The temporal characteristics of the early and late stages of the L- and M-cone pathways that signal color.

Flickering long-wavelength light appears more yellow than steady light of the same average intensity. The hue change is consistent with distortion of the visual signal at some nonlinear site (or sites) that produces temporal components not present in the original stimulus (known as distortion products). We extracted the temporal attenuation characteristics of the early (prenonlinearity) and late (post-nonlinearity) filter stages in the L- and M-cone chromatic pathway by varying the input stimulus to manipulate the distortion products and the measuring of the observers' sensitivity to them. The early, linear, filter stage acts like a band-pass filter peaking at 10-15 Hz with substantial sensitivity losses at both lower and higher frequencies. Its characteristics are consistent with nonlinearity being early in the visual pathway but following surround inhibition. The late stage, in contrast, acts like a low-pass filter with a cutoff frequency around 3 Hz. The response of the early stage speeds up with radiance, but the late stage does not. A plausible site for the nonlinearity, which modelling suggests may be smoothly compressive but with a hard limit at high input levels, is after surround inhibition from the horizontal cells.