The idea that humans learn and maintain accurate speech by carefully monitoring auditory feedback is widely held. But this view neglects the fact that auditory feedback is highly correlated with somatosensory feedback during speech production. Somatosensory feedback from speech movements could be a primary means by which cortical speech areas monitor the accuracy of produced speech. We tested this idea by placing the somatosensory and auditory systems in competition during speech motor learning. To do this, we combined two speech learning paradigms to simultaneously alter somatosensory and auditory feedback in real-time as subjects spoke. Somatosensory feedback was manipulated by using a robotic device that altered the motion path of the jaw. Auditory feedback was manipulated by changing the frequency of the first formant of the vowel sound and playing back the modified utterance to the subject through headphones. The amount of compensation for each perturbation was used as a measure of sensory reliance. All subjects were observed to correct for at least one of the perturbations, but auditory feedback was not dominant. Indeed, some subjects showed a stable preference for either somatosensory or auditory feedback during speech.

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...

Speech production is dependent on both auditory and somatosensory feedback. Although audition may appear to be the dominant sensory modality in speech production, somatosensory information plays a role that extends from brainstem responses to cortical control. Accordingly, the motor commands that underlie speech movements may have somatosensory as well as auditory goals. Here we provide evidence that, independent of the acoustics, somatosensory information is central to achieving the precision requirements of speech movements. We were able to dissociate auditory and somatosensory feedback by using a robotic device that altered the jaw's motion path, and hence proprioception, without affecting speech acoustics. The loads were designed to target either the consonant- or vowel-related portion of an utterance because these are the major sound categories in speech. We found that, even in the absence of any effect on the acoustics, with learning subjects corrected to an equal extent for both kinds of loads. This finding suggests that there are comparable somatosensory precision requirements for both kinds of speech sounds. We provide experimental evidence that the neural control of stiffness or impedance--the resistance to displacement--provides for somatosensory precision in speech production.

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 on its own 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 with training implant subjects progressively adapted to the mechanical perturbation. Moreover, the corrections we observed were for movement deviations that were exceedingly small, on the order of millimetres, indicating that speakers have precise somatosensory expectations. Speech motor learning is significantly dependent on somatosensory input.

Nasir SM, Darainy M, Ostry DJ. Sensorimotor adaptation changes the neural coding of somatosensory stimuli. J Neurophysiol 109: 2077-2085, 2013. First published January 23, 2013 doi:10.1152/jn.00719.2012.-Motor learning is reflected in changes to the brain's functional organization as a result of experience. We show here that these changes are not limited to motor areas of the brain and indeed that motor learning also changes sensory systems. We test for plasticity in sensory systems using somatosensory evoked potentials (SEPs). A robotic device is used to elicit somatosensory inputs by displacing the arm in the direction of applied force during learning. We observe that following learning there are short latency changes to the response in somatosensory areas of the brain that are reliably correlated with the magnitude of motor learning: subjects who learn more show greater changes in SEP magnitude. The effects we observe are tied to motor learning. When the limb is displaced passively, such that subjects experience similar movements but without experiencing learning, no changes in the evoked response are observed. Sensorimotor adaptation thus alters the neural coding of somatosensory stimuli. motor learning; somatosensory evoked potential; reaching movement IS THE NEUROPLASTICITY THAT is associated with motor learning limited to motor areas of the brain, or do the effects of learning extend into nonmotor areas and notably into sensory systems? It is known that there are substantial anatomical interconnections linking the brain's motor and somatosensory regions. Cortical motor areas receive direct inputs from primary (Darian-Smith et al. 1993;Jones et al. 1978) and second somatosensory cortex (Cipolloni and Pandya 1999;Krubitzer and Kaas 1990) and from parietal areas 5 and 7 (Ghosh and Gattera 1995;Petrides and Pandya 1984). Somatosensory areas get direct cortical inputs from primary motor cortex (Darian-Smith et al. 1993;Jones et al. 1978;Krubitzer and Kaas 1990), premotor cortex (Cipolloni and Pandya 1999), and from supplementary motor area (Cipolloni and Pandya 1999;Jones et al. 1978). A change in somatosensory function in association with motor learning would seem to be a natural by-product of this anatomical connectivity. However, apart from behavioral studies (Cressman and Henriques 2009;Haith et al. 2008;Ostry et al. 2010;Wong et al. 2011) and a recent analysis of changes to resting-state networks in association with motor learning (Vahdat et al. 2011), there is little direct evidence that motor learning produces changes in sensory systems. Here we show that motor learning indeed alters the response of somatosensory areas of the brain. The changes we observe are substantially linked to motor learning in the sense that they vary in magnitude with motor learning, and they are not obtained when subjects passively experience the same movement kinematics, but do not experience learning.We studied motor learning using a force-field adaptation paradigm (Shadmehr and Mussa-Ivaldi 1994) in which subjects had to re...