Second-site adaptation in the red-green chromatic pathways.

On different chromatic adapting fields, thresholds were measured with a 1.2 deg flash consisting of simultaneous incremental and decremental red and green components that stimulate the M and L cones in any desired ratio. Thresholds were plotted in normalized coordinates in which the quantal change in the M and L cones due to the flash was divided by the quantal catch due to the field. Detection contours for a wide range of test flashes provide evidence for luminance and chromatic mechanisms that respectively respond to the sum and difference of the M and L cone signals. Field color has little influence on the luminance mechanism but strongly affects chromatic detection, with sensitivity being maximal on yellow fields and declining slightly on green fields and declining strongly on red fields. Similar effects were obtained for long (200 msec) and very brief flashes, although the shape of the contours differed considerably. The results provide evidence for a second adaptation site within the red-green chromatic pathways, similar to the second-site in the S cone pathways. Chromatic fields (green and red) polarize the site and reduce sensitivity to chromatic flashes.