Cortical mechanism of communicative speech production.

Communicative speech requires conformity not only to linguistic rules but also to behavior that is appropriate for social interaction. The existence of a special brain mechanism for such behavioral aspects of communicative speech has been suggested by studies of social impairment in autism, and it may be related to communicative vocalization in animals. We used functional magnetic resonance imaging (fMRI) to measure cortical activation while normal subjects casually talked to an actor (communication task) or verbally described a situation (description task) while observing video clips of an action performed by a familiar or an unfamiliar actor in a typical daily situation. We assumed that the communication task differed from the description task in the involvement of behavioral aspects of communicative speech production, which may involve the processing of interaction-relevant biographical information. Significantly higher activation was observed during the communication task than during the description task in the medial prefrontal cortex (polar and dorsal parts), the bilateral anterior superior temporal sulci, and the left temporoparietal junction. The results suggest that these regions play a role in the behavioral aspects of communicative speech production, presumably in understanding of the context of the social interaction. The activation of the polar part of the medial prefrontal cortex during the communication task was greater when the actor was familiar than when the actor was unfamiliar, suggesting that this region is involved in communicative speech production with reference to biological information. The precuneus was activated during the communication task only with the familiar actor, suggesting that this region is related to access to biographical information per se.