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Expectation of sensory consequences explains inability to tickle oneself

By Richard A. Schmidt, PhD, and Timothy D. Lee, PhD.


Most of us have certain areas of our bodies that are particularly susceptible to tickling. Stimulation of these areas results in a mixture of feelings that often includes both excitement and discomfort. Perhaps of some surprise, the act of tickling has become a study of interest for motor control scientists! The reason is simple: Although we respond to tickles that are given by someone else, we do not respond when we attempt to tickle ourselves (Claxton, 1975; Weiskrantz, Elliott, & Darlington, 1971), unless done under special circumstances. Why is this so?

Some researchers suggest that the inability to tickle ourselves is related to predictive processes involved in motor control. According to Blakemore, Wolpert, and Frith (1998, 2000), the act of producing a movement that would normally stimulate a ticklish part of the body produces an expectation of both motor and sensory consequences of the act. This expectation has the effect of canceling the sensations that would normally arise if the area had been externally stimulated. Indeed, functional magnetic resonance imaging investigations of the brain activity during tickles show less activation under self-generated than externally generated tickles (Blakemore et al., 2000).

In another study, however, Blakemore, Frith, and Wolpert (1999) employed a robotic device that could be used to externally generate a tickling sensation or could be manipulated by the individual. As expected, higher ratings of “tickliness” were found on external- than on self-generated tickle trials. However, in some trials the robot did not execute the movement until a brief period of time (100, 200 or 300 ms) had elapsed after the subject had commanded the tickle. The 100-ms lag resulted in a greater feeling of tickliness compared to immediately experienced self-tickles. Moreover, lags of 200 and 300 ms produced as much sensation of tickliness as externally produced tickles. One explanation for this finding was that the predictive information that was fed forward in expectation of certain sensory consequences had a rather short “shelf life.” Perhaps this is due to rapid decay in memory quality that is associated with feedforward information. Other explanations are equally plausible, however, and further research is warranted to sort out this interesting phenomenon.

This is an excerpt from Motor Control and Learning, Fourth Edition.




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