The positive and negative symptoms of schizophrenia reflect impairments in the perception and initiation of action.

The CNS maintains a fundamental distinction between actions elicited by external stimuli and actions elicited by internal goals (acts of will). As a result the intact organism can monitor centrally three aspects of its own actions: (1) the action appropriate to current external stimulation (stimulus intention or meaning); (2) the action appropriate to current goals (willed intention); and (3) the action which was actually selected (corollary discharge). In Type I (acute) schizophrenic patients, intentions of will lead to actions, but these willed intentions are not monitored correctly. This apparent discrepancy between will and action gives rise to experiential (1st rank) positive symptoms (e.g. delusions of control and passivity). In Type II (chronic) patients, intentions of will are no longer properly formed and so actions are rarely elicited via this route. This gives rise to behavioural negative signs (e.g. poverty of speech). The behaviour of Type II schizophrenics has surface similarities to that shown by patients with Parkinson's disease and patients with frontal lobe lesions in that all three types of patient show a relative deficit of actions elicited by willed intentions. Dopamine blocking drugs reduce positive symptoms in Type I patients precisely because they induce Parkinsonism, i.e. reduce the likelihood of actions being initiated by willed intentions. This in turn reduces the likelihood that actions will occur for which the patient had no awareness of his intention to act.