Cortical Response Variation with Social and Non-Social Affective Touch Processing in the Glabrous and Hairy Skin of the Leg: A Pilot fMRI Study

Abstract

Despite the crucial role of touch in social development and its importance for social interactions, there has been very little functional magnetic resonance imaging (fMRI) research on brain mechanisms underlying social touch processing. Moreover, there has been very little research on the perception of social touch in the lower extremities in humans, even though this information could expand our understanding of the mechanisms of the c-tactile system. Here, variations in the neural response to stimulation by social and non-social affective leg touch were investigated using fMRI. Participants were subjected to slow a (at 3–5 cm/s) stroking social touch (hand, skin-to-skin) and a non-social touch (peacock feather) to the hairy skin of the shin and to the glabrous skin of the foot sole. Stimulation of the glabrous skin of the foot sole, regardless of the type of stimulus, elicited a much more widespread cortical response, including structures such as the medial segment of precentral gyri, left precentral gyrus, bilateral putamen, anterior insula, left postcentral gyrus, right thalamus, and pallidum. Stimulation of the hairy skin of the shin elicited a relatively greater response in the left middle cingulate gyrus, left angular gyrus, left frontal eye field, bilateral anterior prefrontal cortex, and left frontal pole. Activation of brain structures, some of which belong to the “social brain”—the pre- and postcentral gyri bilaterally, superior and middle occipital gyri bilaterally, left middle and superior temporal gyri, right anterior cingulate gyrus and caudate, left middle and inferior frontal gyri, and left lateral ventricle area, was associated with the perception of non-social stimuli in the leg. The left medial segment of pre- and postcentral gyri, left postcentral gyrus and precuneus, bilateral parietal operculum, right planum temporale, left central operculum, and left thalamus proper showed greater activation for social tactile touch. There are regions in the cerebral cortex that responded specifically to hand and feather touch in the foot sole region. These areas included the posterior insula, precentral gyrus; putamen, pallidum and anterior insula; superior parietal cortex; transverse temporal gyrus and parietal operculum, supramarginal gyrus and planum temporale. Subjective assessment of stimulus ticklishness was related to activation of the left cuneal region. Our results make some contribution to understanding the physiology of the perception of social and non-social tactile stimuli and the CT system, including its evolution, and they have clinical impact in terms of environmental enrichment.