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This study compares task outcome and various dialogue parameters between situations in which task participants either could or could not see each other. The results establish that the visibility of one's conversational partner improves information transfer and the management of turn taking in a transactional problem solving task. The greater efficiency of the dialogues between participants who could see each other was attributed to the exchange of visually transmitted, non-verbal signals. attempting to compensate for the lack of this additional channel of communication, pairs of subjects who could not see each other demonstrated flexibility and versatility in communicating. They interrupted their partners more frequently and used more back channel responses to provide their partners with increased verbal feedback. The analysis of one specific non-verbal behaviour, gaze, for a subsample of the dialogues, suggested that gaze plays a role in aiding communication.

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Speakers are thought to articulate individual words in running speech less carefully whenever additional nonacoustic information can help listeners recognize what is said (Fowler & Housum, 1987;Lieberman, 1963).Comparing single words excerpted from spontaneous dialogues and control tokens of the same words read by the same speakers in lists, Experiment 1 yielded a significant but general effect of visual context: Tokens introducing 71 new entities in dialogues in which participants could see one another's faces were more degraded (less intelligible to 54 naive listeners) than were tokens of the same words from dialogues with sight lines blocked. Loss of clarity was not keyed to momentto-moment visual behavior. Subjects with clear sight lines looked at each other too rarely to account for the observed effect. Experiment 2 revealed that tokens of 60 words uttered while subjects were looking at each other were significantly less degraded (in length and in intelligibility to 72 subjects) vis-a-vis controls than were spontaneous tokens of the same words produced when subjects were looking elsewhere. Intelligibilityloss was mitigated only when listeners looked at speakers. Two separate visual effects are discussed, one of the global availability and the other of the local use of the interlocutor's face.No two tokens ofthe same word form are ever fully identical, even when they are uttered within seconds of each other by the same speaker. Explanations for these differences lie at every level oflinguistic description. Although most spoken words are fully recognizable within their linguistic contexts, some differences in pronunciation affect the intelligibility ofa word when it is excerpted from that context and presented as an isolated acoustic form. It has been argued that many such differences in pronunciation are based on the speaker's view of the listener's needs

The HCRC Map Task corpus has been collected and transcribed in Glasgow and Edinburgh, and recently published on CD-ROM. This effort was made possible by funding from the British Economic and Social Research Council. The corpus is composed of 128 two-person conversations in both high-quality digital audio and orthographic transcriptions, amounting to 18 hours and 150,000 words respectively. The experimental design is quite detailed and complex, allowing a number of different phonemic, syntactico-semantic and pragmatic contrasts to be explored in a controlled way. The corpus is a uniquely valuable resource for speech recognition research in particular, as we move from developing systems intended for controlled use by familiar users to systems intended for less constrained circumstances and naive or occasional users. Examples supporting this claim are given, including preliminary evidence of the phonetic consequences of second mention and the impact of different styles of referent negotiation on communicative efficacy.