Identifying critical language planning regions with direct electrical brain stimulation

Many brain areas exhibit activity correlated with language planning, but how such dynamics contribute to spoken interaction remains unclear. We employed direct electrical stimulation to perturb cortical function in neurosurgical patient-volunteers performing a question-answer task. Stimulating regions underlying speech perception and orofacial movement evoked diverse sensorimotor deficits, while errors consistent with disrupted response planning resulted from perturbations of numerous, largely frontotemporal loci. We then measured reaction time as an indicator of planning effort and found that stimulation could significantly alter answer latency. Sites displaying stimulation-induced slowed behavior were primarily located in caudal middle and inferior frontal gyri, areas that exhibited preparatory-related activity during conversational turn-taking in previous intracranial recordings. Taken together, our findings indicate that these areas constitute a network critical for planning speech. One-sentence summary Perturbations of caudal inferior and middle frontal gyri slow reaction times on an interactive speech task, implicating these areas in planning.