The diagnosis of schizophrenia is largely based on reports of bizarre experiences such as having alien thoughts inserted into one's mind. Many patients with this diagnosis show a marked intellectual decline and particular problems with tasks involving certain kinds of memory or requiring mental flexibility. Similar patterns of performance can be seen in patients with damage in the prefrontal cortex. However, patients with schizophrenia show a very varied pattern of impairments relating to their current mental state. Chronic patients with negative features, such as poverty of speech, are most likely to show poor test performance, while the presence of severe hallucinations and delusions need not be associated with any impairment. A cognitive approach suggests that hallucinations and delusions result from the patient attributing his own actions to an external agency. This error is due to an inability to distinguish between external events and perceptual changes caused by his own actions. The basis of this failure could be a functional disconnection between frontal brain areas concerned with action and posterior areas concerned with perception.