hemianopic hallucination

   Also known as hemianoptic hallucination, hemi-anoptic optical hallucination, hemiopic hallucination, hemioptic hallucination, and hemihallucination. The term hemianopic hallucination is indebted to the Greek words hèmi (half), an (not), and opsis (seeing). It translates loosely as 'a hallucination co-occurring with blindness on one side'. Hemianopic hallucinations are "visual hallucinations occurring in individuals suffering from " hemianopia. They typically restrict themselves to the amaurotic hemifield. Hemianopic hallucinations can be "unformed, "formed, or even " complex in nature. An article published in 1886 by the French-American neurologist Édouard-Constant Séguin (1843-1898) is often referred to as the first biomedical description of hemi-anopic hallucinations. Although hemianopic hallucinations tend to manifest themselves in the impaired visual field, they can also present in the intact field of vision. Visual hallucinations concomitant to hemianopia have also been described as filling the whole field of vision, with or without a line of fracture at the border between the amaurotic and intact hemifields. In addition, there are a few case reports of "autoscopy (i.e. the perception of a hallucinated "double of oneself) occurring in the hemianopic field. The pathophysiology of hemianopic hallucinations localized in the amaurotic field of vision may be similar to that of the "Charles Bonnet syndrome (CBS). They are traditionally regarded as " release phenomena, i.e. as hallucinations arising from spontaneous endogenous activity in subcortical brain areas. A competing explanatory model is known as the " deafferentiation hypothesis. However, the literature is inconclusive as to the exact pathophysiology of hemianopic hallucinations. In individual cases it is generally possible to determine the cause of hemianopia with the aid of localizing techniques such as "EEG and " MRI, but it is still unclear whether the existing lesions are also responsible for mediating any hallucinatory activity. Since the time of Séguin various hypotheses have been tested, focusing on the involvement oflocal epileptiform activity emanating from damaged brain tissue (as in " epileptic aura), perceptual release activity, or " reperception, and the involvement of the temporal, parietal, and occipital lobes. In a sample of 120 individuals with homonymous hemi-anopia or " quadrantanopia, due mainly to lesions within the occipital lobe, the German neurologist Hans Wolfgang Kölmel found 16 cases which were complicated by " complex visual hallucinations within the hemianopic field. On the basis of an analysis ofthese cases, Kölmel tentatively concludes that "complex visual hallucinations in the hemianopic field may be interpreted as combined stimulation and release phenomena and differentiated from pure stimulation phenomena such as the aura of epileptic seizures as well as from pure release phenomena appearing as visual hallucinations in cases of extracerebral visual disorders."

   References

   Bhaskaran, R., Kumar, A., Nayar, P.C. (1990). Autoscopy in hemianopic field. Journal of Neurology, Neurosurgery and Psychiatry, 53, 1016-1017.

   Freiman, Th.M., Surges, R., Vougioukas, V.I., Hubbe, U., Talazko, J., Zentner, J., Honeg-ger, J., Schulze-Bonhage, A. (2004). Complex visual hallucinations (Charles Bonnet syndrome) in visual field defects following cerebral surgery. Report of four cases. Journal of Neurosurgery, 101, 846-853.

   Kölmel, H.W. (1985). Complex visual hallucinations in the hemianopic field. Journal of Neurology, Neurosurgery andPsychiatry, 48, 29-38.

   Lamy, H. (1895). Hémianopsie avec hallucinations dans la partie abolie du champ de la vision. Revue Neurologique, 3, 129-135.

   Pick, A. (1904). The localizing diagnostic significance of so-called hemianopic hallucinations with remarks on bitemporal scintillating sco-tomata. American Journal of the Medical Sciences, 78, 586-587.

   Séguin, E.C. (1886). A contribution to thepathol-ogy of hemianopsis of central origin (cortex-hemianopsia). Journal ofNervous and Mental Disorders, 13, 1-38.