Exp Brain Res (2008) 185:399–409 DOI 10.1007/s00221-007-1162-2 123 RESEARCH ARTICLE Inference of complex human motion requires internal models of action: behavioral evidence Ghislain Saunier · Charalambos Papaxanthis · Claudia D. Vargas · Thierry Pozzo Received: 26 April 2007 / Accepted: 28 September 2007 / Published online: 23 October 2007  Springer-Verlag 2007 Abstract Previous behavioral investigation from our laboratory (Pozzo et al. in Behav Brain Res 169:75–82, 2006) suggests that the kinematic features inXuence the subject’s capacity to estimate the Wnal position of simple arm movement in which the last part of the trajectory is hid- den. The authors argue the participation of internal infor- mation, as the kinematic parameters, to compensate the lack of the visual input. The purpose of this report was to verify if the dependency of visual motion inference to bio- logical displays can be generalized for intransitive and complex human motions. To answer this question, the sub- jects were asked to estimate the vanishing and Wnal position of the shoulder trajectory of Sit to Stand (STS) or Back to Sit (BTS) motion performed in the sagittal plane, according to a biological or nonbiological kinematics. The last part of the trajectory (i.e., 35%) was occluded. We observed a kinematic eVect on the precision of individuals’ estimation. The subjects were more precise and less variable to estimate the end trajectory with biological velocity proWles. Moreover, impoverished visual information appeared suYcient to evaluate the Wnal position of an intransitive complex human motion. These results suggest the participation of internal representations to infer the Wnal part of complex motion. We discuss the results in the light of possible neural substrates involved during the inference task. Keywords Motion inference · Internal models · Simulation · Mirror neurons · Complex intransitive motion Introduction How do we visually extrapolate the Wnal position of a bio- logical motion like for instance the Wnal position of a hand that reaches an object located behind a wall? In other words, what is the human ability to reconstruct and esti- mate the hidden part of a moving target? Neurophysiologi- cal evidence (Umiltà et al. 2001) suggests the participation of motor repertoires involving the mirror neurons (MNs) during observation of an occluded action. Localized in the monkey’s pre-motor area (area F5), these neurons have the characteristic of discharging both during action perception of goal directed task and the production of the same action (di Pellegrino et al. 1992; Gallese et al. 1996; Rizzolatti et al. 1996). The coexistence of motor and sensory proper- ties in the same neuron suggests that the motor cortex not only executes actions but also participates in the construc- tion of their representation (Fadiga et al. 2000). From this perspective, action observation would imply both in an implicit simulation and in implementation of motor pro- grams necessary to perform the same action (Jeannerod 2001). Indirect evidences point to the existence of a human MN system within the ventral pre-motor cortex and inferior parietal lobule (for reviews see Rizzolatti and Craighero G. Saunier · C. Papaxanthis · T. Pozzo INSERM-U887, Motricité-Plasticité, Campus Universitaire, BP 27877, 21078 Dijon, France C. Papaxanthis · T. Pozzo UFRSTAPS Campus Universitaire, Université de Bourgogne, BP 27877, 21078 Dijon, France G. Saunier (&) · C. D. Vargas Laboratório de Neurobiologia II, Instituto de Biofísica Carlos Chagas Filho, Universidade Federal de Rio de Janeiro, Rio de Janeiro, Brazil e-mail: ghis@biof.ufrj.br T. Pozzo Italian Institute of Technology, Genova, Italy