David Ostry, Dept. of Psychology, McGill University, Montreal, Canada, and Haskins Laboratories, New Haven, CT
When we speak, we get correlated sensory feedback from speech sounds and the muscles and soft tissues of the vocal tract. While speech motor control may appear to be largely dependent on auditory input, this can only be tested if the contributions of auditory and somatosensory feedback can be dissociated. Here we identify the brain networks that underlie the somatic contribution to speech motor learning. The technique uses a robotic device that selectively alters somatosensory inputs during speech in combination with resting-state fMRI scans that reveal learning related changes in functional connectivity. A partial correlation procedure is applied to the functional connectivity map to identify connections that are altered with learning and cannot be explained by activity in other areas that are active in speech perception and production. In a behavioral analysis, subjects altered their speech movements to compensate for the somatosensory perturbation. Subjects’ classification of the speech sounds also changed even though the perturbation had no measurable effect on speech acoustics. The partial correlation analysis of the neuroimaging data revealed changes in connectivity related to both improvements in movement and to changes in auditory perception. It also revealed novel roles of cortical sensory and motor regions in speech motor adaptation. After removing the contribution of activation patterns attributable to other parts of the speech perception / production network, it was found that improvements in speech movements over the course of learning were due to connectivity changes in non-motor areas of brain, in particular, to a strengthening of connectivity between auditory and somatosensory cortex and between pre-supplementary motor area and the inferior parietal lobule. In contrast, changes in connectively associated with changes in auditory perception were restricted to speech motor areas, in particular, between primary motor cortex and the inferior frontal gyrus. Overall, it is seen that the somatosensory system on its own is responsible for extensive changes to the brain’s sensorimotor network over the course of speech motor adaptation.