The orofacial somatosensory system is modulated during speech planning and production

Purpose: In our previous studies, we showed that the brain modulates the auditory system, and the modulation starts during speech planning. However, it remained unknown whether the brain uses similar mechanisms to modulate the orofacial somatosensory system. Here, we developed a novel behavioral paradigm to (a) examine whether the somatosensory system is modulated during speech planning and (b) determine the somatosensory modulation’s time course during planning and production. Method: Participants (N = 20) completed two experiments in which we applied electrical current stimulation to the lower lip to induce somatosensory sensation. In the first experiment, we used a staircase method (one-up, four-down) to determine each participant’s perceptual threshold at rest (i.e., the stimulus that the participant detected on 85% of trials). In the second experiment, we estimated each participant’s detection ratio of electrical stimuli (with a magnitude equivalent of their perceptual threshold) delivered at various time points before speaking and during a control condition (silent reading). Results: We found that the overall detection ratio in the silent reading condition remained unchanged relative to the detection ratio at rest. Approximately 536 ms before speech onset, the detection ratio in the speaking condition was similar to that in the silent reading condition; however, the detection ratio in the speaking condition gradually started to decrease and reached its lowest level at 58 ms before speech onset. Conclusions: Overall, we provided compelling behavioral evidence that, as the speech motor system prepares speech movements, it also modulates the orofacial somatosensory system in a temporally specific manner.