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Feeling ticklish? The part of the brain that registers tickling has been identified in rats, and simply activating cells there is enough to make them giggle.

It has long been known that rats love to be tickled on the belly, and that it makes them laugh. These giggles take the form of ultrasound shrieks, above the level of human hearing. “It’s remarkable the similarities between rats and humans – the fact they vocalise and clearly enjoy tickling so much,” says Michael Brecht at the Humboldt University of Berlin, Germany.

By implanting electrodes into the somatosensory cortex – the brain region that registers touch – Brecht and his team have identified the neurons that activate such sensations. “We managed to pinpoint the ticklish spot in the brain,” says Brecht, who also found that rats enjoy being tickled on their feet.

Unexpectedly, the researchers also discovered that rats only enjoy being tickled if they are unstressed and in the right mood for it. While standing under a bright light, a rat’s “tickle” cells were much less active and their response more muted. “This is a new finding,” says Brecht. “We have relatively little knowledge of how mood dictates responses of the brain.”

Social bonding

The same neurons were also active when the researchers played a game in which the rats chased a researcher’s hand after being tickled. The animals jumped joyfully after tickling, says Brecht. “They hop around and jump up really high.”

This seems to link tickling with play, leading Brecht to suggest that tickling might have an evolutionary role in social interaction and bonding. “Tickling really is a weird phenomenon, so is it a trick by the brain to encourage sociality and playfulness?” he asks.

Brecht’s research may also explain why we can’t tickle ourselves. He and his team saw that the cells involved became active moments before tickling actually started, and he thinks this anticipation may only occur during social interaction with another individual.

Other studies in people have suggested that the cerebellum may be involved in blocking tickling sensations in individuals who try to tickle themselves. “I think the tickle response is profoundly social, aimed at others, not yourself,” says Brecht.

We still don’t know why only certain animals – including dogs and apes too – respond joyfully to tickling. “We tried to get mice to respond, but they wouldn’t,” says Brecht. “That was surprising, given the reactions of rats.”

Journal reference: Science, DOI: