The influence of stimulus size on L- and M-cone driven electroretinograms

Abstract

Purpose: To determine the ERG responses to isolated L- and M-cone stimuli as a function of stimuli size and temporal frequency, and correlate them with post-receptoral pathways properties.

Methods: Flicker ERGs were recorded monocularly from four trichromatic subjects using corneal fiber electrodes. Sinusoidal stimuli were presented in a Ganzfeld bowl, containing four colored light-emitting diodes (LED). The mean luminance of the white background was 284 cd/m2. A triple “silent substitution” method was applied, resulting in either L-cone or M-cone isolating stimuli each at 10% cone contrast. Flicker ERG measurements were repeated at five temporal frequencies (8, 12, 30, 36 and 48 Hz), in 14 different spatial stimulus configurations implemented by black cardboard field stops: one full-field stimulus, seven circular stimuli varying in size between 10° and 70° in 10° steps, and six annular stimuli with 70° outer diameter and inner diameters between 10° and 60° varying in 10° steps. Fast Fourier Transform (FFT) was used to extract amplitude and phase from the fundamental component. L/M ratio and phase difference were estimated from the averaged responses.

Results: At 8 and 12 Hz, the averaged amplitudes were constant for all stimuli configurations, and the L/M ratio was close to unity. In 30, 36 and 48 Hz, ERG amplitude increased with increasing stimulus size. L-cone driven ERGs were slightly larger than M-cone driven ERGs. The L/M ratio varied with stimuli size and was particularly large for full field stimuli. The L/M ratio differed for 30, 36 or 48 Hz frequencies.

Conclusions: The data confirm results that we presented at last year’s ARVO using reddish backgrounds and suggest that ERG responses to low and high temporal frequencies stimuli are processed by different post-receptorals mechanisms, probably parvocellular and magnocellular pathways. In addition, the L/M ratio for the high frequency mechanism depends upon stimulus size but also on temporal frequency.

Keywords: 510 electroretinography: non-clinical • 688 retina • 471 color vision