Isolation of opponent-colour mechanisms at increment threshold.

An experimental examination was made of some paradigms designed to isolate the opponent-colour system at increment threshold. The effectiveness of a uniform white conditioning field spatially coincident with a 1.05-deg uniform test field was assessed by measuring intensity thresholds for simple detection and for colour discrimination. Values were obtained both by a method of adjustment and by a two-interval forced-choice procedure. For sufficiently high luminances of the conditioning field (3000 td or greater) little or no difference was found between simple-detection and colour-discrimination thresholds over the critical test-flash spectral range 520-620 nm, implying that the paradigm produced almost complete isolation of the opponent-colour system at increment threshold. A control experiment in which thresholds were obtained for a conditioning field larger than the test field gave less satisfactory isolation; near 580 nm the luminance system was found to be at least 0.3 log unit more sensitive than the opponent-colour system. A comparison was also made of the spatially coincident field paradigm with a paradigm in which a modified test stimulus of low temporal and spatial frequency content was presented on a large conditioning field. Test spectral sensitivity curves for simple detection obtained by a method of adjustment showed little difference in effectiveness in opponent-colour isolation.