Abstract

Flicker modulation sensitivity measurements made on high intensity orange steady backgrounds indicate that signals from short-wavelength sensitive cones (S-cones) have access to two pathways. At low S-cone adaptation levels the frequency response falls quickly with increasing frequency, but at higher adaptation levels it extends to much higher frequencies. At these higher S-cone adaptation levels, the following procedures can selectively expose either a process sensitive to low frequencies or one more sensitive to higher frequencies: (1) at high flicker frequencies, the S-cone signal can be nulled by a long-wavelength sensitive cone (L-cone) signal of suitable amplitude and phase, but at low frequencies a residual flicker persists; the modulation sensitivity for the residual flicker is lowpass in shape with a rapid decline in sensitivity with increasing flicker frequency; (2) sensitivity to flicker in the presence of a 17 Hz S- or L-cone mask is also lowpass with a similarly steep loss of high frequency sensitivity; yet (3) sensitivity to flicker during transient stimulation of the S-cones at 0.5 Hz is comparatively wideband (and slightly bandpass) in shape. The S-cone signal produced by the high frequency process is almost as well-maintained towards high frequencies as M- and L-cone signals. Furthermore, it is capable of participating in flicker photometric nulls with M- and L-cone signals. At low frequencies, however, when the low frequency S-cone signal is also present, satisfactory nulls can not be found. From these and phenomenological considerations, we identify the low and high frequency S-cone processes as S-cone inputs to the chromatic and luminance pathways, respectively. The phase adjustments needed to optimize flicker photometric nulls reveal that the S-cone input to the luminance pathway is actually inverted, but this is demonstrable only at relatively low frequencies: at medium or high frequencies the S-cone influence can be synergistic with that of the other cone types because of a delay in the transmission of S-cone signals.