Speech motor learning in profoundly deaf adults

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Abstract
Speech production relies on both somatosensory input from the vocal tract and auditory input. Nasir and Ostry now show that in deaf individuals, somatosensory input alone can support speech motor learning. Speech production, like other sensorimotor behaviors, relies on multiple sensory inputs—audition, proprioceptive inputs from muscle spindles and cutaneous inputs from mechanoreceptors in the skin and soft tissues of the vocal tract. However, the capacity for intelligible speech by deaf speakers suggests that somatosensory input alone may contribute to speech motor control and perhaps even to speech learning. We assessed speech motor learning in cochlear implant recipients who were tested with their implants turned off. A robotic device was used to alter somatosensory feedback by displacing the jaw during speech. We found that implant subjects progressively adapted to the mechanical perturbation with training. Moreover, the corrections that we observed were for movement deviations that were exceedingly small, on the order of millimeters, indicating that speakers have precise somatosensory expectations. Speech motor learning is substantially dependent on somatosensory input.