Control of movement precision in speech production

This chapter reviews evidence that somatosensory precision in speech is also important to the nervous system and appears to be achieved through impedance or stiffness control. A robotic device was used to apply lateral loads to the jaw that altered the motion path and hence somatosensory feedback without affecting speech acoustics. The loads were designed to maximally affect the consonant or vowel-related portion of an utterance. With training subjects corrected for both vowel and consonant-related loads, such that the motion path and presumably the associated somatosensory input returned to that normally experienced under no-load conditions. A control study was run in which subjects first trained with vowel-related loads and then following adaptation the direction of load was reversed unexpectedly. The reversal of the load resulted in deflections that were comparable in magnitude to those observed at the end of adaptation. The findings indicate that even in the absence of any effect on speech acoustics, somatosensory precision is equally important for vowel-related movements and for consonants. The adaptation observed here was achieved by impedance control. The results are consistent with the idea that impedance control is used in attaining the precision requirements of orofacial movement in speech.