Why can't you tickle yourself?

One way that is guaranteed to cheer up a tired, cross child or to reduce an adult to a simpering fit of giggles is a few light strokes on the sole of the foot or the armpit. Some people are more tickly than others, and ticklishness varies from situation to situation.1 But 1 question has baffled minds for years: why can't you tickle yourself? Go on—try it. The sensation produced will never rival that of an external stimulus. The sensation of tickle is perceived by the part of the brain called the somatosensory cortex (also called S1). Studies using functional magnetic resonance imaging, a technique that shows changes in brain activity in different situations according to cerebral blood flow, have shown increased S1 activity in response to an externally produced tactile stimulus. However, the same activity is not produced after an identical, self-produced stimulus.2 We need to turn our attention away from the cortical areas of the brain to the cerebellum, a structure attached to its base, to understand why this is the case. The cerebellum has a role in coordinating movement and receives both sensory input from the body (through the spinal cord) and motor input from the higher cortical areas that command movement. In other words, 1 of its roles is the comparison of the expected response to a stimulus (relying on descending commands from the motor cortex of the brain) with the actual response (relying on sensory feedback from the receptors in the skin). If the 2 responses match, the cerebellum kicks in a mechanism to suppress the effects of the stimulation, and the end result is diminished activity in the somatosensory cortex and with it, diminished perception of the tickle. This cancellation of the sensory consequences of a motor command is a phenomenon known as reafference.3