Table 1. Characteristics of visual hallucinations in selected diseases.

Disease process	Phenomenology	Associated features	Possible mechanism(s)	Contribution of socio-cultural factors?
Occipital lobe pathology (stroke, seizures)	Simple: elementary objects, repeating patterns, stereotyped, often lateralised (field defect)	Hemianopia, may have other posterior cerebral or upper brainstem signs	Abnormal release of cortical activity or seizure activity	−
Migraine	Usually simple: elementary objects, repeating patterns (e.g. ‘fortification spectra’), evolving in stereotyped fashion; occasionally complex	Visual scotoma or hemianopia, may have other visual distortions (e.g. ‘Alice-in-Wonderland’ syndrome), neurological deficits, headache	Altered cortical excitability	−
Delirium (acute brain syndrome)*	Variable: often insects or vermin, mobile, often ill-defined, especially in low light, often threatening	Heightened or reduced awareness / motor activity, disorientation, carphology (picking at bedclothes), formication (crawling sensations)	Release of ascending controls on cerebral cortex, impaired attention	±
Midbrain pathology (‘peduncular hallucinations’)	Often complex: vivid, often tiny (Lilliputian) figures, scenes, often diurnal variation	May have evidence of oculomotor or other upper brainstem pathology, somnolence, sometimes hallucinations in other sensory modalities	Release of ascending controls on cerebral cortex	+
Visual loss (Charles Bonnet syndrome)	Often complex: vivid, unfamiliar people (often children), non-threatening; may have simple patterns	Usually evidence of acquired peripheral visual pathology; especially elderly	Deafferentation of visual cortex	+
Lewy body dementia / Parkinson’s disease dementia	Often complex: vivid, people and/or small animals, often transient or ‘emerge’ from visual environment (objects, patterns) in low light, extracampine (sense of a presence beyond the field of vision), non-threatening	Cognitive impairment, parkinsonism, fluctuations	Deficiency of cortical acetylcholine	+
Temporal lobe epilepsy	Complex: vivid, complex scenes, discrete episodes	May have hallucinations in other sensory modalities, altered awareness, déjà vu, automatisms, overt seizures	Seizure activity, may be reactivation of old memories	++
Psychosis	Complex: vivid and often unpleasant, persecutory, bizarre	Auditory hallucinations, delusions, thought disorder	Uncertain: ?altered gating of external sensory inputs vs internally generated imagery	++

^*including drug intoxications

Hallucinosis syndromes are ordered here according to increasing complexity and proposed potential for modulation by social and cultural factors. The Table does not include visual hallucinations in otherwise healthy people under certain circumstances, e.g. sleep onset (hypnagogic), sensory / sleep deprivation, grief reactions; such hallucinations are also likely to be modulated by socio-cultural factors and recent experience