Response saturation of short-wavelength cone pathways controlled by color-opponent mechanisms

Response saturation of the short-wavelength cone pathways was measured with foveal, violet test flashes on steady or flashed violet fields. These violet stimuli were presented on an intense steady yellow “auxiliary” field that suppressed the Π 4 and Π 5 mechanisms. At given violet field intensities the increment threshold curve became very steep, demonstrating response saturation of the short-wavelength pathways. This response saturation could be greatly reduced or eliminated by flashing the yellow auxiliary field, rather than having it steady, or by adding a flashed yellow field to the steady yellow auxiliary field. The response saturation was strongly promoted by presenting a negative yellow field flash (i.e. a decrement of the steady yellow field) with the positive violet field flash. The results show that the response saturation is controlled, not by independently acting short-wavelength cones, but by cancellative mechanisms that receive signals of opposite sign from the short-wavelength cones and the middle- and long-wavelength cones.