The effect of refractive blur on the detection sensitivity to light offsets in the central visual field

Detection sensitivity to light decrements created on a cathode ray tube was measured along the nasal horizontal meridian of the central visual fields of 5 normal subjects. Decremental (light offset = negative contrast = dark-on-bright) static stimuli were presented at 6, 12, 18, 24 and 30 degrees eccentricity. Detection thresholds to rectangular stimuli of both 4 and 16 mm2 (Goldmann equivalent size III and IV, respectively) were determined with ascending staircase single-crossing technique under various Snellen visual acuity levels. Detection sensitivity diminished with increasing amounts of plus lens-induced refractive blur. One log unit decrease in visual acuity resulted in elevation of detection thresholds for an average of 3.8 contrast decibels (dB) using size III offset stimuli. The effect of blur was less with size IV offset stimuli which resulted in an average of 0.9 dB elevation in detection thresholds. The larger size decremental stimuli provided a wider dynamic range, less interindividual variation in stimulus visibility and more resistance to refractive error than those obtained with the smaller size stimuli.