EUROSPEECH 2001 Scandinavia
When people in conversation refer to things, they achieve a joint focus of attention that enables them to be confident that they are both talking about the same thing. When they cannot see one another, such as over a telephone, they must use verbal means to do so. For instance, they may reuse the same expression upon repeated referring, marking the mutual belief that they have achieved a joint perspective; this process has been called entrainment. In contrast, when they can see one another, such as when they are physically co-present in the same environment, they can use visual evidence and deictic (pointing) strategies to formulate and ground references to objects. In this way, visual co-presence shapes conversation.
Bibliographic reference. Brennan, Susan E. (2001): "How visual co-presence and joint attention shape speaking", In EUROSPEECH-2001, K11-12.