visual hallucination

   Also referred to as vision. Both terms are indebted to the Latin noun visio, which means sight. They are used to denote a hallucination of sight. Historically, visual hallucinations have been divided into a multitude of types. Using their perceived complexity as a guiding principle, they are commonly classified as * elementary (or * simple), * geometric, and * complex visual hallucinations. Those replacing the entire sensory environment are referred to as * scenic or *panoramic hallucinations. Those perceived as being located outside the visual field (such as a person 'seen' from the back of the head) are called * extracampine hallucinations. Visual hallucinations going hand in hand with hallucinations in any of the other sensory modalities are referred to as * compound hallucinations. Using their perceived shape as a guiding principle, visual hallucinations are classified as * formed (or * organized) and * unformed hallucinations. Unformed visual hallucinations are also designated as * photopsia, while formed visual hallucinations are known as *morphopsia. Using their perceived size as a guiding principle, visual hallucinations have been divided into *macroptic and * microptic hallucinations. Visual hallucinations which feature a faithful image ofoneselfare known as *autoscopic hallucinations; less faithful images are called * heautoscopic hallucinations. Those depicting animals are referred to as *zoopsia, whereas those depicting ghosts or dead people have traditionally been referred to as * apparitions. Visual hallucinations depicting a deceased loved one are referred to as *bereavement hallucinations, post-bereavement hallucinations, or grief hallucinations. Visual hallucinations mediated by an ophthalmic structure (such as * muscae volitantes and certain types of * photopsia) are traditionally referred to as *entoptic phenomena. In some classifications entoptic phenomena are considered subtypes of visual hallucinations, whereas in others they are excluded from the class of * hallucinations proper Visual hallucinations occurring in the context of visual impairment, as in * Charles Bonnet syndrome, are designated as * opthalmopathic hallucinations. Using the purported presence or absence of an extracorporeal * point de repère as a guiding principle, visual hallucinations have been divided into visual hallucinations proper and * visual illusions. Visual hallucinations occurring in response to a regular sensory percept in any of the other sensory modalities (such as * sound seeing) are referred to as * synaesthesias. When visual hallucinations are accompanied by a compelling sense of objectivity, they are said to have a high degree of *xenopathy. The literature on the neurophysiological correlates ofvisual hallucinations is as diverse as these phenomeno-logical descriptions. In 1890, the Swedish neurologist Salomon Eberhard Henschen (1847-1930) was the first to report on visual hallucinations occurring in an individual who had a tumour within the occipital cortex. It is now generally accepted that visual hallucinations can be mediated by any part of the visual system, although not every type of visual hallucination can be mediated by any of its parts. However, the rule of thumb that the simpler types of hallucinations are associated with activity in the more peripheral structures and the more complex ones with central activity has proved to be an oversimplification. It does not do justice to complex entop-tic phenomena mediated primarily by the retina, for example, and photopsias mediated primarily by the optic radiation. Another rule of thumb, which states that visual hallucinations perceived as being located in external space are associated with activity in specialized occipital cortical areas, whereas those perceived 'with the mind's eye' are not, is equally questionable. However, the rule of thumb that all visual hallucinations co-occur with aberrant neurophysiological activity in the occipital cortex is still in force. Having multiple areas of relative specialization for different visual attributes (such as contrast, texture, orientation, luminance, movement, and so on), various parts of the visual association cortex are believed to be activated in accordance with the attributes hallucinated. The occurrence of visual hallucinatory activity is attributed not only to direct stimulation of occipital areas (as in *deafferentiation and * experiential hallucinations, for example), but also to indirect stimulation of these occipital areas by ascending activity originating from the limbic system (as in * reperception), the thalamus, the pedunculus cerebri and its adjacent structures (as in * peduncular hallucinations), the primary sensory pathways, or visually specialized regions of the temporal and frontal lobes. Etiological factors capable of affecting these regions are believed to include epileptic seizures, migraine, degenerative neuronal cell loss, stroke, infections, trauma, metabolic disturbances, the use of certain therapeutics, * hallucinogens or other substances (as in * drug-related hallucinations), and *electromagnetic field disturbances. Organic risk factors for the development of visual hallucinations include *sensory deprivation, eye diseases which diminish the transmission of light into the eye (such as cataracts and corneal scarring), retinal diseases, and Parkinson's disease. Generally speaking, visual hallucinations have been found in 11-57% of individuals with visual pathway lesions located anywhere between the retina and the primary visual cortex. Psychosocial risk factors have been found to include stress (in 85% of cases), tiredness (in 60%), loneliness (in 55%), and relational problems (in 50%). As to their co-occurrence with psychiatric disorders, visual hallucinations are found quite often in individuals with a clinical diagnosis of *delirium, *alcoholic hallucinosis, Alzheimer's disease and other types of dementia, *schizophrenia, and *flashbacks (both drug-related and in the context of post-traumatic stress disorder (PTSD)). They are encountered less often in individuals with a clinical diagnosis of mood disorder or borderline personality disorder. Because migraine has a lifetime prevalence of 12-28%, and 10-40% of all individuals acquainted with migraine are also acquainted with * aurae, it has been suggested that the visual aura may well constitute the most prevalent type ofvisual hallucination.

   References

   Berrios, G.E., Brook, P. (1984). Visual hallucinations and sensory delusions in the elderly. British Journal of Psychiatry, 144, 662-664.

   ffytche, D.H., Howard, R.J., Brammer, M.J., David, A., Woodruff, P., Williams, S. (1998). The anatomy of conscious vision: An fMRI study of visual hallucinations. Nature Neuroscience, 1, 739-742.

   Gauntlett-Gilbert, J., Kuipers, E. (2003). Phenomenology ofvisual hallucinations in psychiatric conditions. Journal of Nervous and Mental Disease, 191, 203-205.

   Mocellin, R., Walterfang, M., Velakoulis, D. (2006). Neuropsychiatry of complex visual hallucinations. Australian & New Zealand Journal of Psychiatry, 40, 742-751.

   Siegel, R.K., Jarvik, M.E. (1975). Drug-induced hallucinations in animals and man.In: Hallucinations. Behavior, experience, and theory. Edited by Siegel, R.K., West, L.J. New York, NY: John Wiley & Sons.

   Weinberger, L.M., Grant, F.C. (1940). Visual hallucinations and their neuro-optical correlates. Archives ofOphthalmology, 23, 166-199.

   Wilkinson, F. (2004). Auras and other hallucinations: Windows onthevisual brain. Progress in Brain Research, 144, 305-320.