An hallucination is a perception in the absence of adequate peripheral stimulus (cf. illusion). Such perceptions are substantial, constant, occur in objective space, and are usually not accompanied by insight. They most usually occur in the visual and auditory domains.
Visual hallucinations may range in complexity. They may be "simple", spots or flashes of light (photopsia, photism, scintillation), or "complex", ranging from patterns (fortification spectra, epileptic aura) to fully formed objects or individuals. They may be transient, such as brief visions of a person or animal (passage hallucinations, for example in Parkinson’s disease) or long lasting. Visual hallucinations may be normal, especially when falling asleep or waking (hypnogogic, hypnopompic). There are many other associations including both psychiatric and neurological disease, including:

Delirium: especially hyperalert subtype
Withdrawal states: e.g., delirium tremens; hypnotics, anxiolytics Drug overdose: e.g., anticholinergic drugs
Neurodegenerative disorders: dementia with Lewy bodies (a diagnostic criterion) more often than Alzheimer’s disease: these may be associated with cholinergic depletion, and improved with cholinesterase inhibitor drugs; idiopathic Parkinson’s disease (with or without treatment).
Narcolepsy-cataplexy Peduncular hallucinosis Migraine aura
Charles Bonnet syndrome (visual hallucinations of the visually impaired)
Epilepsy: complex partial seizures "Alice in Wonderland" syndrome

Different mechanisms may account for visual hallucinations in different conditions: defective visual input and processing may occur in visual pathway lesions, whereas epilepsy may have a direct irritative effect on brain function; visual hallucinations associated with brainstem lesions may result from neurotransmitter abnormalities (cholinergic, serotonergic).
Auditory hallucinations may be simple (tinnitus) or complex (voices, music) and may be associated with focal pathology in the temporal cortex. Third person hallucinations, commenting on a person’s actions, are one of the first rank symptoms of schizophrenia.


Ashton H. Delirium and hallucinations. In: Perry E, Ashton H, Young A (eds.). Neurochemistry of consciousness: neurotransmitters in mind. Amsterdam: John Benjamins, 2002: 181-203
Barodawala S, Mulley GP. Visual hallucinations. Journal of the RoyalCollege of Physicians of London 1997; 31: 42-48
Manford M, Andermann F. Complex visual hallucinations. Clinical and neurobiological insights. Brain 1998; 121: 1819-1840

Tekin S, Cummings JL. Hallucinations and related conditions. In: Heilman KM, Valenstein E (eds.). Clinical neuropsychology (4th edition). Oxford: OUP, 2003: 479-494


Cross References

"Alice in wonderland" syndrome; Anwesenheit; Charles bonnet syndrome; Delirium; Fortification spectra; Illusion; Narcolepsy; Photism; Photopsia