Cortical dynamics underlying speech planning in rapid conversational turn-taking

Abstract

During conversation, silent gaps between speakers are typically 200ms or less—suggesting that speech planning often occurs while speakers listen to their partners’ turns. Though the psycholinguistic mechanisms of planning during turn-taking are well-studied, its neural underpinnings are largely unknown. Using intracranial electrocorticography in neurosurgical patient-volunteers, we delineated planning-related activity from dynamics underlying sensorimotor processes using an interactive question–answer paradigm (adapted from Bögels et al., 2015) and found that planning activity is largely restricted to a left hemisphere network centered on caudal inferior and middle frontal gyri. In a separate task, participants performed both speech and non-speech actions, and we found that this planning network is most active while planning spoken output. We then examined neural activity during unconstrained conversation and found that the identified planning circuit is active prior to participant turn onset and while participants listen to the turns of conversational partners. Finally, in preliminary direct cortical stimulation experiments, we found that perturbation of this planning network results in significantly longer reaction times and lexical errors but not gross articulatory disruptions. In conclusion, using controlled interactive language tasks, we uncovered a speech-selective cortical planning circuit that is active during natural conversation and required to execute rapid turn-taking.