False perceptions arising without an adequate external stimulus (as opposed to illusions, which are misinterpretations of stimuli arising from an external object). Hallucinations come from ‘within’, although the affected individual may see them as coming from ‘without’. Nevertheless, they may occur at the same time as real perceptions, and may affect any sense (vision, hearing, smell, taste, touch, etc).
Causes
They may be the result of intense emotion or suggestion, sensory deprivation (for example, overwork or lack of sleep), disorders of sense organs, or disorders of the central nervous system. They are often an indication of a MENTAL ILLNESS. They may be deliberately induced by the use of HALLUCINOGENS.