Abstract

Field additivity depends on the spatial relations between test and field. By means of a 2AFC method, 20% subadditivity was found in detecting a 1° test flashed for 200 ms on one degree fields. This held for a 530 nm test (at one log unit above absolute threshold) on fields of 481 nm, 622 nm, and a mixture of the two. It also held for a 500 nm test on fields of 530 nm, 610 nm, and their mixture. This agrees with Foster's (1981) hypothesis that coincident edges elicit processing at opponent sites ("spectral sharpening"). Additivity was found with 1° tests on 10° backgrounds (Stiles' conditions), and 3.6' arc tests on both 8.6' arc and 10° backgrounds. In the spatially coincident, subadditive conditions, the variance of the threshold was greater on the mixed than on the pure fields. Correspondingly, the psychometric function was shallower on the mixed than on either pure field. The 530 nm 3.6' arc test flashed on its 10° background(s) produced additivity, but it also produced a shallower slope on the mixed field. This implies that under some spatial configurations the psychometric curve is only additive over a limited range.

© 1990 Optical Society of America