It is generally accepted that a robot should exhibit a contingent behavior, adaptable to the needs of each individual user, to achieve a more natural and pleasant interaction. In this paper we have evaluated whether this general rule applies also when the robot plays a leading role and needs to motivate the human partner to keep a certain pace, as during training or teaching. Also among humans, in schools or factories, structured interaction is often guided by a predefined rhythm, which facilitates the coordination of the partners involved and is thought to maximize their efficiency. On the other hand, a pre-established timing forces all participants to adjust their natural speed to the external, sometimes not appropriate, timing requirement. Where does the optimal trade-off between these two paradigms lie? We have addressed this question in a dictation scenario where the humanoid robot iCub plays the role of a teacher and dictates brief English or Italian sentences to the participants. In particular we compare a condition in which the dictation is performed at a fixed timing with a condition in which iCub monitors subjects' gaze to adjust its dictation speed. The results are discussed both in terms of participants' subjective evaluation and their objective performance, by highlighting the advantages and drawbacks of the choice of contingent robot behavior.
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