Why is it impossible to tickle yourself?

Question

Accepted Answer

It is impossible to tickle yourself because the human brain is wired to distinguish between expected and unexpected sensations. The mechanism behind this phenomenon is primarily located in the cerebellum, the part of the brain responsible for motor control and predicting sensory consequences.

Here is the scientific breakdown of why this occurs:

Sensory Prediction: When you move your hand to tickle yourself, your brain’s motor cortex sends two signals simultaneously. The first signal directs your muscles to move, while the second—known as a "corollary discharge"—informs the somatosensory cortex (the area that processes touch) about the movement you are about to perform. 
   Cancellation Effect: Because the brain predicts the exact sensation your fingers will produce, it effectively cancels out or suppresses the tactile response. The brain classifies the touch as "self-generated" rather than an external threat, rendering the tickle sensation ineffective.
   Lack of Surprise: Ticklishness is an evolutionary defense mechanism designed to protect vulnerable areas of the body (such as the neck, armpits, and ribs) from insects or predators. Because you are the one initiating the movement, there is no element of surprise or external threat, so the brain does not trigger the involuntary "laugh reflex" associated with tickling.

Exceptions to the Rule:
   Pathological Conditions: Some individuals with specific neurological conditions, such as schizophrenia, have been shown to be able to tickle themselves. This is believed to be due to a disruption in the brain's ability to accurately attribute self-generated actions, causing them to perceive the self-touch as coming from an external source.
   Mechanical Aids: You can sometimes simulate the sensation of being tickled by using a machine or a device to tickle you at irregular, unpredictable intervals, as this bypasses the brain’s ability to predict the exact timing and location of the stimulus.