Perception of biological motion from limited-lifetime stimuli

The visual perception of human movement from sparse point-light walkers is often believed to rely on local motion analysis. We investigated the role of local motion in the perception of human walking, viewed from the side, in different tasks. The motion signal was manipulated by varying point lifetime. We found the task of coherence discrimination, commonly used in biological motion studies, to be inappropriate for testing the role of motion. A task requiring temporal information showed a strong performance drop when fewer points were used or when the image sequence was sampled and displayed at a reduced frame rate. Irrespective of the frame rate, performance did not vary with point lifetime. We concluded that local motion is not required for the perception of tested biological movements, suggesting that the analysis of biological motion does not benefit from examining local motion. The reliance of perception on the number of displayed points and frames supports the idea that biological motion is perceived from a sequence of spatiotemporally sampled forms.     

Perception of biological motion: A stimulus set of human point-light actions

We present a set of stimuli representing human actions under point-light conditions, as seen from different viewpoints. The set contains 22 fairly short, well-delineated, and visually “loopable” actions. For each action, we provide movie files from five different viewpoints as well as a text file with the three spatial coordinates of the point lights, allowing researchers to construct customized versions. The full set of stimuli may be downloaded fromwww.psychonomic.org/archive/.     

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/.     

Perception of biological motion in common marmosets (Callithrix jacchus): by females only

The ability to perceive biological motion (BM) has been demonstrated in a number of species including humans but the few studies of non-human primates have been relatively inconclusive. We investigated whether common marmosets (Callithrix jacchus) are able to perceive biological motion, using a novel method to test non-human primates. Marmosets (7 male and 7 female) were trained to remove a cover from a container and look inside it, revealing a computer screen. Then they were presented with images on this computer screen consisting of a novel BM pattern (a walking hen) and 4 manipulations of that pattern (a static frame of this pattern and inverted, scrambled, and rotating versions of the pattern). The behavioural responses of the marmosets were recorded and used to assess discrimination between stimuli. BM was attended to by females but not males, as shown by active inspection behaviour, mainly movement of the head towards the stimulus. Females paid significantly less attention to all of the other stimuli. This indicates the females’ ability to attend to biological motion. Females showed slightly more attention to the inverted BM than to the static, scrambled, and rotating patterns. The males were less attentive to all of the stimuli than were the females and, unlike the females, responded to all stimuli in a similar manner. This sex difference could be due to an inability of males to recognise BM altogether or to a lesser amount of curiosity. Considered together with the findings of previous studies on chicks and humans, the results of the present study support the notion of a common mechanism across species for the detection of BM.     

Perception of motion affects language processing

Recently developed accounts of language comprehension propose that sentences are understood by constructing a perceptual simulation of the events being described. These simulations involve the re-activation of patterns of brain activation that were formed during the comprehender's interaction with the world. In two experiments we explored the specificity of the processing mechanisms required to construct simulations during language comprehension. Participants listened to (and made judgments on) sentences that described motion in a particular direction (e.g. "The car approached you"). They simultaneously viewed dynamic black-and-white stimuli that produced the perception of movement in the same direction as the action specified in the sentence (i.e. towards you) or in the opposite direction as the action specified in the sentence (i.e. away from you). Responses were faster to sentences presented concurrently with a visual stimulus depicting motion in the opposite direction as the action described in the sentence. This suggests that the processing mechanisms recruited to construct simulations during language comprehension are also used during visual perception, and that these mechanisms can be quite specific.     

Perception of three-dimensional structure from motion

The ability to perceive the 3-D shape of objects solely from motion cues is referred to as structure-from-motion perception. Recent experiments indicate how this remarkable perceptual attribute is computed by the brains of primates. This computation proceeds in at least two stages, one in which motion measurements are made and another in which moving surfaces are reconstructed. The middle temporal area (MT) in the macaque monkey appears to play a pivotal role in the latter step and suggests a previously unappreciated function for this well-known cortical region, which had previously been thought to play a more rudimentary role in simply signaling the direction of motion of images.     

Perception of motion in equiluminous kinematograms

Two fields of random dots that were identical except for a slight shift in a central square region were presented in rapid alternation. This produced a vivid impression of a square moving back and forth above the background. When the kinematogram is presented in equiluminous red/green, the motion of the central region can still be seen, although over a narrower range of alternation rates, interstimulus intervals, and displacements than for black/white presentation. The perception of motion for equiluminous stimuli indicates that colour and motion can be analyzed conjointly by the visual system. However, as originally reported by Ramachandran and Gregory, the segregation of the oscillating central square from the background is lost at equiluminance. This segregation process therefore appears to be colour-blind.     

Perception of chromatic motion requires luminance interaction

There is an ongoing debate related to whether chromatic motion perception arises as a consequence of a chromatic signal only (eg Wandell et al 1999 Neuron 24 901-909) or a signal that is essentially based on luminance processes (luminance artifacts) (Mullen et al 2003 Vision Research 43 1235-1247). These two views conform to the idea that colour and luminance processes are physiologically independent (Livingstone and Hubel 1988 Science 240 740-749), but according to other reports many primary cortical 'V1' cells respond to both colour and luminance contrast (eg Vidyasagar et al 2002 European Journal of Neuroscience 16 945-956). A psychophysical task was designed to test whether possible interaction between luminance and chromatic contrast could account for perception of chromatic motion. It is shown that subjects respond in a manner that reflects involvement of both processes.     

