Why you can't tickle yourself

You've probably already observed it: someone who can't tickle themselves, while the very same stroke from another person makes them burst out laughing. An athlete who knows, before the end of their jump, how the landing will go. A person who guesses their coach's movement a fraction of a second before it begins.

Every time, the brain tells apart what comes from itself and what comes from outside.

The word of the day: efference copy

When the cortex sends a motor command to a muscle, it sends a copy in parallel to other structures, notably the cerebellum: this is the efference copy (efference copy in the glossary). The cerebellum uses it to build in advance the sensation that should come back up from the body. When your hand moves at your request, the system already knows what it is supposed to feel.

If the incoming sensation (reafference) matches the prediction, the brain quietly files the matter: it's me, nothing to report. If it differs, it triggers an error signal, which produces surprise and is used to correct the next predictions (the mismatch between prediction and feedback).

In 1998, Sarah-Jayne Blakemore and her colleagues showed it with imaging: when you tickle yourself, the touch areas of the brain activate less than when another person produces exactly the same movement. The brain dampened the sensation in advance, because the efference copy announced that it came from you.

Why a touch from outside can change something

If the brain filters out a large part of what comes from itself, then what can surprise it and recalibrate it must often come from outside. When a practitioner places their hands in the right spot, with a contact you could never have applied to yourself, the brain could not predict it, so it did not dampen it: it has to process it, and that processing can help update the internal model your body was using (the skin's sensors).

That is also why you make progress when a coach touches you, guides you or sets a constraint you couldn't create yourself: outside stimulation produces error signal, and that signal is the fuel of calibration. What remains is to check that the effect holds: an immediate change proves nothing until it has passed a retest after some delay, as shown by the example of the ticklishness that disappears after thirty seconds of pressure.

The brain dampens what it has predicted, and learns from what surprises it.

To understand how predictions and errors build learning, the course "How we learn to move" is free, as is the introduction to RNP, Neuro-Postural Reprogramming.

You are a movement professional: here is the training. You want to take stock for yourself: request an assessment.