Rod and cone responses in the flicker ERG

Abstract

Psychophysical studies have shown that the rod system can reduce the flicker sensitivity of the cone system at temporal frequencies near 10 Hz, apparently through the interference of out-of-phase rod and cone signals. Light-adapting or bleaching the rod system improves flicker sensitivity. We have examined an electrophysiological correlate of this rod-cone interaction by measuring ERG responses to a long-wavelength (Wratten No. 23A) Ganzfeld stimulus flickering at 10 Hz, either in the dark-adapted eye, against a steady Ganzfeld background, or following a Ganzfeld bleach. Consistent with the psychophysical finding, the amplitude of the cone component of the b wave is larger if the eye is moderately light-adapted or partially bleached rather than completely dark-adapted. A comparison of the ERG responses to scotopically matched short- and long-wavelength Ganzfeld flickering stimuli shows that in the dark-adapted eye, a negative-going rod response subtracts from the positive cone b-wave response, reducing its amplitude. Light adaptation shortens the implicit time of the rod response and reduces its amplitude, so that its negative influence on the cone b wave is reduced. The effect of light adaptation on the cone flicker ERG is therefore predictable from the algebraic summation of phase-shifted rod and cone ERG subcomponents.