In our previous studies of visual perception in hemiplegia1-4 we were repeatedly struck by the fact that the perceptual accuracy of judgments, of the visual vertical, horizontal, median plane, and object distance made by hemiplegic patients were significantly inferior to those made by non-brain-injured controls when the judgments were made in the dark. However, no significant inferiority was manifested when the judgments were made under conditions of normal illumination. Thus, when both hemiplegic and non-brain-injured control patients were compared with respect to the accuracy of their judgments of the true vertical, using a luminous rod in a dark light-proofed room, statistically significant differences in accuracy of judgment resulted. But when the same subjects were compared on data derived from testing judgments of verticality made under conditions of normal illumination, no statistically significant differences in accuracy of judgment were obtained. Related findings were obtained when the judgment was of
BIRCH HG, BELMONT I, REILLY T, BELMONT L. Visual Verticality in Hemiplegia: Visual Influences on Perception. Arch Neurol. 1961;5(4):444–453. doi:10.1001/archneur.1961.00450160094007
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