J. Gratch et al. (Eds.): IVA 2006, LNAI 4133, pp. 28 – 41, 2006. © Springer-Verlag Berlin Heidelberg 2006 Imitation Learning and Response Facilitation in Embodied Agents Stefan Kopp and Olaf Graeser Artificial Intelligence Group, University of Bielefeld P.O.Box 100131, D-33501 Bielefeld, Germany {skopp, ograeser}@techfak.uni-bielefeld.de Abstract. Imitation is supposedly a fundamental mechanism for humans to learn new actions and to gain knowledge about another’s intentions. The basis of this behavior seems to be a direct influencing of the motor system by the perceptual system, affording fast, selective enhancement of a motor response already in the repertoire (response facilitation) as well as learning and delayed reproduction of new actions (true imitation). In this paper, we present an approach to attain these capabilities in virtual embodied agents. Building upon a computational motor control model, our approach connects visual representa- tions of observed hand and arm movements to graph-based representations of motor commands. Forward and inverse models are employed to allow for both fast mimicking responses as well as imitation learning. 1 Introduction Human children and adults effortlessly mimic and imitate others. They do so in a variety of situations and different types of imitative behavior can be distinguished [6]. Stimulus enhancement refers to the increased probability to act upon an object, on which another individual has acted in a similar way before. Response facilitation is the selective enhancement of a motor response already in the repertoire simply by seeing a conspecific performing an action. Finally, (true) imitation refers to the case in which an action that has not been part of the own repertoire before is learnt by observing others and can be reproduced with a possible time delay. One can observe all of these kinds of behavior already in human infants [12]. Yet, the question why we imitate each other so often is hard to answer. Many rationales or purposes have been put forward, among them the acquisition of new behaviors, the realization and signaling of the fact that another one is like me [11], or the mutual alignment and convergence in social interactions [13]. In addition, imitative behaviors such as mimicking gestures were suspected to gain knowledge about the intentions of conversational partners [4]. One central mechanism that seems to underlie imitative behavior is the selective influencing of the motor system by the perceptual system. These links open the possibility to equate an observed action with one’s own actions through an implicit form of simulating them in the motor system. With the finding of ‘mirror regions’ with neurons that respond to both self-generated actions and the actions of others [8],