Auditory plasticity and speech motor learning

Abstract

Is plasticity in sensory and motor systems linked? Here, in the context of speech motor learning and perception, we test the idea sensory function is modified by motor learning and, in particular, that speech motor learning affects a speaker's auditory map. We assessed speech motor learning by using a robotic device that displaced the jaw and selectively altered somatosensory feedback during speech. We found that with practice speakers progressively corrected for the mechanical perturbation and after motor learning they also showed systematic changes in their perceptual classification of speech sounds. The perceptual shift was tied to motor learning. Individuals that displayed greater amounts of learning also showed greater perceptual change. Perceptual change was not observed in control subjects that produced the same movements, but in the absence of a force field, nor in subjects that experienced the force field but failed to adapt to the mechanical load. The perceptual effects observed here indicate the involvement of the somatosensory system in the neural processing of speech sounds and suggest that speech motor learning results in changes to auditory perceptual function.sensorimotor adaptation ͉ speech perception ͉ speech production A s a child learns to talk, or as an adult learns a new language, a growing mastery of oral fluency is matched by an increase in the ability to distinguish different speech sounds (1-5). Although these abilities may develop in isolation, it is also possible that speech motor learning alters a speaker's auditory map. This study offers a direct test of this hypothesis, that speech motor learning, and, in particular, somatosensory inputs associated with learning, affect the auditory classification of speech sounds (6-8). We assessed speech learning by using a robotic device that displaced the jaw and modified somatosensory input without altering speech acoustics (9-11). We found that even though auditory feedback was unchanged over the course of learning, subjects classify the same speech sounds differently after motor learning than before. Moreover, the perceptual shift was observed only in subjects that displayed motor learning. Subjects that failed to adapt to the mechanical load showed no perceptual shift even though they experienced the same force field as subjects that showed learning. Our findings are consistent with the idea that speech learning affects not only the motor system but also involves changes to sensory areas of the brain. ResultsTo explore the idea that speech motor learning affects auditory perception, we trained healthy adults in a force-field learning task (12, 13) in which a robotic device applied a mechanical load to the jaw as subjects repeated aloud test utterances that were chosen randomly from a set of four possibilities (bad, had, mad, sad) (Fig. 1). The test utterances were displayed on a computer monitor that was placed in front of the subjects. The mechanical load was velocity-dependent and acted to displace the jaw in a protrusion direction, al...

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