Auditory and somatosensory memory and speech motor learning
Résumé
Motor learning requires that one maintains sensory information about prior movements. In the case of speech motor learning, improvements in performance are likely reliant on both auditory and somatosensory memory, however their contributions to learning are presently unknown. To evaluate their role, we examined whether sensory memory performance explains differences between subjects in speech motor learning. We hypothesized that individual differences in auditory and somatosensory memory can predict speech motor adaptation. We carried out a speech motor adaptation task and somatosensory and auditory memory tasks using a within-subject design. We examined whether there was a correlation in performance between adaptation and memory tasks. As the stimulus utterances, we focused on a vowel // since this vowel can be changed acoustically to the vowels /e/ or /a/ simply by manipulating the first formant frequency, and those acoustical change can be related to vertical movement of speaking. For the speech motor learning task, we used an experimental model of speech adaptation that involved altered auditory feedback. In this procedure the first formant of the produced speech sound was altered in real-time and the altered sound was played back to speakers through headphones. The speech task was to produce the syllable /t/ which was altered so as to sound like /te/. Participants were expected to change their produced syllable towards the vowel /ta/ so as to maintain the feedback sound as /t/. In the auditory memory test, discriminable variants of the vowel /e/ were tested. In the somatosensory memory test, the orofacial skin was gently stretched in vertical direction. Somatosensory and auditory memory capacity were tested in separate sessions, but using the same experimental procedure, in which participants were required to identify whether a test stimulus had been presented earlier in a memory set of stimulations. As shown in previous work, we found that the magnitude of speech motor adaptation varied across the participants. Differences in adaptation were correlated with individual differences in somatosensory memory performance, but not auditory memory performance. The results suggest that even though speech is largely auditory in nature, adaptation itself may be more reliant on somatosensory inputs and memory processing.