Technique to investigate the temporal phase shift between L- and M-cone inputs to the luminance mechanism

We describe a technique to estimate the intrinsic phase shift between long-wavelength-cone (L-cone) and middle-wavelength-cone (M-cone) signals in the luminance mechanism with minimal contamination by chromatic mechanism(s). The technique can also estimate, simultaneously with the phase shift, the weight ratio of L and M cones for the luminance mechanism. We measured motion identification thresholds for a 1.0 cycle/deg, 12.0-Hz sinusoidal grating representing different vector directions in L- and M-cone contrast space. The physical phase of the L- and M-cone signals was varied over a broad range between -150 deg and +150 deg to investigate the effect on the threshold contours. The slope of the threshold contour in cone contrast space varied as a function of the physical phase. Estimates of the intrinsic phase shift between L and M cones are based on the change in slope of the threshold contour. The estimates are consistent with previous reports and show that whereas the L-cone signal lags behind the M-cone signal by approximately 35 deg for an orange background, the M-cone signal lags behind the L-cone signal by approximately 8 deg for a green background.