Detection and discrimination of small, brief lights: variable tuning of opponent channels.

There is disagreement as to how closely the properties of the pathways that detect small spots resemble the properties of the achromatic system. Spectral sensitivities for small, brief lights show a single, relatively broadband peak similar to the achromatic spectral sensitivity. The lights also appear relatively desaturated near threshold. However, recent field sensitivity data (Finkelstein and Hood, 1981, 1982) have shown that opponent mechanisms are either directly involved in detecting small spots, or they interact with, and desensitize mechanisms are either directly involved in detecting small spots, or they interact with, and desensitize, a strictly achromatic detection pathway. The test sensitivity, test mixture, and wavelength discrimination data of the present study support the first alternative; they implicate opponent (chromatic) mechanisms in the detection of small, brief lights. However, the data cannot be fit by the same opponent systems used to fit large-test data. They suggest instead an hypothesis in which large and small lights are detected by opponent mechanisms that change their spectral tuning with changes in stimulus parameters.