Speech motor learning without hearing

Abstract

The nervous system receives both auditory and somatosensory information as we talk. Little is known about the role of somatosensory input in speech motor control. It is indeed a longstanding puzzle how post-lingually deaf speakers, in the absence of any substantial auditory input, maintain speech intelligibility. In this study we have assessed the role of somatosensory feedback in subjects with cochlear implants who were tested with their implants turned off. We used a robotic device to alter speech movements, and hence somatosensory feedback, by delivering mechanical loads to the jaw. We found that with training subjects corrected for the loads, such that the motion path approached that normally experienced under no-load conditions. Thus even under no auditory feedback, somatosensory input mediates speech movements in post-lingually deaf adults. In this study all five subjects with cochlear implants showed varying degrees of adaptation compared to only four out of six subjects in an age matched control group. This suggests a prominent role for somatosensory feedback in individuals with hearing loss. Finally, movement kinematic patterns for subjects with hearing loss were no different than those of subjects with normal hearing, which further underscores the reliance on somatosensory feedback in deaf adults.