A hallucination is the experience of a sensation in the absence of a provoking stimulus. The hallucinated sensation can be visual, auditory, tactile, and sometimesolfactoryor gustatory. For example, a tactile hallucination is when you feel something crawling on your skin but nothing is there.

This is not to be confused with an illusion, which is the distortion or misinterpretation of a real perception: if you thought a plant in your living room was a girl in a green sweater, for example.

A hallucination is usually very vivid and feels real. Most people with hallucinations do not realize they aren’t real. While some hallucinations can be pleasant, others can be very scary and disruptive.

Hallucinations can occurin three main settings:

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Tensed Woman With Head In Hands Sitting On Chair By Window At Home

Diseases of the Eye

In 1760, Charles Bonnet, a Swiss naturalist, and philosopher, first described the intriguing case of his 87-year-old grandfather, who had severecataracts. His father still had his full mental capacities, yet he saw people, birds, animals, and buildings while being nearly blind in both eyes.

He gave his name to the syndrome Charles Bonnet Syndrome, which describes the presence of visual hallucinations (and only visual with no other sensory modality affected) in elderly people with various eye diseases: retinal detachment,macular degeneration, cataracts and damage to the optic nerve and pathways.

The mechanism is not well understood. Some scientists have proposed that there is a “release” of the brain areas that normally process images. The visual stimuli sent from our retina to our brain usually inhibits our brain from processing any image other than the one that is currently in front of our eyes.

For example, if you were bored and daydreaming at work, you would still see the screen of your computer in front of you as opposed to the beach that you can only attempt to visualize. When the eye is sick, visual stimulation is absent and this control is lost, hence “releasing” the brain from the confinement of reality. In these cases, the person usually realizes that these are not real.

Diseases of the Brain

Hallucinations are the manifestations of many diseases of the brain (and the mind, if you are Cartesian about it), although their mechanism is poorly understood:

Adverse Effect of Drugs

Hallucinogenic drugs, including LSD (lysergic acid diethylamide) and PCP (phencyclidine), act on a chemical receptor in the brain to induce altered perceptions and sometimes frank hallucinations. In addition, many drugs that are available in the market have side effects that include hallucinations.

8 SourcesVerywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.Jan T, Del Castillo J.Visual hallucinations: charles bonnet syndrome.West J Emerg Med. 2012;13(6):544–547. doi:10.5811/westjem.2012.7.12891Teeple RC, Caplan JP, Stern TA.Visual hallucinations: differential diagnosis and treatment.Prim Care Companion J Clin Psychiatry. 2009;11(1):26–32. doi:10.4088/pcc.08r00673National Institute on Aging.What Is Lewy Body Dementia?Morenas-rodríguez E, Camps-renom P, Pérez-cordón A, et al.Visual hallucinations in patients with acute stroke: a prospective exploratory study.Eur J Neurol.2017;24(5):734-740. doi:10.1111/ene.13278Plaze M, Paillère-Martinot ML, Penttilä J, et al. “Where do auditory hallucinations come from?"–a brain morphometry study of schizophrenia patients with inner or outer space hallucinations.Schizophr Bull. 2011;37(1):212–221. doi:10.1093/schbul/sbp081Weissenstein A, Luchter E, Bittmann MA.Alice in Wonderland syndrome: A rare neurological manifestation with microscopy in a 6-year-old child.J Pediatr Neurosci. 2014;9(3):303–304. doi:10.4103/1817-1745.147612Waters F, Blom JD, Dang-Vu TT, et al.What Is the Link Between Hallucinations, Dreams, and Hypnagogic-Hypnopompic Experiences?Schizophr Bull. 2016;42(5):1098–1109. doi:10.1093/schbul/sbw076Duarte mangas M, Martins Y, Bravo L, Matos pires A.Misdiagnoses of Epilepsy as Ekbom Syndrome, Mood Instability, and Nocturnal Visual Hallucinations.Case Rep Psychiatry. 2017;2017:3968751. doi:10.1155/2017/3968751

