The primary objective of this study was to investigate contributions of the auditory and somatosensory modalities to speech motor control. In Task I, a sensorimotor adaptation experiment was conducted by perturbing one of these two sensory modalities or both modalities simultaneously. During productions of monosyllabic CVC words, the first formant frequency (F1) in the auditory feedback was shifted up by a real-time digital processor and/or the extent of jaw opening was increased or decreased with a force field applied by a robotic device (and, thus, the associated somatosensory feedback was perturbed). All eight subjects lowered F1 in their productions to compensate for the up-shifted F1 in the feedback signal regardless of whether or not the jaw was perturbed. These adaptive changes in subjects' acoustic output resulted from adjustments in the articulatory movements of the jaw or tongue. However, adaptation in jaw opening extent in response to the mechanical perturbation occurred only when no auditory feedback perturbation was applied or when the direction of adaptation to the force was compatible with the direction of adaptation to a simultaneously applied acoustic perturbation. In Tasks II and III, subjects' auditory precision and accuracy for F1 frequency and somatosensory precision and accuracy for jaw position were estimated using a psychophysical approach (method of adjustment). Correlation analyses showed that the relationships (a) between F1 adaptation extent and auditory acuity for F1 and (b) between jaw position adaptation extent and somatosensory acuity were weak and statistically not significant for the present sample size.
Overall, findings suggest that, in speech production, sensorimotor adaptation updates the underlying control mechanisms in such a way that vowel-related articulatory movements aim to achieve primarily acoustic goals rather than somatosensory goals.
