The physiology of tickling Ed!

Yesterday, my coursemates and I took advantage of the last of the warming rays of sunshine from the recent good weather, and ventured into Holland Park for a picnic to celebrate the birthdays of the beautiful Camilla and Thea.

I don’t know whether it was the sun, the Pimms, or the fact that we weren’t revising, but we regressed… a lot! We played ‘duck, duck, goose’ (which is much more violent than I remember…!), ‘stuck in the mud’ and made human pyramids. I also discovered that another coursemate, Ed Prosser, is ticklish… very ticklish.

Did you know that there are two types of tickle? They were given names by psychologist Stanley Hall in 1897; there is the subtle feather-type tickle knismesis (light tickle) and the laughter-inducing tickle gargalesis (heavy tickle). But what is it that your are physically feeling when you are tickled? And why can’t we tickle ourselves? Aristotle even pondered on this!

“Is it because one also feels tickling by another person less if one knows beforehand that it is going to take place, and more if one does not foresee it? A man will therefore feel tickling least when he is causing it and knows that he is doing so. Now laughter is a kind of derangement and deception . . . that which comes unawares tends to deceive, and it is this also which causes the laughter, whereas one does not make oneself laugh.”

Tickling has also puzzled some of the greatest minds in scientific history from Plato to Bacon as well as Galileo and Darwin! We know that within the phenomenal organ that is our skin there is a plethora of receptors which are responsible for detecting the full range of sensations that we experience from searing heat to a feather-like touch. But the activation of these receptors only goes part of the way to explaining that squirming uncomfortable sensation of being tickled (uh oh I think I just admitted to being ticklish too…!). I came across this wonderful article by Dr Christine Harris from 1999 which is well worth a read. Here is my favourite section for your reading pleasure!

An Involuntary Response?

If ticklish laughter is not the same as our response to humor and not inherently an interpersonal response, then what is it? Some ideas are more biological than psychological. One possibility is that gargalesis is akin to a reflex, a view suggested at the turn of the century by G. Stanley Hall and advocated by physicians in more recent writings (Stearns 1977, Black 1982).

This brings us back to the question of why I cannot tickle myself. One might think that if the response is a reflex, capable of being elicited by a machine, we should be able to stimulate ourselves into paroxysms of laughter. We can, after all, produce a perfectly respectable knee-jerk reflex by tapping our own knees. There is, however, another phylogenetically ancient reflex that one definitely cannot elicit in oneself: startle. The violent startle reaction produced by a loud sound requires unpredictability, and it can be inhibited by even a very faint warning signal (Dawson et al. 1997). It may be that ticklish laughter, too, requires appropriate and vigorous stimulation that cannot be anticipated in advance, just as Aristotle proposed. In our tickling experiments, subjects laughed more when they had their eyes closed than when they had their eyes open.

Another possible explanation for why one cannot induce gargalesis in oneself is a neurological process observed in vision. The world doesn’t appear to jump every time you move your eyes, because the brain has taken into account the fact that it issued the command to move. Similarly, perhaps when the brain issues the command to tickle, it cancels out the sensation of ticklishness.

One study has examined the neural responses to self-generated tickle of the hand. Using functional magnetic-resonance imaging (fMRI), the investigators found that responses exhibited in the brain’s somatosensory cortex were different from those seen when the tickling was externally produced (Blakemore, Wolpert and Frith 1998). The study may or may not tell us about the specific inability to produce ticklish laughter in oneself, because it used light tickle (laughter would disrupt MRI scanning).

Given the unusual properties of tickle, though, some theorists have suggested that tickle is too complicated to be viewed as a simple reflex. Perhaps, as Robert Provine has suggested, it is a species-typical stereotyped motor pattern that requires a particular releasing stimulus or a fixed action pattern. It is a characteristic of reflexes that the response increases as stimulation becomes more intense, whereas fixed action patterns have an all-or-none quality. Although one study (McKimmin 1990) found that more intense tickling produced greater self-reports of ticklishness, it is unclear whether ticklish laughter follows the same pattern.

In sum, the results from the handful of studies done on tickle suggest that the inability to tickle oneself may reflect the inhibition of neural impulses at a relatively low physiological level—although the mechanism is undetermined. Our machine-tickle experiment hints that this inability does not have a merely interpersonal explanation, making physiological explanations more intriguing.

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