Phase-dependent sensitivity to heterochromatic flicker.

We measured modulation sensitivity to a pair of equally luminous sinusoidally modulated lights (568 and 630 nm) as a function of their relative phase. Measurements were made for 2, 3, 6, and 12 Hz at a retinal illuminance of 100 Td. The data indicated that two processes were active and their outputs combined by a vector summation rule. There was a phase shift of -18 degrees to -20 degrees (630 nm leads 568 nm) at 6 Hz, no phase shift at 12 Hz, an equivocal shift at 2 Hz, and an indeterminate shift at 3 Hz. At frequencies where a phase shift was observed, our analysis indicated that the phase shift affected sensitivities measured at all relative phase settings. These results are inconsistent with models postulating equal contributions of long-wavelength- and middle-wavelength-receptors to centers and surrounds of processes responsible for the detection of luminance flicker.