See What Happens When You Tickle a Rat | National Geographic

Researchers at Humboldt University of Berlin have been trying to find out what happens in the brain when we're tickled. In 1999, scientists found young rats also vocalize when they're tickled. Are they actually laughing? What does a rat's voice sound like?

The typical hearing range of a human is 20 Hertz to 20 kilohertz. Rats vocalize at 20 to 100 kilohertz. They make long calls at 22 kilohertz when they're scared or annoyed, and when they're happy, they make short calls at 50 kilohertz. For example, when they interact with other rats and they are given food, a bit like when human voices go higher when we're excited or having fun.

So here's the sound a rat makes if you tickle its back. The sound has been converted to a lower frequency that we humans can hear, but the source frequency is mainly 50 kilohertz. Apparently, the rat likes being tickled not just because of the frequency of its calls, but because when the tickling stops, the rat doesn't turn away but looks to see where the hand has gone.

Rats recognize the researcher's hand as their playmate and approach the hand to be tickled more. Eventually, they start chasing the hand rapidly while vocalizing at 50 kilohertz. [Applause] Signals from the body surface reach a brain area called the somatosensory cortex, like a map of the entire body with distinct regions for each body part: trunk, front legs, back legs.

The researcher tickled the rats and measured the activity in the trunk region, which is the most ticklish part of a rat's body. Here is the activity of neurons in the trunk region when a rat is tickled. Thin vertical lines indicate electrical activity in the neuronal cells. [Applause] [Music] [Applause] Signals from the trunk are being sent to this brain region. Does this neural activity represent ticklishness?

If it is the case, activation of these neurons without actually tickling the body must be enough for the rat to feel ticklish. To test it, the researcher stimulated the neurons electrically. [Applause] Foreign calls at 50 kilohertz when its brain is stimulated. Thus, the researcher found out that ticklishness is represented by activity of the neurons in the somatosensory cortex.

I am worried that a lot of these guys are disappearing, and nowhere else on Earth does this organism exist. You have to go to the backs of valleys or distant ridge lines or isolated mountain ranges.