When the Filter Misfires

Some studies find that people with schizophrenia are more likely to experience self-generated touch as ticklish. One possible reason is that the brain's prediction system cancels self-made sensations less reliably, so the touch can feel more external than expected. Researchers connect similar prediction problems to some experiences of hearing one's own thoughts as outside voices, though schizophrenia is far more complex than one filter.

Brain scans back this up. When healthy people try to tickle themselves, the somatosensory cortex barely responds. In people with schizophrenia, the same region lights up fully, as if another hand were doing it. The line between self and other gets blurred.