Underconstrained perception: a theoretical approach to the nature and function of verbal hallucinations.

We do not see the world as it is. Perception forms a subjective image of the world in a language that has proven to be adaptive to our interaction with the external world. Perception is mainly determined by current needs of the organism and goals of behavior. Sensory processing itself does not culminate in perception and is not essential for perception, since perception derives from representations of internal symbols and their features. The current stimulation of sensory organs does, however, constrain our perception. Perception might be less constrained by the external world in cases of (1) increased attention, (2) decreased sensory stimulation, or (3) facilitated formation of cortical associations between representations of expectations and internal symbols. Hallucinations are perceptions that are underconstrained by external sensory stimulation. Verbal hallucinations that allow the patient to infer about his self-image might constitute the core psychopathology in a subset of schizophrenia. Commenting and discussing voices might be perceived under the pressure of increased attention to environmental factors that relate to the patient's social fears and wishes. Secondarily, delusions about the possession of thoughts and disorders of self-experience may develop.