Ten normal subjects underwent static threshold visual field testing of both eyes with the Humphrey perimeter, with one eye tested twice. The mean sensitivity of the field seemed virtually identical in the two eyes, with the average difference between the right and left eyes (0.65 decibels [dB]) being no greater than the testing error as reflected in the difference between the same eye tested twice (0.7 dB). The authors provide the mathematical basis for recognizing that a right eye-left eye difference in mean sensitivity might be abnormal. Additional information is needed about the variance of the right eye-left eye difference in th population at large, but present information suggests that a 2-dB difference may be meaningful on a single examination. A 1.5-dB difference is statistically significant if confirmed on a second test, and a difference as small as 1 dB may be meaningful if shown consistently in a series of four examinations. In all cases, nonglaucomatous causes of field abnormality needs to be ruled out, and the generalized asymmetry is most meaningful if it is consistent with asymmetry of cupping or intraocular pressure. Several cases are reported in which a mild (1 dB) generalized depression of the visual field is the only recognizable abnormality in the visual field in eyes with early glaucoma.
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