Difference in visual information between face to face and telephone dialogues

We analyzed conversations between a pair of subjects, under two conditions. One is face to face conversation that has visual contact, and the other is conversation through telephone lines that has not. From the recorded videotape we extracted the subject's actions especially focusing on the head movements. By comparing the dialogues under two conditions, it seems that there are two types of head movements, one is intended to give a response to his partner and the other is to send some signal. We analyze how the visual information contributes in spoken dialogue perception, and the possibility of adopting it in a multi-modal human interface.