This fMRI study examined which structures of a proposed dorsal stream system are involved in the auditory-motor integration during fast overt repetition. We used a shadowing task which requires immediate repetition of an auditory-verbal input and is supposed to elicit unconscious imitation effects of phonologically irrelevant speech parameters. Subjects' responses were recorded in the scanner. To examine automated auditory-motor mapping of speech gestures of others onto one's own speech production system we contrasted the shadowing of pseudowords produced by multiple speakers (men, women, and children) with the shadowing of pseudowords produced by a single speaker. Furthermore, we asked whether behavioral variables predicted changes in functional activation during shadowing. Shadowing multiple speakers compared to a single speaker elicited increased bilateral activation predominantly in the superior temporal sulci. These regions may mediate acoustic-phonetic speaker normalization in preparation of a translation of perceptual into motor information. Additional activation in Broca's area and the thalamus may reflect motor effects of the adaptation to multiple speaker models. Item-wise correlational analyses of response latencies with BOLD signal changes indicated that longer latencies were associated with increased activation in the left parietal operculum, suggesting that this area plays a central role in the actual transfer of auditory-verbal information to speech motor representations. A multiple regression of behavioral with imaging data showed activation in a right inferior parietal area near the temporo-parietal boundary which correlated positively with the degree of speech rate imitation and negatively with response latency. This activation may be attributable to attentional and/or paralinguistic processes.
