Structural, neural and mental underpinnings of rat ticklishness

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The primary objective of this proposal is to determine the structural, neural, and mental underpinnings of rat ticklishness. Tickling, a peculiar and amusing form of social touch, has been discussed by philosophers and scientists for more than two millennia. Despite a historical interest in tickling as a topic, fundamental questions remain unanswered: Why does tickling induce laughter? Why do body parts differ in ticklishness? Why can we not tickle ourselves? Our previous work suggests that activity in layer 5 of trunk somatosensory cortex might be sufficient for evoking ticklishness in rats. We will build on these findings with three main goals in mind. (i) We will further corroborate that layer 5 of trunk somatosensory cortex harbors the neural substrate of ticklishness using pharmacological and optogenetic blocking techniques. (ii) We will identify the microcircuits mediating ticklishness and attempt to understand why some body parts are preferentially ticklish. (iii) We will ask, in neural terms, what makes tickling fun? We will investigate if rats will voluntarily self-administer electric tickling, i.e. microstimulation or optogenetic activation of the somatosensory cortex. Our electrical/optogenetic brain stimulation approach will allow imposing the same brain activity without external cues, in the presence of a weak cue (a still hand) or in the presence of a strong cue (a hand pretending to touch). This way the proposed experiments will elucidate how rats generate the idea of a tickling agent, a mental construct that greatly affects ticklish sensations.