The spectral properties of the two rod pathways

Abstract

Psychophysical and electroretinographic observations in normal and achromat observers suggest that rod flicker signals have access to at least two retinal pathways: one (q,), slow and sensitive, predominating at scotopic luminance levels, the other (a;), 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 s;, but have comparatively little effect on A,,. The z; field sensitivities derived from these measurements have been fitted with linear combinations of the scotopic luminosity function, Vi, the M-cone spectral sensitivity function, M,, and the L-cone function, L,. These fits demonstrate a clear cone influence on ni, 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 x; in the deuteranope and an M-cone influence in the protanope. This suggests that both cone types can affect the sensitivity of I$ The finding that in the steady-state cone signals reduce the sensitivity of u; but have Little effect on a,, could suggest that n; signals travel through a faster cone pathway (with its own gain control at which both rod and cone signals can reduce rod threshold), while rc,, signals travel through a separate rod pathway. However, it could simply reflect the fact that nt predominates at higher luminances than 1~~ where the cone excitation level is inevitably greater. To examine the influence of the cones on u. more closely, we: (i) produced transient cone excitation by alternating rodquated 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 A~. Rods Temporal sensitivityFlicker Spectral sensitivity Field sensitivity Rodxone interaction INTRODUCTIONPsychophysical data (Conner & MacLeod, 1977;Conner, 1982;Sharpe, Stockman & MacLeod, 1989) and electroretinographic (ERG) recordings (Stockman, Sharpe, Zrenner & Nordby, 1991) in the normal and achromat observer demonstrate that rod flicker signals have access to both a slow and a fast retinal pathway. The slower pathway [labeled a,, after the notation of Stiles (1978)] is more sensitive and dominates from absolute threshold up to low mesopic levels. The faster pathway (labeled nh) is less sensitive and takes over flicker detection only at moderate and high mesopic levels.The two pathways are most clearly revealed in the double-branching of scotopic critical flicker fusion (CFF) vs intensity functions (Hecht, Shlaer, Smith, Haig & Peskin, 1938;Hecht, Shlaer, Smith, Haig & Peskin, 1948;Conner & MacLeod, 1977 et al., 1989). To explain the restricted range of the null, we assume that the intensity dependencies of x0 and ...

An extended 15 Hz ERG protocol (1): the contributions of primary and secondary rod pathways and the cone pathway

“…However, for a flicker of 15 Hz, the perception of flicker diminishes for flash strengths above the flicker threshold. At higher flash strengths, the perception of flicker reappears and becomes more pronounced [8][9][10][11][12]. In later studies, this disappearance of flicker perception was found to correlate with a minimum in the amplitude versus flash strength curve of 15 Hz flicker electroretinograms (ERGs) [13,14].…”

Into the twilight zone: the complexities of mesopic vision and luminous efficiency

“…Inset shows the experimental conditions: a 6 deg diameter, 500 nm target was presented in the centre of an 11.5 deg diameter, 668 nm background at an eccentricity of 13 deg. Based on data from Sharpe et al. (1993b).…”

Motion perception at scotopic light levels

“…5,[27][28][29][30][31][32][33] However, psychophysical investigations suggest that there is little or no difference between the spatial extents of the neural excitation pools of the two signals. 28,31 Moreover, if the differences between rod and cone motion is largely due to filtering by the rod photoreceptors themselves or to the early spatial and temporal summing of their signals (or both), rather than to the subsequent retinal pathway characteristics, then no change of perceived velocity with a change in luminance should be expected.…”

“…[22][23][24][25][26] Psychophysical and electrophysiological studies in man support an analogous rod duality in the human visual system; the clearest signature of which is a loss of flicker visibility and a corresponding reduction in electroretinographic response amplitude at frequencies near 15 Hz and at intensities near 0 log scotopic td caused by destructive interference between slow (low intensity) and fast (high intensity) rod signals. 5,[27][28][29][30][31][32][33] Are these slow and fast rod flicker signals correlated with the two cortical motion systems? Does the rod visual system at low intensities, at which only the slow retinal pathway is functioning, have access to the fast cortical motion system?…”