Peripheral mechanisms of pleasurable touch

were first studied by Zotterman in cats and suggested that knismesis is carried in part by pain fibers (21). C-fibers, unmyelinated afferents, are putatively involved in pleasurable touch in rodents (22). Central mechanisms of tickling were investigated by functional magnetic resonance imaging (fMRI) in human brains (9); that study, which used tickling stimuli evoking knismesis and observedsomatosensorycortexactivation, suggested that self-tickle suppression might be mediated by the cerebellum. Recent human fMRI identified activation of the lateral hypothalamus, parietal operculum, amygdala, cerebellum, and somatosensory cortex by ticklish laughter (10). Four of our results localize tickle processing to the somatosensory cortex: (i) We found that tickling can evoke intense neuronal activity in the somatosensory cortex (Fig. 2). Moreover, play behavior, which induces anticipatory vocalizations in rats (Fig. 1, F to H) (17) and humans (23), evoked neuronal activity similar to the activity evoked by tickling (fig. S2E). (ii) We observed “mood-dependent” alteration of activity in the trunk somatosensory cortex, specifically an activity increase after tickling phases (Fig. 2, B and C), anxiogenic suppression of responses (Fig. 3, D and E), and a reduction of microstimulation thresholds for evoking calls after tickling. Such “mood-dependent” modulation of the somatosensory cortex is unexpected, as it is nontactile and there is little evidence to date for mood effects in other cortical areas. (iii) The strong callrelated activation of the trunk somatosensory cortex points to an involvement in ticklingevoked vocalizations. Call-related firing in the somatosensory cortex is much stronger than call-evoked activity in the auditory cortex (18). (iv) Microstimulation-evoked vocalizations suggest that deep-layer but not superficial-layer cortical activity is sufficient to trigger vocalizations. The short latencies of evoked calls indicate few intervening processing steps between trunk activity and calls. Electrical stimulation in various brain areas is known to evoke laughter with or without mirth (24–27), but stimulation-evoked ticklish laughter has not been reported so far, and our results might be different from previously reported laughter evoked by brain stimulation. In line with lesion evidence, our observations suggest a neural link among tickling, play, and the somatosensory cortex (28). Other findings have implicated the somatosensory cortex in social information processing (29–31). The observation that the somatosensory cortex is involved in the generation of tickling responses suggests that this area might be more closely involved in emotional processing than previously thought. Identification of the neural correlates of ticklishness will allow us to frame questions about tickling in neural terms and thus help us to understand this mysterious sensation.