Orofacial somatosensory inputs change the auditory categorization in perceptual adaptation training
Résumé
Orofacial somatosensory system intervenes in the perception of speech sounds. Recent study also showed that somatosensory input during speech motor learning can play an important role in the change of speech perception. However, it is still unclear that repeated exposure of somatosensory stimulation during speech perception can also induce an adaptive change of speech representation. This study examined whether speech perception system can be modified by perceptual training involving repeated exposure of orofacial somatosensory stimulation. We focused on the perceptual threshold between /e/ and /a/, and examined whether this perceptual threshold is changed between prior to and following the perceptual adaptation training. We tested 15 native speakers of French. The experiment consisted of three sessions. The first session was the test to measure baseline level of perceptual threshold. The second session was the perceptual adaptation training with somatosensory stimulation associated with facial skin deformation. The third session was the perceptual test again to measure aftereffects of the training. In the first and the third session, we applied an adaptive method based on the maximum-likelihood procedure to detect perceptual threshold by small number of trials. Seventeen trials are used to determine a perceptual threshold. We repeated four times of this test in one session. In the second session, we applied method of constant stimuli using 10 continuum between /e/ and /a/ in order to be exposed all stimulus variants equal number of times. We presented each stimulus in a pseudo-random order for 50 times. In this session, we also applied somatosensory stimulation when the stimulus sound was presented. Given that the target vowels (/e/ and /a/) are characterized by articulatory vertical movements, facial skin deformation was applied in the upward direction. In all sessions, subjects’ task was to identify whether the presented sound was /e/ or /a/ by pressing a key on a keyboard as quickly as possible. The stimuli were presented through headphones at a comfortable volume level. Perceptual thresholds between /e/ and /a/ were identified as the 50% point of the psychometric function estimated by fitting a logistic function. The threshold prior to perceptual adaption was obtained using responses of the last two blocks in the first session and the one following adaptation was obtained using the first two blocks in the third session. Two subjects were excluded since their changes were out of the range for average ± 3SD of the rest of group. When we compared perceptual thresholds prior to and following perceptual adaptation, the threshold changed to the direction of /e/ (-5.46 ± 1.04, average ± s.e.), indicating that the subjects perceived /a/ more than /e/ as an aftereffect of training. One-way ANOVA showed significantly different between sessions prior to and following the training (F(1, 12) = 27.83, p < 0.001). Our data suggest that repetitive exposure of somatosensory inputs during speech perception changes perceptual system and can support the idea that somatosensory input plays a role in speech adaptation.