Researchers studying rats have identified neurons in the brain tied to ticklishness and laughter, and they were able to elicit a chuckle from the furry creatures by stimulating those same neurons in additional experiments. The rats were found to only "enjoy" ticklishness when they were in a good mood, similar to what happens in humans. Interestingly, the results hint that the somatosensory cortex - the brain region where the neurons were located - may play some role in mood (in the past, this brain region has been associated mainly with the sensation of touch). Though tickling is not mysterious to those on its receiving end, mechanisms underlying how this sensation reaches our brain are unclear. Building upon previous research in which it was discovered that rats being tickled emit laughter that can't be heard by humans, Shimpei Ishiyama and Michael Brecht decided to monitor neuron activity in the rat somatosensory cortex, a region particularly sensitive to touch, to gain more insights into this sensation. Consistent with earlier claims that tickling is rewarding, rats rapidly approached the tickling hand, and exhibited tickling-induced, unsolicited jumps ("joy jumps"), accompanied by ultrasonic giggling, when tickled. They also exhibited both increased laughter and neural firing rates in the deep layers of the somatosensory cortex. Stimulating somatosensory cortex neurons when the rats were not being tickled also evoked laughter, the researchers found. However, when the rats were in an anxious mood (raised high up on a platform, for example), tickling-evoked laughter and neural firing were significantly suppressed. The authors note that these findings support Darwin's idea that "the mind must be in a pleasurable condition" for ticklish laughter to occur.
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Science