Greta. a Believable Embodied Conversational Agent

A wide area of research on Autonomous Agents is presently devoted to the construction of ECAs, Embodied Conversational Agents (Cassell et al. 2000; Pelachaud & Poggi, 2001). An ECA is a virtual Agent that interacts with a User or another Agent through multimodal communicative behavior. It has a realistic or cartoon-like body and it can produce spoken discourse and dialogue, use voice with appropriate prosody and intonation, exhibit the visemes corresponding to the words uttered, make gestures, assume postures, produce facial expression and communicative gaze behavior. An ECA is generally a Believable Agent, that is, one able to express emotion (Bates, 1994) and to exhibit a given personality (Loyall & Bates, 1997). But, according to recent literature (Trappl & Payr, in press; de Rosis et al., in press a), an Agent is even more believable if it can behave in ways typical of given cultures, and if it has a personal communicative style (Canamero & Aylett, in press; Ruttkay et al., in press). This is, in fact, what makes a human a human. More, an ECA must be interactive, that is, take User and context into account, so as to tailor interaction onto the particular User and context at hand. In an ECA that fulfils these constraints the communicative output, that is, the particular combination of multimodal communicative signals displayed (words, prosody, gesture, face, gaze, body posture and movements) is determined by different aspects: a. contents to communicate, b. emotions, c. personality, d. culture, e. style, f. context and User sensitivity. At each moment of a communicative interaction, all of these aspects combine with each other to determine what the Agent will say, and how. In this paper we show how these aspects of an ECA can be modeled in terms of a belief and goal view of human communicative behavior. We then illustrate Greta, an ECA following these principles which is being implemented in the context of the EU project MagiCster.