Decreased retinal illumination (such as can be caused by pupillary constriction or light absorption by ocular media opacities) was simulated with a randomly ordered series of neutral density filters in front of the right eyes of five subjects with dilated pupils. Threshold measurements were performed on Humphrey and Octopus perimeters at 0, 5, 10, 15, 20, and 25 degrees nasally along the 180-degree meridian. A 0.6-log unit neutral density filter, which reduces retinal illumination the equivalent of halving the pupillary diameter, decreased the mean Humphrey thresholds by 1.1 ± 0.8 decibels (dB) (mean ± standard deviation) and the mean Octopus thresholds by 1.7 ± 1.4 dB. Statistically significant (P ≤ .05, Dunnett's test) threshold depressions were observed at all eccentricities with a 1.5-log unit neutral density filter on the Humphrey perimeter (-4.5 ± 0.7 dB) and with a 1.0-log unit neutral density filter on the Octopus perimeter (-3.5 ± 1.0 dB).
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