Red-green color discrimination as a function of stimulus field size in peripheral vision

Red-green color-discrimination thresholds were measured at eccentricities of 10 and 25 deg in the nasal retina. Thresholds were measured as a function of stimulus field size both during the cone plateau and after dark adaptation. During the cone plateau, threshold decreased with increasing field size, but the effect of field size was dependent on the color of the test stimulus. The decrease in threshold was greater for yellow and orange test stimuli than for red and green tests. Two factors, summation and opponent-mechanism adaptation, appear to affect the relation between threshold and field size. An equation suggested by Boynton and Kambe in 1980 [Color Res. Appl. 5, 13 (1980)] provides a good description of the variation in thresholds with field size and eccentricity. After dark adaptation, thresholds increased for all test colors, suggesting that rod signals reduce discrimination. The dark-adapted thresholds could be described well by the addition of a rod term to the Boynton-Kambe equation.