Schizophrenia is a serious illness affecting around 1% of the population, and is responsible for significant suffering for sufferers and carers alike. Although there is substantial evidence for a biological basis, the diagnosis is still based on symptoms that characteristically involve hallucinations (false perceptions) and delusions (false beliefs). Over the past few years, I've been trying to understand these symptoms at the cognitive, neural and experiential levels: what does it feel like to have such symptoms?
Many of the main symptoms of schizophrenia seem to reflect a confusion between effects caused by the self as agent and effects caused by external agents, for example when the subject hears his own thoughts spoken aloud, or believes that foreign forces are causing his own actions. The latter experience, known as an illusion of control, has been extensively studied. This symptom is less a disorder of action control than a disorder of action awareness, particularly in the sense of agentivity. It was first hypothesized that this was a problem of self-control. The patient is unaware of his intentions due to a failure of the corollary discharge (or reafference copy) responsible for indicating that a movement is about to occur. As a result, movement is perceived as caused by external forces.
More recently, this idea has been expressed in terms of aforward model. When we perform an action, we anticipate the consequences of the action in terms of behavioral and sensory effects. Numerous experiments have produced results consistent with the idea that patients with illusions of control are unable to make these predictions. At the neural level, this inability is associated with a dysfunction of the mechanism responsible for attenuating the neural activity that reflects the sensory consequences of self-generated movements. These observations imply that, for a patient with control illusions, an active movement is truly perceived as a passive movement.