8 Sources

Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.Jan T, Del Castillo J.Visual hallucinations: charles bonnet syndrome.West J Emerg Med. 2012;13(6):544–547. doi:10.5811/westjem.2012.7.12891Teeple RC, Caplan JP, Stern TA.Visual hallucinations: differential diagnosis and treatment.Prim Care Companion J Clin Psychiatry. 2009;11(1):26–32. doi:10.4088/pcc.08r00673National Institute on Aging.What Is Lewy Body Dementia?Morenas-rodríguez E, Camps-renom P, Pérez-cordón A, et al.Visual hallucinations in patients with acute stroke: a prospective exploratory study.Eur J Neurol.2017;24(5):734-740. doi:10.1111/ene.13278Plaze M, Paillère-Martinot ML, Penttilä J, et al. “Where do auditory hallucinations come from?"–a brain morphometry study of schizophrenia patients with inner or outer space hallucinations.Schizophr Bull. 2011;37(1):212–221. doi:10.1093/schbul/sbp081Weissenstein A, Luchter E, Bittmann MA.Alice in Wonderland syndrome: A rare neurological manifestation with microscopy in a 6-year-old child.J Pediatr Neurosci. 2014;9(3):303–304. doi:10.4103/1817-1745.147612Waters F, Blom JD, Dang-Vu TT, et al.What Is the Link Between Hallucinations, Dreams, and Hypnagogic-Hypnopompic Experiences?Schizophr Bull. 2016;42(5):1098–1109. doi:10.1093/schbul/sbw076Duarte mangas M, Martins Y, Bravo L, Matos pires A.Misdiagnoses of Epilepsy as Ekbom Syndrome, Mood Instability, and Nocturnal Visual Hallucinations.Case Rep Psychiatry. 2017;2017:3968751. doi:10.1155/2017/3968751

Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.

Jan T, Del Castillo J.Visual hallucinations: charles bonnet syndrome.West J Emerg Med. 2012;13(6):544–547. doi:10.5811/westjem.2012.7.12891Teeple RC, Caplan JP, Stern TA.Visual hallucinations: differential diagnosis and treatment.Prim Care Companion J Clin Psychiatry. 2009;11(1):26–32. doi:10.4088/pcc.08r00673National Institute on Aging.What Is Lewy Body Dementia?Morenas-rodríguez E, Camps-renom P, Pérez-cordón A, et al.Visual hallucinations in patients with acute stroke: a prospective exploratory study.Eur J Neurol.2017;24(5):734-740. doi:10.1111/ene.13278Plaze M, Paillère-Martinot ML, Penttilä J, et al. “Where do auditory hallucinations come from?"–a brain morphometry study of schizophrenia patients with inner or outer space hallucinations.Schizophr Bull. 2011;37(1):212–221. doi:10.1093/schbul/sbp081Weissenstein A, Luchter E, Bittmann MA.Alice in Wonderland syndrome: A rare neurological manifestation with microscopy in a 6-year-old child.J Pediatr Neurosci. 2014;9(3):303–304. doi:10.4103/1817-1745.147612Waters F, Blom JD, Dang-Vu TT, et al.What Is the Link Between Hallucinations, Dreams, and Hypnagogic-Hypnopompic Experiences?Schizophr Bull. 2016;42(5):1098–1109. doi:10.1093/schbul/sbw076Duarte mangas M, Martins Y, Bravo L, Matos pires A.Misdiagnoses of Epilepsy as Ekbom Syndrome, Mood Instability, and Nocturnal Visual Hallucinations.Case Rep Psychiatry. 2017;2017:3968751. doi:10.1155/2017/3968751

Jan T, Del Castillo J.Visual hallucinations: charles bonnet syndrome.West J Emerg Med. 2012;13(6):544–547. doi:10.5811/westjem.2012.7.12891

Teeple RC, Caplan JP, Stern TA.Visual hallucinations: differential diagnosis and treatment.Prim Care Companion J Clin Psychiatry. 2009;11(1):26–32. doi:10.4088/pcc.08r00673

National Institute on Aging.What Is Lewy Body Dementia?

Morenas-rodríguez E, Camps-renom P, Pérez-cordón A, et al.Visual hallucinations in patients with acute stroke: a prospective exploratory study.Eur J Neurol.2017;24(5):734-740. doi:10.1111/ene.13278

Plaze M, Paillère-Martinot ML, Penttilä J, et al. “Where do auditory hallucinations come from?"–a brain morphometry study of schizophrenia patients with inner or outer space hallucinations.Schizophr Bull. 2011;37(1):212–221. doi:10.1093/schbul/sbp081

Weissenstein A, Luchter E, Bittmann MA.Alice in Wonderland syndrome: A rare neurological manifestation with microscopy in a 6-year-old child.J Pediatr Neurosci. 2014;9(3):303–304. doi:10.4103/1817-1745.147612

Waters F, Blom JD, Dang-Vu TT, et al.What Is the Link Between Hallucinations, Dreams, and Hypnagogic-Hypnopompic Experiences?Schizophr Bull. 2016;42(5):1098–1109. doi:10.1093/schbul/sbw076

Duarte mangas M, Martins Y, Bravo L, Matos pires A.Misdiagnoses of Epilepsy as Ekbom Syndrome, Mood Instability, and Nocturnal Visual Hallucinations.Case Rep Psychiatry. 2017;2017:3968751. doi:10.1155/2017/3968751

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