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   The eponym Charles Bonnet syndrome (CBS) refers to the Swiss naturalist and philosopher Charles Bonnet (1720-1792). It was introduced in 1936 by the Swiss neurologist Georges de Mor-sier (1894-1982) to denote a hallucinatory state or syndrome characterized by *visual hallucinations occurring in "senile syndromes with ocular lesions", or, as de Morsier rephrased it in 1938, "in the elderly with intact cognition". Historically, the eponym CBS has variously been used to denote (1) visual hallucinations occurring in the cognitive intact elderly, (2) visual hallucinations occurring in the context of eye disease, and (3) visual hallucinations occurring in the presence of preserved insight. The syndrome that was later to be named CBS was first described in 1760 by Bonnet, whose grandfather Charles Lullin suffered from this type of hallucinations. As rendered by Bonnet in his book Essai Analytique sur les Facultés de ï Ame, Lullin had suffered from loss of visual acuity due to a bilateral cataract. Eight years after a first cataract operation (and 1 year after a second operation, after which his visual acuity had become even worse), Lullin reported seeing vivid images of scaffolding and brickwork patterns, a multitude of particles ("atoms") whirling about, clover patterns covering the walls and furniture, as well as people, birds, carriages, buildings, and other objects. According to Bonnet these images were not accompanied by hallucinations in any of the other sensory modalities. Nor did Lullin, a retired magistrate, show any signs of cognitive impairment. Reportedly, Lullin was well aware that his visions were "fictions" of his brain, and showed himself intrigued and amused by them. Near the end ofhis life, Bonnet's own visual acuity deteriorated as well, after which he, too, began to experience visual hallucinations. The hallucinations occurring in CBS have also been referred to as * ophthalmopathic hallucinations, and as * positive spontaneous visual phenomena (PSVP). Their complexity can range from * simple or * geometric to * complex, although the operational definition of CBS, as issued by the psychiatrists Jorge Manuel Ribeiro Damas-Mora et al. in 1982, requires the presence of complex visual hallucinations. Due to their *xenopathic character, hallucinations occurring in the context of CBS tend to have a highly realistic appearance. However, individuals in possession of proper reality monitoring strategies usually recognize the hallucinations at hand as non-sensory percepts. Their onset can be at any age, although CBS has been found to be most prevalent at old age. Estimates as to the prevalence of CBS among elderly individuals with impaired visual acuity range from 10 to 30%. Risk factors for the development of CBS include such conditions as poor visual acuity due to corneal degeneration, age-related mac-ular degeneration, glaucoma, and cataract, as well as solitude, fatigue, poor general physical health, and the use ofbeta blockers. CBS can also occur in association with partial disturbances of vision such as *hemianopia, *quadrantanopsia, central scotoma, and *amblyopia, as well as in temporary disturbances of vision (i.e. * amaurosis fugax). Visual hallucinations occurring in the context of hemianopia or quadrantanopsia tend to manifest themselves in the impaired visual field, but they can also present in the intact field of vision. Pathophysiologically, the hallucinations concomitant to CBS tend to be regarded as falling into the class of * release hallucinations, i.e. hallucinations mediated by spontaneous electrophysiological activity originating from subcortical brain areas such as the thalamus, the pedunculus cerebri, and the limbic system. A competing explanatory model, known as the * deafferentiation hypothesis, attributes the mediation of hallucinations in CBS to the increased excitability of the visual pathways and/or the visual cortex, due to a lack of inhibitory afferent impulses. Brain regions considered capable of mediating spontaneous visual percepts include the retina, the lateral genicu-late nucleus, the primary visual cortex, and the visual association cortex. Conceptually as well as phenomenologically (and perhaps pathophysio-logically as well), visual hallucinations occurring in the context of CBS appear to display some overlap with * bereavement hallucinations and * phantom vision. A variant of Charles Bonnet syndrome was reported in 1953 by the American neurologist and psychiatrist Walter Jackson Freeman (1895-1972) and his colleague Jonathan M. Williams, which involved visual * hallucinations in braille experienced by a virtually blind woman. In occultism and mysticism, individuals suffering from CBS are sometimes referred to as 'Bonnet people', and their capacity to hallucinate as 'a portal to a parallel reality'.

   References

   Bonnet, Ch. (1760). Essai analytique sur les facultés de l'âme. Copenhague: Frères Cl. & Ant. Philibert.

   De Morsier, G. (1936). Les automatismes visuels (Hallucinations visuelles rétro-chiasmatiques). Schweizerische Medizinische Wochenschrift, 66, 700-703.

   ffytche, D.H. (2005). Visual hallucinations and the Charles Bonnet syndrome. Current Psychiatry Reports, 7, 168-179.

   Menon, G.J., Rahman, I., Menon, S.J., Dutton, G.N. (2003). Complex visual hallucinations in the visually impaired: The Charles Bonnet syndrome. Survey of Ophthalmology, 48, 58-72.

   Teunisse, R.J., Cruysberg, J.R., Hoefnagels, W.H., Verbeek, A.L., Zitman, F.G. (1996). Visual hallucinations in psychologically normal people: Charles Bonnet's syndrome. Lancet, 347, 794-797.