line with the concept of age-dependent variability in the recruitment of cognitive inhibitory networks, complementing the former predominant focus on frontal lobes.

Spatiotemporal dynamics of responses to biological motion in the human brain

We sought to understand the spatiotemporal characteristics of biological motion perception. We presented observers with biological motion walkers that differed in terms of form coherence or kinematics (i.e., the presence or absence of natural acceleration). Participants were asked to discriminate the facing direction of the stimuli while their magnetoencephalographic responses were concurrently imaged. We found that two univariate response components can be observed around ~200 msec and ~650 msec post-stimulus onset, each engaging lateral-occipital and parietal cortex prior to temporal and frontal cortex. Moreover, while univariate responses show biological motion form-specificity only after 300 msec, multivariate patterns specific to form can be well discriminated from those for local cues as early as 100 msec after stimulus onset. By finally examining the representational similarity of fMRI and MEG patterned responses, we show that early responses to biological motion are most likely sourced to occipital cortex while later responses likely originate from extrastriate body areas.

Heritability of reflexive social attention triggered by eye gaze and walking direction: common and unique genetic underpinnings

BACKGROUND

Social attention ability is crucial for human adaptive social behaviors and interpersonal communications, and the malfunction of which has been implicated in autism spectrum disorder (ASD), a highly genetic neurodevelopmental disorder marked by striking social deficits.

METHODS

Using a classical twin design, the current study investigated the genetic contribution to individual variation in social and non-social attention abilities, and further probed their potential genetic linkage. Moreover, individual autistic traits were further measured in an independent group of non-twin participants to examine the hypothetical link between the core social attention ability and ASD.

RESULTS

We found reliable genetic influences on the social attentional effects induced by two distinct cues (eye gaze and walking direction), with 91% of their covariance accounted for by common genetic effects. However, no evidence of heritability or shared genetic effects was observed for the attentional effect directed by a non-social cue (i.e. arrow direction) and its correlation with the social attention ability. Remarkably, one's autistic traits could well predict his/her heritable core social attention ability extracted from the conventional social attentional effect.

CONCLUSIONS

These findings together suggest that human social attention ability is supported by unique genetic mechanisms that can be shared across different social, but not non-social, processing. Moreover, they also encourage the identification of 'social attention genes' and highlight the critical role of the core human social attention ability in seeking the endophenotypes of social cognitive disorders including ASD.

The neural correlates of orienting to walking direction in 6-month-old infants: An ERP study

The ability to detect social signals represents a first step to enter our social world. Behavioral evidence has demonstrated that 6-month-old infants are able to orient their attention toward the position indicated by walking direction, showing faster orienting responses toward stimuli cued by the direction of motion than toward uncued stimuli. The present study investigated the neural mechanisms underpinning this attentional priming effect by using a spatial cueing paradigm and recording EEG (Geodesic System 128 channels) from 6-month-old infants. Infants were presented with a central point-light walker followed by a single peripheral target. The target appeared randomly at a position either congruent or incongruent with the walking direction of the cue. We examined infants' target-locked event-related potential (ERP) responses and we used cortical source analysis to explore which brain regions gave rise to the ERP responses. The P1 component and saccade latencies toward the peripheral target were modulated by the congruency between the walking direction of the cue and the position of the target. Infants' saccade latencies were faster in response to targets appearing at congruent spatial locations. The P1 component was larger in response to congruent than to incongruent targets and a similar congruency effect was found with cortical source analysis in the parahippocampal gyrus and the anterior fusiform gyrus. Overall, these findings suggest that a type of biological motion like the one of a vertebrate walking on the legs can trigger covert orienting of attention in 6-month-old infants, enabling enhancement of neural activity related to visual processing of potentially relevant information as well as a facilitation of oculomotor responses to stimuli appearing at the attended location.

Heritable aspects of biological motion perception and its covariation with autistic traits

The ability to detect biological motion (BM) and decipher the meaning therein is essential to human survival and social interaction. However, at the individual level, we are not equally equipped with this ability. In particular, impaired BM perception and abnormal neural responses to BM have been observed in autism spectrum disorder (ASD), a highly heritable neurodevelopmental disorder characterized by devastating social deficits. Here, we examined the underlying sources of individual differences in two abilities fundamental to BM perception (i.e., the abilities to process local kinematic and global configurational information of BM) and explored whether BM perception shares a common genetic origin with autistic traits. Using the classical twin method, we found reliable genetic influences on BM perception and revealed a clear dissociation between its two components-whereas genes account for about 50% of the individual variation in local BM processing, global BM processing is largely shaped by environment. Critically, participants' sensitivity to local BM cues was negatively correlated with their autistic traits through the dimension of social communication, with the covariation largely mediated by shared genetic effects. These findings demonstrate that the ability to process BM, especially with regard to its inherent kinetics, is heritable. They also advance our understanding of the sources of the linkage between autistic symptoms and BM perception deficits, opening up the possibility of treating the ability to process local BM information as a distinct hallmark of social cognition.

