- visual allachaesthesia
- Also known as optical allachaesthesia, optical alloaesthesia, and fata morgana of the visual sphere. The first three expressions are indebted to the Greek words attache (elsewhere) and aisthanesthai (to notice, to perceive). They translate loosely as 'seeing in a different place'. All four terms are used to denote the illusory projection of a regular sense perception from one quadrant of the visual field to the diagonally opposed quadrant (hence the name fata morgana of the visual sphere). For example, a person with visual allachaesthesia may perceive a visitor standing on the right as standing in an inverted position on the left, as if this person were trodding on the ceiling. The German term optische Alloästhe-sie (i.e. optical alloaesthesia) was introduced in or shortly before 1928 by the Austrian neurologist and psychiatrist Otto Pötzl (1877-1962) and his colleague Georg Hermann. The phenomenon itself was described as early as 1895, as witness the autobiographical recording of this phenomenon by the German neurologist Ernst Beyer. As Beyer reports, he himself experienced visual allachaesthesia in the wake of one of his migraine attacks. He saw houses and a church in his left visual field, which were actually present on his right-hand side. In Beyer's case, the phenomenon was accompanied by a blurring of vision in the lower left quadrant and a * scotoma developing from the periphery towards the central field of vision. Visual allachaesthesia tends to occur in the context of visuospatial neglect, usually due to a lesion to the right parietal lobe. However, it has also been described in hysteria, in the context of *aurae preceding migraine attacks, and in association with lesions affecting the occipital lobe. Visual allachaesthesia is commonly classified as a *metamorphopsia, which is itself classified as a *sensory distortion. Visual allachaesthesia should not be confused with *inverted vision, a condition in which objects of fixation, or the entire extracorporeal environment, are perceived as if rotated 180o.ReferencesBeyer, E. (1895). Uber Verlagerungen im Gesichtsfeld bei Flimmerskotom. Neurologische Zentrallblatt, 14, 10-15.Critchley, M. (1939). Visual and auditory hallucinations. British Medical Journal, 2, 634-639.Halligan, P.W., Marshall, J.C., Wade, D.T. (1992). Left on the right: Allochiria in a case of left visuo-spatial neglect. Journal of Neurology, Neurosurgery and Psychiatry, 55, 717-719.Hermann, G., Pötzl, O. (1928). Die optische Alloästhesie. Berlin: S. Karger.Klee, A., Willanger, R. (1966). Disturbances of visual perception in migraine. Acta Neurolog-ica Scandinavica, 42, 400-414.
Dictionary of Hallucinations. J.D. Blom. 2010.