S-cones, flicker and luminance

Abstract

The poor temporal resolution of S-cone vision is often attributed to the confinement of S-cone signals to sluggish chromatic pathways, but it could also reflect a deficit in the S-cones themselves. This question can be resolved psychophysically if the S-cone signal can be measured through the faster achromatic pathway. This is possible on intense orange fields, on which the S-cone response can be followed to nearly 30 Hz. Moreover, if S-cone flicker is combined with suprathreshold L-cone flicker at a slightly mistuned frequency, visible beats at the difference frequency can be detected for frequencies as high as 40 Hz. On these intense fields, we can distinguish two S-cone pathways: one with a low-pass frequency response that detects little above 12 Hz, and a second with a more extended bandpass response. The bandpass signal is capable of participating in flicker photometric nulls with M- and L-cone signals and shows a high frequency sensitivity loss that is not greatly different from the other cones, suggesting that S-cones are brisk and can feed the achromatic luminance channel. The luminance input is negative, but this is complicated by phase lags that can reach 180° at 18 Hz.