Speech Motor Adaptation Without Auditory Feedback

The nervous system receives both auditory and somatosensory information while we talk. Speech production entails sensorimotor control and as such relies on multiple sensory inputs. The role of somatosensory input in speech motor control is little understood. Previous studies seeking to identify a somatosensory basis to speech motor function have done so in the presence of auditory inputs. Hence any effects that were observed there could be attributed to the presence of the auditory signal. Here we show that somatosensory input on its own may underlie speech production and speech motor learning. This is done by studying speech learning in cochlear implant recipients, tested with their implants turned off. Speech motor learning was assessed using a robotic device that applied forces and thus displaced the jaw and altered somatosensory feedback during speech. We found that with training implant subjects gradually adapted to the mechanical perturbation. The observed corrections were for movement deviations that were rather small, in the range of a few millimetres. This indicates that speakers have precise somatosensory expectations independent of auditory goals.