Abstracts

Rationale: Insular cortex is a common activation in fMRI studies of speech and language and through lesion studies has been linked to several speech and orofacial motor control disorders including apraxia of speech (AOS). Lesions with a greater overlap with the insula are linked to a greater number of speech errors suggesting insula is involved in pre-articulatory planning. However, recent studies have challenged this, suggesting the inferior frontal gyrus is the true region responsible for this phenomenon. Here we investigate the role of the human insula in speech production in epileptic patients undergoing intracranial seizure mapping. Methods: Broadband gamma activity (70-150 Hz) was measured from stereotactically placed depth electrodes in the insulae of epileptic patients (n=24) undergoing extraoperative seizure mapping for pharmacoresistant epilepsy. Patients underwent two speech production tasks, repeating words or naming common objects from pictures. From this we analysed insular activity during speech production and compared activity during mono and multi-syllabic word articulations. Patients were also tested while listening to speech and during silent mouth movements to decompose the different aspects of speech. Results: We find that during speech production, posterior insula, particularly the anterior long gyrus showed no significant pre-articulatory activity and instead activates after the onset of articulation, at a similar timescale to the superior temporal gyrus. The posterior insula was also responsive while listening to externally produced speech and during non-speech mouth movements suggesting both auditory and somatosensory sensitivity. By contrast, the main body of the anterior insula showed little overall activity, limited mostly to the superior sulcus and the frontal operculum. Neither region showed a distinction relating to the complexity of the speech articulation. Conclusions: Our results suggest posterior insula is involved in processing both auditory and somatosensory feedback during speech production and may act as a multi-sensory integration region. Anterior insula, however, does not appear to show substantial functional involvement during speech production. These results are important for the understanding of the neural pathways of speech production, suggesting insula is not a preparatory region but instead part of the feedback network, which in turn has implications for rehabilitation of people with speech disorders such as AOS or patients who have undergone neurosurgical treatment of insular epilepsy. Funding: This work was supported by the NIH National Institute on Deafness and Other Communication Disorders grants NS098981 and DC014589.