Side-participants (SPs) in multiparty dialogue establish and maintain their status as currently non-contributing, but integrated partners of the conversation by continuing to track, and be seen to be tracking, the conversation. To investigate strategies for realising such 'active side-participant' behaviour, we constructed an experimental setting where a humanoid robot appeared to track (overhear) a two-party conversation coming out of loudspeakers. We equipped the robot with ‘eyes’(small displays) with movable pupils, to be able to separately control head-turning and gaze. Using information from the pre-processed conversations, we tested various strategies (random, reactive, predictive) for controlling gaze and head-turning. We asked human raters to judge videos of such tracking behaviour of the robot, and found that strategies making use of independent control of gaze and head direction were significantly preferred. Moreover, the 'sensible' strategies (reactive, predictive) were reliably distinguished from the baseline (random turning). We take this as indication that gaze is an important, semi-independent modality, and that our paradigm of off-line evaluation of overhearer behaviour using recorded interactions is a promising one for cost-effective study of more sophisticated tracking models, and can stand as a proxy for testing models of actual side-participants (whose presence would be known, and would influence, the conversation they are part of).
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