The ability to derive the facing direction of a spatially scrambled point-light walker relies on the motions of the feet and is impaired if they are inverted. We exploited this local inversion effect in three experiments that employed novel stimuli derived from only fragments of full foot trajectories. In Experiment 1, observers were presented with stimuli derived from a single fragment or a pair of counterphase fragments of the foot trajectory of a human walker in a direction discrimination task. We show that direction can be retrieved for displays as short as 100 ms and is retrieved in an orientation-dependent manner only for stimuli derived from the paired fragments. In Experiment 2, we investigated direction retrieval from stimuli derived from paired fragments of other foot motions. We show that the inversion effect is correlated with the difference in vertical acceleration between the constituent fragments of each stimulus. In Experiment 3, we compared direction retrieval from the veridical human walker stimuli with stimuli that were identical but had accelerations removed. We show that the inversion effect disappears for the stimuli containing no accelerations. The results suggest that the local inversion effect is carried by accelerations contained in the foot motions.

The perception of biological motion is subserved by both a global process that retrieves structural information and a local process that is sensitive to individual limb motions. Here, we present an experiment aimed to characterize these two mechanisms psychophysically. Naive observers were tested on one of two tasks. In a walker detection task designed to address global processing, observers were asked to discriminate coherent from scrambled walkers presented in separate intervals. In an alternate direction discrimination task designed to address primarily local processing, observers were asked to discriminate walking direction from both coherent and spatially scrambled displays. In both tasks, we investigated performance-specificity to human (versus non-human) motion and the effects of mask density and learning on task performance. Performance in the walker detection task was best for the human walker, was susceptible to learning, and was heavily hindered by increasing mask densities. In contrast, performance on the direction discrimination task, in particular for the scrambled walkers, was unaffected by walker type, did not show a learning trend, and was relatively robust to masking noise. These findings suggest that the visual system processes global and local information contained in biological motion via distinct neural mechanisms that have very different properties.

We present three experiments that investigated the perception of animacy and direction from local biological motion cues. Coherent and scrambled point-light displays of humans, cats, and pigeons that were upright or inverted were embedded in a random dot mask and presented to naive observers. Observers assessed the animacy of the walker on a six-point Likert scale in Experiment 1, discriminated the direction of walking in Experiment 2, and completed both the animacy rating and the direction discrimination tasks in Experiment 3. We show that like the ability to discriminate direction, the perception of animacy from scrambled displays that contain solely local cues is orientation specific and can be well-elicited within exposure times as short as 200 ms. We show further that animacy ratings attributed to our stimuli are linearly correlated with the ability to discriminate their direction of walking. We conclude that the mechanisms responsible for processing local biological motion signals not only retrieve locomotive direction but also aid in assessing the presence of animate agents in the visual environment.

SummaryVisually guided behavior depends on (1) extracting and (2) discriminating signals from complex retinal inputs, and these perceptual skills improve with practice [1]. For instance, training on aerial reconnaissance facilitated World War II Allied military operations [2]; analysts pored over stereoscopic photographs, becoming expert at (1) segmenting pictures into meaningful items to break camouflage from (noisy) backgrounds, and (2) discriminating fine details to distinguish V-weapons from innocuous pylons. Training is understood to optimize neural circuits that process scene features (e.g., orientation) for particular purposes (e.g., judging position) [3–6]. Yet learning is most beneficial when it generalizes to other settings [7, 8] and is critical in recovery after adversity [9], challenging understanding of the circuitry involved. Here we used repetitive transcranial magnetic stimulation (rTMS) to infer the functional organization supporting learning generalization in the human brain. First, we show dissociable contributions of the posterior parietal cortex (PPC) versus lateral occipital (LO) circuits: extracting targets from noise is disrupted by PPC stimulation, in contrast to judging feature differences, which is affected by LO rTMS. Then, we demonstrate that training causes striking changes in this circuit: after feature training, identifying a target in noise is not disrupted by PPC stimulation but instead by LO stimulation. This indicates that training shifts the limits on perception from parietal to ventral brain regions and identifies a critical neural circuit for visual learning. We suggest that generalization is implemented by supplanting dynamic processing conducted in the PPC with specific feature templates stored in the ventral cortex.