Predicting human action with human and non-human movement kinematics: behavioural and fMRI findings

Abstract

Human observers perceive movement with typical human kinematics more sensitively than movement with non-human kinematics, even when its velocity profile is more uniform. In a behavioural experiment and a subsequent fMRI study, we tested the influence of movement kinematics on the accuracy of predicting the time course of another individual’s actions. A temporal occlusion paradigm was used in which participants observed short video sequences that were briefly occluded. The videos either showed a human point-light shape that was animated with human movement (natural condition) or with artificial movement (artificial condition). For generating the artificial movement, original human motion-capture data were manipulated to obtain movement that was more uniform regarding velocity profiles and trajectories. During brief occlusions, the participants predicted the actions and judged after occlusion whether the actions were continued coherently in time or shifted to an earlier or later frame. The results revealed higher error rates in the artificial compared to the natural condition, suggesting a perceptual advantage for movement with a human velocity profile. This corresponds to the notion of a close interaction between observed and executed movement. This notion was challenged in an fMRI experiment which employed the same stimuli in a prediction and in an observation condition. The observation condition required answering questions about movement details. Prediction in contrast to observation induced right premotor activation whereas observation strongly activated the left premotor cortex. The results suggest that the involvement of the motor system was modulated by task instructions. Interestingly, activation in several cortical areas, including the visual system, was modulated by the increasing divergence from human movement characteristics. An interpretation in the framework of the simulation account of action perception and its limitations will be discussed.