An absolute threshold defect in the inferior retina.

Absolute thresholds were checked in six meridians in 18 patients at 10 and 20 degrees eccentricity, with VA degree and 1 degree targets. Thresholds averaged higher in the inferior than in the superior retina. The highest threshold was obtained from the inferior vertical meridian. Area-intensity functions were obtained from 3 subjects for the superior and inferior vertical meridians. The thresholds were generally higher for the inferior meridians. The results were discussed in terms of possible retinal factors. The difference in function between the inferior and superior retinal meridians is probably embryological in origin and presumably is secondary to delayed retinal differentiation in the region of the embryonic fissure.