Validation of novel naturalistic limb movement stimuli for studying biological motion perception in adults.

Introduction: Understanding how we perceive and interpret the movements of other living beings is fundamental to social interaction, survival, and navigating our dynamic environment. Traditional studies of biological motion perception have predominantly employed simplified stimuli, such as point-light displays, which sacrifice ecological validity for experimental control. Methods: To overcome this limitation, we created a novel stimulus set featuring naturalistic videos of isolated hand and leg movements. These stimuli incorporated variations in viewing perspective (first-person vs. third-person) and the presence or absence of object interactions, thereby increasing their real-world relevance. We recorded electroencephalography (EEG) from 28 adult participants as they viewed these goal-directed movements alongside nature scenes that served as control stimuli. Neural engagement was assessed through two complementary measures: inter-subject correlation (ISC), which captures shared neural responses across viewers, and intra-subject correlation (IaSC), which reflects temporal consistency of responses within individual viewers. We hypothesized that (1) biological limb movements would elicit stronger neural synchrony than non-biological motion, (2) movement type (arm vs. leg), object interaction, and visual perspective would differentially modulate ISC based on distinct affordance-related processing systems, and (3) these effects would replicate upon second exposure. Results: Our findings revealed that lower-limb movements elicited significantly greater ISC compared to both upper-limb movements and nature scenes. This effect proved robust, replicating consistently when participants viewed the same experimental conditions a second time. Furthermore, specific stimulus features modulated neural synchronization in domain-specific ways: first-person perspective enhanced ISC specifically for leg videos, whereas object interaction increased ISC specifically for arm videos. Notably, IaSC showed no differences across conditions, revealing a dissociation between cross-individual neural alignment and within-individual temporal consistency. Discussion: These findings provide new insights into how particular features of naturalistic movement systematically modulate shared neural engagement, advancing our understanding of biological motion perception in ecologically valid contexts. The dissociation between ISC and IaSC suggests that cross-individual neural synchronization and within-individual temporal consistency reflect distinct neural processes.