Efficiencies for parts and wholes in biological-motion perception

People can reliably infer the actions, intentions, and mental states of fellow humans from body movements (Blake & Shiffrar, 2007). Previous research on such biological-motion perception has suggested that the movements of the feet may play a particularly important role in making certain judgments about locomotion (Chang & Troje, 2009; Troje & Westhoff, 2006). One account of this effect is that the human visual system may have evolved specialized processes that are efficient for extracting information carried by the feet (Troje & Westhoff, 2006). Alternatively, the motion of the feet may simply be more discriminable than that of other parts of the body. To dissociate these two possibilities, we measured people's ability to discriminate the walking direction of stimuli in which individual body parts (feet, hands) were removed or shown in isolation. We then compared human performance to that of a statistically optimal observer (Gold, Tadin, Cook, & Blake, 2008), giving us a measure of humans' discriminative ability independent of the information available (a quantity known as efficiency). We found that efficiency was highest when the hands and the feet were shown in isolation. A series of follow-up experiments suggested that observers were relying on a form-based cue with the isolated hands (specifically, the orientation of their path through space) and a motion-based cue with the isolated feet to achieve such high efficiencies. We relate our findings to previous proposals of a distinction between form-based and motion-based mechanisms in biological-motion perception.

Action Congruency Influences Crowding When Discriminating Biological Motion Direction

Identification and discrimination of peripheral stimuli are often difficult when a few stimuli adjacent to the target are present (crowding). Our previous study showed that crowding occurs for walking direction discrimination of a biological motion stimulus. In the present study, we attempted to examine whether action congruency between the target and flankers would influence the crowding effect on biological motion stimuli. Each biological motion stimulus comprised one action (e.g., walking, throwing wastepaper, etc.) and was rotated in one of five directions around the vertical axis. In Experiment 1, observers discriminated between the directions of the target stimulus actions, which were surrounded by two flankers in the peripheral visual field. The crowding effect was stronger when the flankers performed the same action as the target and the directions differed. The congruency of action type enhanced the crowding effect in the direction-discrimination task. In Experiment 2, observers discriminated between action types of target stimuli. The crowding effect for the action-discrimination task was not modulated by the congruency of action direction. Thus, identical actions induced a larger crowding effect for action-direction discrimination, but congruent directions did not influence crowding for action-type discrimination. These results suggest that the processes involved in direction discrimination of biological motion are partially distinct from action discrimination processes.

Response Priming with More or Less Biological Movements as Primes

Response priming in general is a suitable tool in cognitive psychology to investigate motor preactivations. Typically, compatibility effects reflect faster reactions in cases in which prime and target suggest the same response (i.e., compatible trials) compared with cases in which prime and target suggest opposite responses (i.e., incompatible trials). With moving dots that were horizontally aligned, Bermeitinger (2013) found a stable pattern of results: with short SOAs, faster responses in compatible trials were found; with longer SOAs up to 250 ms, faster responses in incompatible trials were found. It is unclear whether these results are specific to the special motion used therein or whether it generalizes to other motions. We therefore used other motions realized by arrangements of dots. In four experiments, we tested point-light displays (biological coherent walkers vs. less biological scrambled/split displays) as primes. In two experiments, eye gaze motions realized by moving dots representing irises and pupils (i.e., biological) versus the same motion either without surrounding face information or integrated in an abstract line drawing (i.e., less biological) were used. We found overall large positive compatibility effects with biological motion primes and also positive-but smaller-compatibility effects with less biological motion primes. Most important, also with very long SOAs (up to 1320 ms), we did not find evidence for negative compatibility effects. Thus, the pattern of positive-followed-by-negative-compatibility effects found in Bermeitinger (2013) seems to be specific to the materials used therein, whereas response priming in general seems an applicable tool to study motion perception.

Developmental tuning of reflexive attentional effect to biological motion cues

The human visual system is extremely sensitive to the direction information retrieved from biological motion. In the current study, we investigate the functional impact of this sensitivity on attentional orienting in young children. We found that children as early as 4 years old, like adults, showed a robust reflexive attentional orienting effect to the walking direction of an upright point-light walker, indicating that biological motion signals can automatically direct spatial attention at an early age. More importantly, the inversion effect associated with attentional orienting emerges by 4 years old and gradually develops into a similar pattern found in adults. These results provide strong evidence that biological motion cues can guide the distribution of spatial attention in young children, and highlight a critical development from a broadly- to finely-tuned process of utilizing biological motion cues in the human social brain.

Life motion signals lengthen perceived temporal duration

Point-light biological motions, conveying various different attributes of biological entities, have particular spatiotemporal properties that enable them to be processed with remarkable efficiency in the human visual system. Here we demonstrate that such signals automatically lengthen their perceived temporal duration independent of global configuration and without observers' subjective awareness of their biological nature. By using a duration discrimination paradigm, we showed that an upright biological motion sequence was perceived significantly longer than an inverted but otherwise identical sequence of the same duration. Furthermore, this temporal dilation effect could be extended to spatially scrambled biological motion signals, whose global configurations were completely disrupted, regardless of whether observers were aware of the nature of the stimuli. However, such an effect completely disappeared when critical biological characteristics were removed. Taken together, our findings suggest a special mechanism of time perception tuned to life motion signals and shed new light on the temporal encoding of biological motion.