Compelling perceptual experiences which may be visual, tactile, olfactory, or auditory, but which lack a physical stimulus.

Although hallucinations are false perceptions, they carry the force of reality and are a definitive sign of mental illness. Hallucinations may be caused by organic deterioration or functional disorders, and can occur in normal people while asleep or awake, or as a result of sensory deprivation. Generally not positive experiences, hallucinations are often described as frightening and distressing. A person under a hallucinatory state may be either alert and intelligent or incoherent, depending on the type and degree of the disturbance.

One...