We all have an inner voice. Most of us know they're not real. But, for up to 15% of the population at some points in their lives, they can take on a different tone, as a terrifying experience that cannot be distinguished from reality. Where do they come from, and what do they say to sufferers? And how can the symptoms be treated? In this special guest episode, the Wellcome Trust's Chris Chapman hears the stories of schizophrenics affected by voices and explores a new approach to giving sufferers control over their experiences...