Cortical somatosensory responses evoked by orofacial skin stretch experimentally applied during speech movement planning in stuttering and nonstuttering adults.

Prior findings indicate that individuals who stutter do not show the typical modulation of auditory processing that is observed during speech movement planning in nonstuttering speakers. We now ask whether this lack of planning-related sensory modulation in stuttering adults is specific to the auditory domain. In this first study (15 stuttering and 15 nonstuttering participants), we implemented the prior stimulation timeline in a paradigm with orofacial skin stretch stimuli. A robotic device applied skin stretches to elicit somatosensory evoked potentials during speech movement planning and a silent reading control condition. We compared the N1 component for and to assess a possible influence of movement planning on somatosensory processing. Both groups showed clear N1 responses, consistent with prior validation of the skin stretch stimulation. However, for the selected timepoint of stimulation (i.e., identical to our auditory stimulation studies) we found no evidence of pre-speech somatosensory modulation in either group. There were no amplitude or latency differences between speaking and control conditions and no between-group differences. Given that even typical participants showed no modulation at the probed times during movement planning, whereas some studies have obtained evidence supporting somatosensory modulation for orofacial movements during speech movement execution, (a) the time course of modulation may differ from that observed for auditory stimuli, or (b) input from the specific facial mechanoreceptors stimulated here may be not subject to pre-speech modulation. Future studies should probe somatosensation at different timepoints before and after speech movement onset and in effectors actively performing the articulatory gestures (lip, tongue).