Scientists have been wondering for decades about the somatosensory consequences of our own movements. By now, we know that the brain (probably the cerebellum) anticipates and cancels incoming information about self-produced movements, creating what is known as an ‘efferent copy’.
An everyday phenomenon that results from these predictive processes is being unable to tickle. Why don’t we laugh trying to tickle ourselves? This is the question Sarah-Jayne Blakemore and her colleagues at the Institute of Neurology at University College London asked themselves a few years ago.
To find the answer, they scanned the brains of 16 people as they tried to tickle themselves on the palms of their hands and feet. And later, they repeated the experiment by getting other subjects to tickle them.
They examined whether involuntary movements induced by self-stimulation and, on the other hand, stimulation induced by a third party (both resulting in standard stimulation of the participant’s bare foot) would be perceived as tickling.
The foot stimulus was only graded as tickling as when the experimenter tickled the participant’s foot, using an identical stimulus as the self-stimulus. They therefore concluded that magnetic stimulation of the motor cortex (foreign stimulation) is unlikely to generate an efferent copy of the motor system output, as self-stimulation does.
Thus, they verified that the areas that respond to touch and pleasure were activated much less when they were done by oneself, according to the magazine NeuroReport. And they came to the conclusion that self-generated tactile stimulation is attenuated because internally the sensory system predicts the sensations that our movements will produce in the same moment that the motor system gives the order to execute them.
The efferent copy
The concept of ‘efferent copy’ is basic in neurophysiology, because it refers to how the individual evaluates reality, internally and externally, and how the individual interacts with himself. It alludes to the concept of anticipation of movements.
To understand it, we can imagine a daily situation: our partner organizes a surprise party for us, and we find out through the indiscretion of a friend or relative. When the party arrives, the surprise and emotional effect diminishes. Why? Because we already knew what was going to happen. In this case, something similar happens: if there is no surprise, there is no tickling either.
Self-actualizing stimuli to ourselves travel in a double way: the brain, on the one hand, is giving the order to carry out said movement; on the other, it is receiving the expected stimulus, the result of that movement. In these cases, the muscles carry a command with them, which is sent to the sensory receptors. There is no surprise.
According to neurologist and musician Arturo Goicochea, the efferent copy refers to the absence of self-consciousness; allows automatic movements , which are classified as harmless, and are tolerated and filtered as such. Furthermore, as the neurologist explains on his blog, given that the efferent copy enables us to predict movements and stimuli, it also allows us to evaluate “the costs and benefits of each movement, depending on each objective and each context”.
More information: Cortex. 2003 Feb; 39 (1): 105-10. A ticklish question: does magnetic stimulation of the primary motor cortex give rise to an ‘efference copy’? Chronicle EP, Glover J.
