Hallucinations Happen When the Brain Fills in the Blanks

Key points

Hallucinations in Parkinson’s and psychedelics may stem from shared neural mechanisms.

When sensory signals weaken, the brain fills in the gaps with its own patterns and predictions.

Psychedelics let scientists watch the balance shift between seeing the world and generating it.

By Nathan H. Heller

“ His first misperceptions occurred when he was in a nightclub; the skin of the other dancers, even their faces, seemed to be covered with tattoos. At first, he thought the tattoos were real, but they started to glow and then to pulse and writhe…”

— Oliver Sacks, Hallucinations

What if the same neural glitch that produced this Parkinson’s patient’s writhing tattoos also drives psychedelic visions? It may seem unlikely that a degenerative brain disorder and a psilocybin trip