Why can’t you tickle yourself? Every time your brain sends a movement command, it also sends a COPY to your senses — a heads-up that says ‘this next sensation is me.’ The predicted feeling is subtracted out, so self-touch feels dull while the exact same touch from someone else feels sharp. Slide from self to other and watch the felt signal jump.
Efference copy (von Holst & Mittelstaedt 1950): a copy of every motor command feeds a forward model that predicts its own sensory consequences (the reafference), which are subtracted from the actual input — so self-generated stimulation is attenuated while external stimulation passes at full strength. This is why self-tickle fails (Blakemore et al. 1998) and why the world looks still when your eyes move. A fail-loud self-check throws unless self-touch is cancelled (~0) while an identical external touch is felt in full. ◆ real sensorimotor neuroscience, node-verified.
A linear actual−predicted cancellation (the exact principle); the real forward model is learned and slightly imperfect, and delay/mismatch restores the tickle — the self-vs-external asymmetry is the exact, testable effect.