Top-down processes like heuristics and gap filling create consistency in normal perception. Sometimes top-down processes cause illusory perceptions. Top-down processes are also involved in the creation of hallucinations, experienced in phenomena like sensory deprivation and phantom limbs. Moreover, it has been argued that the left hemisphere is important in the creation of hallucinations, since it also creates consistency. The influence of the left hemisphere in creating hallucinations is evidenced by an experiment concerning anosognosia (denial of illness). In schizophrenia, top-down processes and the left hemisphere both seem to be involved in the creation of hallucinations. It is argued that hallucinations are instigated for creating consistency and thereby have a functional character.