Does This Tickle?

Aristotle was puzzled by an interesting fact: Why can't humans tickle themselves? We have all experienced being tickled at some point or other, but do we understand how tickling works? This is something that some scientists are striving to understand, but not only because of simple curiosity. One tell-tale sign of Schizophrenia is the ability to tickle oneself. Also, tickling is linked to our ability to laugh, play, and simply feel good, so researching how tickling works adds to our understanding of positive and negative emotions, even depression.

Neuroscientists performed an experiment in which they tickled rats and studied their physical and neural responses. They found that the rats learned to enjoy the tickling and even came to think of the hand doing the tickling as their "playmate." The rats displayed a universal expression of positive emotion called "joy jumps" that are also seen in human children, dogs, foxes, and guinea pigs (to name just a few). To record neural activity, the scientists inserted electrodes into the somatosensory cortex of the rats. Cells in this area of the brain increased firing while the rats were being tickled, but also after the tickling as the rats chased the hand and "giggled." Interestingly, when the researchers applied an electrical current to these same cells, this stimulated the rats to behave as if they were being tickled (giggling and jumping playfully). This is important evidence that shows these cells are responsible for ticklishness.

As you might already know from experience, ticklishness also depends on mood. If we are in good moods we are more likely to laugh when being tickled and when we are in bad moods we are likely to complain when we are tickled. They also tested this on the rats by exposing nocturnal rats to a bright light (making them unhappy or anxious) and observing their responses to being tickled. They found the rats were much less ticklish and did not display the signs of playfulness that they did in the previous experiment. Also, the cells in that somatosensory cortex were suppressed, adding evidence to the finding that these cells are required for the typical tickle response. The researchers also came to the conclusion that our brains must form hard-wired connections early in life for the tickling sensation to be learned, and even potentially enjoyed. If we don't experience tickling when we are young, then we are much less likely to enjoy tickling later in life.

Link to video and review of this research:
http://www.sciencemag.org/news/2016/11/watch-these-ticklish-rats-laugh-and-jump-joy