In this programme, Geoff Watts meets researchers attempting to unlock the mysteries of hallucination as well as some of those who experience the phenomenon. Geoff visits Dr Dominic Ffytche of the Institute of Psychiatry in London, and undergoes a stroboscopic experiment designed to induce hallucinations in subjects whilst their brains are being scanned.

We hear some of the vivid accounts from hallucinators, including Doris, who has macular degeneration. Over the last year, her failing eyesight has resulted in an array of objects and images appearing before her with startling clarity, from relatively benign baskets of flowers to the rather more distressing sight of dark, haunting figures sitting by her bed. Her condition is known as Charles Bonnet Syndrome and Dr Ffytche estimates that over two million people suffer from this in the UK alone, mostly in silence, due to the fear of being labelled as 'mad'.

Geoff also visits Kelly Diederen's lab at Cambridge University, which is investigating the origin of auditory hallucinations - hearing voices. Common in people with schizophrenia, Dr Diederen is instead, scanning the brains of so-called "healthy hallucinators," individuals who otherwise lead perfectly functional lives save for the fact that they hear voices on a daily basis. Could they hold the key to understanding and treating a key symptom of psychosis?

(Image: Close up of the face of an anonymous male. Credit: BBC)