ERG signals driven by chromatic and luminance processing: Variation as a function of retinal eccentricity

Abstract

In order to investigate the how L- and M-cone input to the chromatic and luminance mechanisms varies as a function of retinal eccentricity we recorded ERGs from human subjects at temporal rates of 12Hz and 30Hz using a range of circular stimuli of different angular subtense which increased in 100 steps up to 70-deg diameter. In addition, we also recorded responses from annuli with a 70-deg outer diameter but gradually ablated from the centre in 100 steps. L- and M-cone isolating (rod silent) responses were obtained from five colour normal subjects using a DTL fibre electrode. Cone contrasts were equalized for each stimulus condition.

Fourier analysis of the ERGs was used to measure the magnitude of the first harmonic of the response. For the 12Hz response the ratio of the L- and M-cone contribution to the ERG signal was found close to unity for all subjects. The group averaged data showed that the L:M ratio remained remarkably constant (close to 1:1) as a function of retinal eccentricity. For the 30Hz response the L- and M-cone ratio was found to vary between 4:1 and 10:1 across observers. Furthermore, in contrast to the 12Hz response, the L:M ratio for the 30Hz exhibited a large degree of variation with retinal eccentricity.

These differences in response properties as a function of retinal eccentricity provide further evidence to support the view that L- and M-cone isolating ERGs reflect the operation of different retinal processing pathways when elicited by either fast (30Hz) or slow (12Hz) temporal stimulation.