However, the long-held belief that cognitive abilities deteriorate over the course of illness has recently been called into question. For example, in 34 schizophrenic patients whose IQs were measured as children and again 19.4 years later, no significant decline in IQ had occurred over the course of nearly two decades, but both the child and the adult IQ in these patients were lower than those of the general population. Psychotic symptoms that characterize schizophrenia include disturbances in perception, abnormalities in the content of thought, and abnormalities in the form of thought. Perceptual disturbances consist of hallucinations and illusions. Hallucinations are false perceptions that occur in the absence of an appropriate stimulus.
Illusions differ from hallucinations in that a sensory stimulus is present but is misinterpreted.
Disturbances of affect commonly occur in schizophrenia. Apparent blunting of affect may also be caused by overmedication with antipsychotic drugs or by an intercurrent depression. Abnormalities in the content of thought include delusions and ideas of reference. Delusions are fixed, culturally inappropriate beliefs that the patient holds despite all reasonable evidence to the contrary. Delusions of reference that are somewhat less strongly held are referred to as ideas of reference.