polyopia

   Also known as polyopsia. Both terms stem from the Greek words polus (much, many) and opsis (seeing). They refer to the perception of an image that repeats itself within the visual field. The ensuing coexistence of various similar images within the field of vision is called * multiplication. The term polyopia has been in use at least since the 1850s. In 1928 the German-American biological psychologist and philosopher Heinrich Klüver (1897-1979) divided polyopia into 'objective' polyopia, hallucinatory polyopia, and imaginal polyopia. In Klüver's system the term 'objective' polyopia refers to the perception of multiple identical images in regular visual perception (i.e. 'ghosting') and the term hallucinatory polyopia to the perception of multiple identical hallucinatory images. Klüver uses the term imaginal polyopia to denote a special form of polyopia in which the perception of part of an object gives rise to the subjective sensation of seeing the whole, and the subsequent idea of a coexistence of multiple versions of that object. Klüver illustrates the latter phenomenon with the following first-person account by one of his test persons. "M. passed me on my left side. I saw nothing but a part of his cloak. Automatically it turned into the whole figure of M.; and I had now some sort of idea that a large number of M.'s moved away from me in a curved line, the M. in the foreground being the smallest one. I was unable to say whether it was a very strong image or vision; phenomenally, the many M.'s were projected into the perceived space of the dark room." Pathophysiological^, polyopia is associated with either peripheral or central nervous system dysfunction. Peripheral variants of polyopia are associated primarily with corneal opacities, corneal irregularities, and lens irregularities. The ensuing polyopic images tend to be stable over time. Central variants of polyopia, also referred to as cerebral polyopia, are associated pathophysiologically with aberrant neuronal discharges affecting the occipital or occipito-parietal lobe. Etiologically, they are associated with a variety of conditions, including encephalitis, trauma, migraine, focal epileptic seizures, and the use of *hallucinogens such as mescaline and LSD. Cases of polyopia occurring in the context of an *aura are referred to as * ictal illusions. The literature on cerebral poly-opia indicates that in this variant the number of image reduplications tends to be restricted to two or four images. Single-image reduplications are referred to as * monocular diplopia. In cases of multiple image reduplications, which may be arranged in a row, a column, or a grid-like pattern, the term * entomopia is used. Polyopia is commonly classified as a rare type of *palinopsia, which itself constitutes a type of * visual perseveration. Visual perseveration, in turn, tends to be classified as a * reduplicative phenomenon or a type of * metamorphopsia.

   References

   ffytche, D.H., Howard, R.J. (1999). The perceptual consequences of visual loss: 'Positive' pathologies of vision. Brain, 122, 1247-1260.

   Klüver, H. (1966). Mescal and Mechanisms of hallucinations. Chicago, IL: University of Chicago Press.

   Lopez, J.R., Adornato, B.T., Hoyt, W.F. (1993). 'Entomopia': A remarkable case of cerebral polyopia. Neurology, 43, 2145-2146.