Two-band model of heterochromatic flicker.

We have attempted to reconcile the results of several recent chromatic flicker studies. By adjusting the relative amplitudes of red and green sine-wave stimuli that were flickering in opposite phase, we obtained conditions varying from purely chromatic (red-green) stimulation, through each "silent-cone" condition, to purely luminous (homochromatic) stimulation. We also tested the effects of adapting backgrounds in each condition. Our results can be explained in terms of a low-frequency band that represents the opponent-color response, and a high-frequency band that represents the achromatic response. These two bands respond in various proportions, depending on the red-green stimulus ratio. Chromatic adaptation generally affects the low- and high-frequency bands differently and hence changes the shape of the flicker sensitivity curve. However, if the temporally varying waveform and the adapting background are both chosen to stimulate the same cone type, then the opponent-color and achromatic bands are both attenuated by the same amount. In this case, the shapes of the silent-red and silent-green flicker curves are preserved under chromatic adaptation. We conclude that none of these flicker curves are controlled by the temporal characteristics of independent cone types.