Differential effects of luminance and contrast on acuity thresholds across the visual field

Abstract

Purpose: To investigate the effects of contrast and luminance on acuity thresholds across the visual field. This may shed some light on the important question of why some visual thresholds change only slowly as a function of eccentricity, whereas others fall off extremely quickly.Methods: Resolution thresholds for Gabor patches were measured in the nasal visual field using a modified yes/no staircase procedure. The effect of contrast was examined by changing the physical contrast of the Gabor stimuli between the values of 3.125 and 100%. The effect of luminance was investigated using high contrast Gabor stimuli but varying luminance over 2.7 log units with the use of neutral density filters. Natural pupils were used throughout.Results: The variation in resolution thresholds as a function of eccentricity was well described by the equation T = T0/[+(E/E2)], where T represents threshold, T0 threshold at the fovea, E is eccentricity and E2 is a parameter which describes the rate of decline in performance, representing the eccentricity at which the foveal resolution threshold halves. Changes in contrast resulted in the same logarithmic change in resolution threshold at all eccentricities, leaving the E2 value unchanged. Thresholds were proportional to the logarithm of contrast, and transforming the data in this way accounted for 98% of the variance in the eccentricity and contrast dependent data. Variations in luminance produced quite different effects, with E2 becoming significantly greater as luminance was reduced. At all eccentricities, resolution thresholds were proportional to the logarithm of retinal illuminance, but the gradient of this relationship changed markedly as a function of eccentricity, becoming close to zero at the higher eccentricities studied.Conclusions: The differential effects of contrast and luminance on acuity thresholds can be understood in terms of a pyramidal model of early vision in which reductions in suprathreshold contrast affect the signal by the same amount in all early visual filters, whereas reductions in luminance have a selective effect on smaller visual filters which are more susceptible to quantal noise.