Speech sequencing in the human precentral gyrus.

Fluent speech production is mediated by serially ordering and preparing motor plans corresponding to target speech sounds, a process known as speech-motor sequencing. Here we used high-density direct cortical recordings while 14 participants spoke utterances with varying phonemic and syllabic sequence complexity after reading a target sequence and a delay period. Phasic activations corresponding to speech production and auditory feedback were observed, but also sustained neural activity that persisted throughout all task phases including the target presentation, the delay period and production of the sequence. Furthermore, sustained activity in a specific area, the middle precentral gyrus (mPrCG), was both modulated by sequence complexity and predicted reaction time, suggesting a role in speech-motor sequencing. Electrocortical stimulation of the mPrCG caused speech disfluencies resembling those seen in apraxia of speech. These results suggest that speech-motor sequencing is mediated by a distributed cortical network in which the mPrCG plays a central role.