Interactions Between Photopic Visual Mechanisms Revealed by Mixing Conditioning Fields

The luminances of conditioning fields that are necessary to reduce the visibility of a previously suprathreshold test flash to an increment threshold are determined for each of two conditioning fields of different color. Normalizing each of these luminances to 1.0, subthreshold amounts (e.g., 0.8, 0.6, etc.) of one field component are preset by the experimenter; the subject then adjusts the other component until threshold is again attained. In an eye for which the adaptive state is controlled by the action of light upon only one spectral class of photopic mechanism, the normalized luminance sum must always equal 1.0. We find however that this sum is consistently less than 1.0, indicating a supersummative interaction of adaptive effect between two or more different mechanisms.