Abstract
To evaluate rod and cone contributions to the dark-adapted 15-Hz flicker electroretinogram (ERG) across a broad range of stimulus luminances by comparing rod-isolating (ERGR), cone-isolating (ERGC), and non-receptor-specific (ERGR+C) responses. Dark-adapted, full-field 15-Hz ERGs were obtained from four normally sighted subjects (ages 29-36years) using a four-primary LED-based stimulating system. The primaries were either modulated sinusoidally in phase (ERGR+C) or were modulated in counter-phase to achieve rod isolation (ERGR) or cone isolation (ERGC) by means of triple silent substitution. Measurements were made for a broad range of luminances (-2.5 to 1.8 log scot. cd/m(2) in 0.2 log unit steps). Fourier analysis was used to obtain the amplitude and phase of the fundamental response component at each stimulus luminance. Stimulus luminance had different effects on response amplitudes and phases under the three paradigms. Specifically, ERGC amplitude and phase increased monotonically as luminance increased. The effects on ERGR+C and ERGR were complex: ERGR+C and ERGR amplitude was small and the phase decreased for low luminances, whereas amplitude and phase increased sharply at moderate luminances. For high luminances, ERGR+C amplitude and phase increased, whereas ERGR amplitude decreased and phase was approximately constant. At low luminances, the ERGR+C and ERGR functions can be attributed to interactions between two rod pathways. At high luminances, the functions can be accounted for by interactions between rod and cone pathways (ERGR+C) or rod insensitivity (ERGR). The ERGR paradigm minimizes cone intrusion, permitting assessment of rod function over a large range of luminance levels.
Highlights
At low luminances, the ERGR+C and ERGR functions can be attributed to interactions between two rod pathways
Electroretinograms (ERGs) elicited by 15-Hz flicker stimuli presented to the dark adapted eye have been used to assess the response of the rod pathway across a broad range of luminance levels [e.g. 1–6]
The present study evaluated rod and cone contributions to the dark-adapted 15-Hz ERG across a broad range of luminance levels using a commercially available four-primary photostimulator that was implemented based on the principle of silent substitution [18,22] to generate three types of temporally modulated stimuli: 1) rod isolating stimuli in which cone excitation was kept constant (ERGR), 2) cone isolating stimuli in which S, M- and L-cone excitations were modulated in phase while keeping the rod excitation constant (ERGC), 3) non-receptor-specific stimuli in which both rod and cone excitations were modulated in phase at the same contrast (ERGR+C)
Summary
Electroretinograms (ERGs) elicited by 15-Hz flicker stimuli presented to the dark adapted eye have been used to assess the response of the rod pathway across a broad range of luminance levels [e.g. 1–6]. The complex relationship has been attributed to interactions between the cone and rod pathways at mesopic luminance levels and between two different rod pathways at scotopic luminance levels [1]. There is evidence for at least two rod pathways in the mammalian retina [7,8,9,10,11,12,13,14]: a “slow” rod pathway that transmits signals from the rod photoreceptors, to rod ON bipolar cells, to AII-amacrine cells, to cone ON and OFF bipolar cells and subsequently to ganglion cells. A second rod pathway, the “fast” pathway, transmits signals from rod to cone photoreceptors via gap junctions, to cone ON and OFF bipolar cells and their ganglion cells. The fast and slow rod pathways differ in response timing as well as the luminance range over which they operate, but previous psychophysical [6, 15,16] and electrophysiological [1,2,3,5,6,17] work has provided evidence that there is a luminance range over which both pathways operate simultaneously
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