Left-hemisphere cortical language regions respond equally to observed dialogue and monologue

Much of the language we encounter in our everyday lives comes in the form of conversation, yet the majority of research on the neural basis of language comprehension has used input from only one speaker at a time. 20 adults were scanned while passively observing audiovisual conversations using functional magnetic resonance imaging. In a block-design task, participants watched 20-second videos of puppets speaking either to another puppet (the "dialogue" condition) or directly to the viewer ("monologue"), while the audio was either comprehensible (played forward) or incomprehensible (played backward). Individually functionally-localized left-hemisphere language regions responded more to comprehensible than incomprehensible speech but did not respond differently to dialogue than monologue. In a second task, participants watched videos (1-3 minutes each) of two puppets conversing with each other, in which one puppet was comprehensible while the others speech was reversed. All participants saw the same visual input but were randomly assigned which characters speech was comprehensible. In left-hemisphere cortical language regions, the timecourse of activity was correlated only among participants who heard the same character speaking comprehensibly, despite identical visual input across all participants. For comparison, some individually-localized theory of mind regions and right hemisphere homologues of language regions responded more to dialogue than monologue in the first task, and in the second task, activity in some regions was correlated across all participants regardless of which character was speaking comprehensibly. Together, these results suggest that canonical left-hemisphere cortical language regions are not sensitive to differences between observed dialogue and monologue.