Abstract
After seeing a moving object suddenly disappear, observers typically mislocate its final position to where that object would have been a few milliseconds later. This “forward displacement” (FD) is thought to reflect online predictions, by the visual system, of the likely future position of moving objects. Here, I will present the results of three studies that collectively demonstrate that FD elicited by body movement perception is modulated by implicit, unconscious, knowledge of actor-specific biomechanical constraints. In these studies, participants watched videos of two actors performing rotations of the right shoulder. In 80% of the trials (familiarization trials) videos depicted movements directed towards the body. Movements of the “flexible” actor started far from the body and movements of the “rigid” actor started closer from the body, reflecting the two actors’ different flexibility. In the remaining 20% of the trials (trials of interest), both actors performed the same movement directed away from the body. This movement was such that it would have been easy to continue for the “flexible” actor, but impossible to continue for the “rigid” actor. Participants had to indicate whether the arm of the actor depicted on a picture displayed shortly after the video disappeared was at the same position as at the end of the video. In the first study, participants were explicitly told that one actor was flexible and one was rigid. In the two other ones, they learned this information implicitly, through familiarization trials. In all three experiments, analyses of responses to the trials of interest indicated that there was more FD when participants observed the flexible than the rigid actor, and the effect was similar for participants who reported that they did not consciously notice a difference of flexibility between actors. Thus, unconscious knowledge of actor-specific biomechanical abilities affects the perceptual prediction of their movements.