Communicative constraints affect oro-facial gestures and acoustics: Whispered vs normal speech

The present paper investigates a relationship between the acoustic signal and oro-facial expressions (gestures) when speakers (i) speak normally or whisper, (ii) do or do not see each other, and (iii) produce questions as opposed to statements. To this end, we conducted a motion capture experiment with 17 native speakers of German. The results provide partial support to the hypothesis that the most intensified oro-facial expressions occur when speakers whisper, do not see each other, and produce questions. The results are interpreted in terms of two hypotheses, i.e., the "hand-in-hand" and "trade-off" hypotheses. The relationship between acoustic properties and gestures does not provide straightforward support for one or the other hypothesis. Depending on the condition, speakers used more pronounced gestures and longer duration compensating for the lack of the fundamental frequency (supporting the trade-off hypothesis), but since the gestures were also enhanced when the listener was invisible, we conclude that they are not produced solely for the needs of the listener (supporting the hand-in-hand hypothesis), but rather they seem to help the speaker to achieve an overarching communicative goal.