Why can't you tickle yourself?

It is well known that you cannot tickle yourself. Here, we discuss the prop osal that such attenuation of self-produced tactile stimulation is due to t he sensory predictions made by an internal forward model of the motor syste m. A forward model predicts the sensory consequences of a movement based on the motor command. When a movement is self-produced, its sensory consequen ces can be accurately predicted, and this prediction can be used to attenua te the sensory effects of the movement. Studies are reviewed that demonstra te that as the discrepancy between predicted and actual sensory feedback in creases during self-produced tactile stimulation there is a concomitant dec rease in the level of sensory attenuation and an increase in tickliness. Fu nctional neuroimaging studies have demonstrated that this sensory attenuati on might be mediated by somatosensory cortex and anterior cingulate cortex: these areas are activated less by a self-produced tactile stimulus than by the same stimulus when it is externally produced. Furthermore, evidence su ggests that the cerebellum might be involved in generating the prediction o f the sensory consequences of movement. Finally, recent evidence suggests t hat this predictive mechanism is abnormal in patients with auditory halluci nations and/or passivity experiences. NeuroReport 11:11-16 (C) 2000 Lippinc ott Williams & Wilkins.