a b s t r a c tSpeakers tend to attenuate information that is predictable or repeated. To what extent is this done automatically and egocentrically, because it is easiest for speakers themselves, and to what extent is it driven by the informational needs of addressees? In 20 triads of naive subjects, speakers told the same Road Runner cartoon story twice to one addressee and once to another addressee, counterbalanced for order (Addressee1/Addressee1/ Addressee2 or Addressee1/Addressee2/Addressee1). Stories retold to the same (old) addressees were attenuated compared to those retold to new addressees; this was true for events mentioned, number of words, and amount of detail. Moreover, lexically identical expressions by the same speaker were more intelligible to another group of listeners when the expressions had been addressed to new addressees than when they had been addressed to old addressees. We conclude that speakers' attenuating of information in spontaneous discourse is driven at least in part by addressees. Such audience design is computationally feasible when it can be guided by a ''one-bit" model (my audience has heard this before, or not).

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a b s t r a c tWe examined whether people spontaneously represent the partner's viewpoint in spatial memory when it is available in advance and whether they adapt their spontaneous descriptions accordingly. In 18 pairs, Directors studied arrays of objects while: (1) not knowing about having to describe the array to a Matcher, (2) knowing about the subsequent description, and (3) knowing the Matcher's subsequent viewpoint, which was offset by 90°, 135°, or 180°. In memory tests preceding descriptions, Directors represented the Matcher's viewpoint when it was known during study, taking longer to imagine orienting to perspectives aligned with it and rotating their drawings of arrays toward it. Conversely, when Directors didn't know their Matcher's viewpoint, they encoded arrays egocentrically, being faster to imagine orienting to and to respond from perspectives aligned with their own. Directors adapted their descriptions flexibly, using partner-centered spatial expressions more frequently when misaligned by 90°and egocentric ones when misaligned by 135°. Knowing their misalignment in advance helped partners recognize when descriptions would be most difficult for Directors (at 135°) and to mutually agree on using their perspective. Thus, in collaborative tasks, people don't rely exclusively on their spatial memory but also use other pertinent perceptual information (e.g., their misalignment from their partner) to assess the computational demands on each partner and select strategies that maximize the efficiency of communication.

Are gesturing and speaking shaped by similar communicative constraints? In an experiment, we teased apart communicative from cognitive constraints upon multiple dimensions of speech-accompanying gestures in spontaneous dialogue. Typically, speakers attenuate old, repeated or predictable information but not new information. Our study distinguished what was new or old for speakers from what was new or old for (and shared with) addressees. In 20 groups of 3 naive participants, speakers retold the same Road Runner cartoon story twice to one addressee and once to another. We compared the distribution of gesture types, and the gestures' size and iconic precision across retellings. Speakers gestured less frequently in stories retold to Old Addressees than New Addressees. Moreover, the gestures they produced in stories retold to Old Addressees were smaller and less precise than those retold to New Addressees, although these were attenuated over time as well. Consistent with our previous findings about speaking, gesturing is guided by both speaker-based (cognitive) and addressee-based (communicative) constraints that affect both planning and motoric execution. We discuss the implications for models of co-speech gesture production.