Abstract

Psychosis can be one of the most distressing and bewildering features of mental illness. It comprises altered states of belief and perception, referred to as Delusions and Hallucinations respectively. The existence of such phenomena poses a very difficult challenge to our understanding. What sorts of psychological disturbance may lead to an individual creating, and becoming entirely immersed in, a world for which there appears to be very little supportive evidence and much that is contradictory?

I will consider this question in the setting of the challenges faced by the human brain in dealing with a world that is complex, unreliable, noisy and ambiguous. I suggest that, in order to make any sense of this world, the brain must form hypotheses about what sorts of causes in the outside world might give rise to its sensory input. In doing so, it uses expectations to resolve ambiguity.

Simply put, expectation shapes experience: even under normal circumstances we are active in creating our experiences. From this perspective, the characteristic phenomena of psychosis are not that far-removed from normal processing and may reflect a subtle alteration in balance between prior expectation and current experience.