Perception of auditory motion affects language processing

Previous reports have demonstrated that the comprehension of sentences describing motion in a particular direction (toward, away, up, or down) is affected by concurrently viewing a stimulus that depicts motion in the same or opposite direction. We report 3 experiments that extend our understanding of the relation between perception and language processing in 2 ways. First, whereas most previous studies of the relation between perception and language processing have focused on visual perception, our data show that sentence processing can be affected by the concurrent processing of auditory stimuli. Second, it is shown that the relation between the processing of auditory stimuli and the processing of sentences depends on whether the sentences are presented in the auditory or visual modality.     

Perception of motion transparency in 5-month-old infants

We investigated the perceptual development of motion transparency in 3- to 5-month-old infants. In two experiments we tested a total of 55 infants and examined their preferential looking behaviour. In experiment 1, we presented transparent motion as a target, and uniform motion as a non-target consisting of random-dot motions. We measured the time during which infants looked at the target and non-target stimuli. In experiment 2, we used paired-dot motions (Qian et al, 1994 Journal of Neuroscience 14 7357-7366) as non-targets and also measured target looking time. We calculated the ratio of the target looking time to the total target and no-target looking time. In both experiments we controlled the dot size, speed, the horizontal travel distance of the dots, and the motion pattern of the dots. The results demonstrated that 5-month-old infants showed a statistically significant preference for motion transparency in almost all stimulus conditions, whereas the preference in 3- and 4-month-old infants depended on stimulus conditions. These results suggest that the sensitivity to motion transparency was robust in 5-month-olds, but not in 3- and 4-month-olds.     

Perception of the temporal pattern of motion in infancy

Two studies explored infants' perception of the temporal pattern of motion. Study 1 used a familiarization-novelty paradigm to demonstrate that 4-month-old infants discriminate two repetitive temporal patterns (viz., || || vs. ||| |). Study 2 used a preferential-looking paradigm longitudinally at 4 and 8 months with all the possible pairs of three repetitive patterns representing three levels of complexity, namely low (|| || || ||), medium (||| | ||| |), and high (||| || | ||). Visual preference shifted from the medium-complexity pattern at 4 months to the high-complexity pattern at 8 months. Moreover, evidence was obtained at 4 months that infants discriminated between all three patterns of motion. Unexpectedly, infants with complicated deliveries (inductions, forceps, or Caesarean sections) did not, as a group, distinguish among the patterns at 4 months.     

Simulating biological and non-biological motion

It is widely accepted that the brain processes biological and non-biological movements in distinct neural circuits. Biological motion, in contrast to non-biological motion, refers to active movements of living beings. Aim of our experiment was to investigate the mechanisms underlying mental simulation of these two movement types. Subjects had to either simulate mentally or to overtly reproduce previously executed or observed movements. Healthy subjects showed a very high timing precision when simulating biological and a strong distortion when simulating non-biological movements. Schizophrenic subjects, however, showed the opposite. Since overt reproduction was precise in any case, this double dissociation shows that processes underlying mental simulation of biological and non-biological movements are separate from each other and from perceptual and motor-control processes.     

Motion induction from biological motion

A new type of motion illusion is described in which ambiguous motion becomes unidirectional on superimposition of a human figure walking on a treadmill. A point-light walker in profile was superimposed on a vertical counterphase grating backdrop. Eleven na?ve observers judged the apparent direction of motion against the grating as left or right in a two-alternative forced-choice task and found that the grating appeared to drift in a direction opposite to the walking. The illusion disappeared when the point lights moved in scrambled configurations. This indicates that the illusion is caused by biological motion that provides recognition of gaits. A human figure walking backwards produced no illusion because of the difficulty in identifying the gait. This indicates that the illusion is determined by translational motion rather than form represented from biological motion.     

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.     

Perception of animacy from the motion of a single object

We demonstrate that a single moving object can create the subjective impression that it is alive, based solely on its pattern of movement. Our displays differ from conventional biological motion displays (which normally involve multiple moving points, usually integrated to suggest a human form) in that they contain only a single rigid object moving across a uniform field. We focus on motion paths in which the speed and direction of the target object change simultaneously. Naive subjects' ratings of animacy were significantly influenced by (i) the magnitude of the speed change, (ii) the angular magnitude of the direction change, (iii) the shape of the object, and (iv) the alignment between the principal axis of the object and its direction of motion. These findings are consistent with the hypothesis that observers classify as animate only those objects whose motion trajectories are otherwise unlikely to occur in the observed setting.     

The Thatcher effect in biological motion

We demonstrate the Thatcher effect in biological-motion displays and show that it is primarily a result of the moving, and not static, cues in the display.