Effects of Transcranial Magnetic Stimulation (TMS) on Somatosensory Perception

National Center For Complementary & Integrative Health

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Abstract

Study 1 tested the general hypothesis that activation of primary somatosensory cortex (S1) is necessary for discriminative aspects of tactile perception (where and when touch occurs), but is not necessary for affective appreciation of touch (the feeling of pleasantness/unpleasantness of touch). Study 2 tests the hypothesis that the posterior insula is necessary for affective touch but not touch discrimination. In Study 1 we used low frequency repetitive transcranial magnetic stimulation (rTMS) to temporarily reduce activity in the right hemisphere S1 of adult human volunteers. We then conducted sensory testing to examine the influence of this deactivation on spatial tactile discrimination and on the perceived intensity and affective valence (pleasantness) of gentle touch (using a soft brush). In Study 2, rTMS is conducted over the posterior insula. Healthy adult volunteers are recruited and pre-screened by telephone and a screening visit is conducted to determine eligibility. Eligible participants participate in a 2-hour MRI and questionnaire session and two rTMS and psychophysical testing sessions. In Session 1, a structural magnetic resonance imaging (MRI) and a functional magnetic resonance imaging (fMRI) scan are collected during hand movement and touch to assist with localization of the hand area for TMS and also to compare how the brain processes slow and fast touch on the palm versus back of the hand. In addition, participants complete several questionnaires about touch and personality traits. In Sessions 2 and 3, rTMS is performed over the target brain area to interfere with sensory processing (active condition) or over the vertex (top of the head; control condition) while tactile stimuli are presented and rated by the participant. If our hypothesis is correct, rTMS over S1, but not over the vertex, will alter sensory discrimination but will NOT alter affective ratings of touch, while rTMS over the posterior insula will alter affective ratings of touch but will not alter touch discrimination. Study 1 was recently published in the Journal of Neuroscience. We demonstrated that rTMS to S1 altered sensory discrimination but did not alter touch affect. In addition, we found a correlation between touch pleasantness and fMRI brain response in the anterior cingulate cortex, but not in S1. Study 2 was recently published in Neuroscience Letters. Our MRI results suggest that the rTMS affected only secondary somatosensory cortex, which overlies the posterior insula, and did not reach the insula. rTMS to S2 decreased intensity ratings of fast brushing, without altering touch pleasantness or spatial discrimination. MRI showed a reduced response to brushing after S2 rTMS. Together, our results show that reducing touch-evoked activity in S2 decreases perceived touch intensity, suggesting a causal role of S2 in touch intensity perception.