The spectral properties of the two rod pathways

Psychophysical and electroretinographic observations in normal and achromat observers suggest that rod flicker signals have access to at least two retinal pathways: one (pi 0), slow and sensitive, predominating at scotopic luminance levels; the other (pi'0), fast and insensitive, predominating at mesopic ones. We have measured steady-state flicker detection sensitivities on background fields ranging from 430 to 640 nm in normal observers. Our results suggest that cone signals can reduce the sensitivity of pi'0, but have comparatively little effect on pi 0. The pi'0 field sensitivities derived from these measurements have been fitted with linear combinations of the scotopic luminosity function, V' lambda, the M-cone spectral sensitivity function, M lambda, and the L-cone function, L lambda. These fits demonstrate a clear cone influence on pi'0, but they cannot tell us unequivocally whether the influence is from the M-cones, from the L-cones or from both. Accordingly, we made similar measurements in dichromats, who lack one of the two longer wavelength cone types. These measurements revealed an L-cone influence on pi'0 in the deuteranope and an M-cone influence in the protanope. This suggests that both cone types can affect the sensitivity of pi'0. The finding that the steady-state cone signals reduce the sensitivity of pi'0 but have little effect on pi 0 could suggest that pi'0 signals travel through a faster cone pathway (with its own gain control at which both rod and cone signals can reduce rod threshold), while pi 0 signals travel through a separate rod pathway. However, it could simply reflect the fact that pi'0 predominates at higher luminances than pi 0 where the cone excitation level is inevitably greater. To examine the influence of the cones on pi 0 more closely, we: (i) produced transient cone excitation by alternating rod-equated 480 and 679 nm fields; and (ii) extended our steady-state measurements to include deep-red backgrounds of 650 and 680 nm. Both experiments revealed a small, but measurable influence of the cones on pi 0.