Hallucinations Happen When the Brain Fills in the Blanks

"In a recent review paper my colleagues and I published in Schizophrenia Bulletin, we explored the surprising overlap between visual hallucinations in Lewy body diseases, such as Parkinson's and dementia with Lewy bodies, and those induced by psychedelics like psilocybin and LSD. Though these conditions differ radically in their cause, the associated hallucinations seem to arise from similar disruptions in how the brain processes visual input."

"While Lewy body diseases are better known for producing motor and cognitive impairments, they also have profound impacts on sensory systems. Up to 75% of patients report visual distortions of size, motion, or form. More complex hallucinations are reported at rates close to 40%. Patients often report minor hallucinations months or years before complex hallucinations emerge. Early on, a patient might experience movement in still objects, fleeting figures in their periphery, or illusory faces half-glimpsed in ambiguous patterns."

Hallucinations in Lewy body diseases and in psychedelic experiences appear to share underlying disruptions in visual processing. Lewy body diseases affect sensory systems profoundly, with up to 75% of patients reporting visual distortions and roughly 40% experiencing complex hallucinations. Minor hallucinations often precede more vivid episodes, beginning with movement in still objects, fleeting peripheral figures, or illusory faces and later progressing to full scenes or beings. Serotonergic modulation and weakened sensory signals allow the brain to fill gaps with internal patterns and predictions. Psychedelics transiently shift the balance between sensory-driven perception and internally generated imagery, revealing this mechanism.

Read at Psychology Today