Hack 65. Why Can’t You Tickle Yourself? (2)
Every time an action is made, the brain generates an efference copy of the actual motor command in parallel. The efference copy is just like a carbon copy, or duplicate, of the real motor command and is used to make a prediction about the effect of the action, for example, the tickling effect of a finger stroke. The predicted sensory effect of the efference copy and the actual sensory effect of the motor command are compared (Figure 6-3). If there is a mismatch, the sensation is labeled as externally generated.
Your accurate prediction of the consequences of the self-tickle reduces the sensory effects (the tickliness) of the action, but this does not happen when someone else tickles you. This explains why the sensation is usually more intense when another person touches your arm compared with when you touch your own arm.
Neuroimaging studies using a tickling machine (Figure 6-4) at University College London1 suggest that the distinction between self and other is hardwired in the brain. This device was used to apply a soft piece of foam to the participant’s left palm. In one condition, the participant self-produced the touch stimulus with his right hand, and in the other condition, the experimenter produced the stimulus. The participant’s brain was scanned during the experiment to investigate the brain basis of self-produced versus externally produced touch. Results show stronger activation of the somatosensory cortex and anterior cingulate, parts of the brain involved in processing touch and pleasure, respectively, when a person is tickled by someone else, compared with when they tickle themselves. The cerebellum, a part of the brain that is generally associated with movement, also responds differently to self-produced and externally produced touch, and it may have a role in predicting the sensory consequences of self-touch but not external touch. (See [Hack #7] for more about these parts of the brain.)
Taken from : Mind Hacks
