The First Time Ever I Saw Your Feet: Inversion Effect in Newborns' Sensitivity to Biological Motion

摘要

Inversion effect in biological motion perception has been recently attributed to an innate sensitivity of the visual system to the gravity-dependent dynamic of the motion. However, the specific cues that determine the inversion effect in na?ve subjects were never investigated. In the present study, we have assessed the contribution of the local gravity-dependent motion (i.e., the orientation of individual dot trajectories) and the global configuration (i.e., the vertical location of the dots representing the feet within the display) in determining the inversion effect for biological motion in humans at birth. Results showed that 2-day-old newborns, at their 1st exposure to point-light displays, preferred a biological motion stimulus representing the legs of a walking animal compared with an identical display in which individual dot trajectories were locally inverted so that the motion violates the gravity force (Experiment 1). Interestingly, the global configuration affected the analysis of the gravity-dependent profile of dots motion. Indeed, the spontaneous preference disappeared when the local dots representing feet were embedded in a more complex global display and all the dots representing feet were located on the ground below the rest of the body (Experiment 2). Finally, results revealed that the orientation of the global configuration per se is not a crucial factor in determining newborns' preference (Experiment 3). These results suggest that humans possess an inborn predisposition about the direction of the gravity force that imprints biological motion and supports the hypothesis that a mechanism for the detection of biological motion is already at work before visual experience.