S-cone contrast sensitivity in glaucoma as a function of mean luminance

Evaluation of tritan defects is potentially useful for early diagnosis and evaluation of treatment of glaucoma, but the clinical utility of tests such as the Farnsworth-Munsell 100 hue has been limited due in part to effects of individual variations in pupil size and extent of lens yellowing. S-cone contrast sensitivity has been proposed as a more useful tritan test, but it is not known precisely how S-cone contrast sensitivity measured with a computer monitor is influenced by variations in mean S-cone quantal catch due to individual differences in pupil size and lens density. We measured S-cone contrast sensitivity of patients with glaucoma, using 1.0 cycle/degree blue grating superimposed on a bright yellow background. Sensitivity was measured at a range of mean luminances, and pupil size was monitored with a closed-circuit video system. Inter-subject variability was reduced by plotting S-cone contrast sensitivity vs. retinal illuminance rather than stimulus luminance, indicating that much of the variability was due to differences in pupil size. Each patient’s data were fit with a threshold versus retinal illuminance (TVR) function. The TVR analysis suggested that additional variability was due to prereceptoral filters. These results indicate that reductions in S-cone contrast sensitivity at luminances available from computer monitors may reflect, in part, effects of pupillary miosis or lens yellowing, rather than optic nerve damage.