Non-verbal visual cues accompany speech to supplement the meaning of spoken words, signify emotional state, indicate position in discourse, and provide back-channel feedback. This visual information includes head movements, facial expressions and body gestures. In this paper we describe techniques for manipulating both verbal and non-verbal facial gestures in video sequences of people engaged in conversation. We are developing a system for use in psychological experiments, where the effects of manipulating individual components of non-verbal visual behaviour during live face-to-face conversation can be studied. In particular, the techniques we describe operate in real-time at video frame-rate and the manipulation can be applied so both participants in a conversation are kept blind to the experimental conditions.
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