The multifocal electroretinogram (mfERG) and cone isolating stimuli: Variation in L- and M-cone driven signals across the retina

Multifocal electroretinograms (mfERG) were recorded from 38 normal trichromats with a pattern-reversing display that modulated only their long-wavelength sensitive (L) or only their middle-wavelength sensitive (M) cones at equal cone contrasts and average quantal catches. The display consisted of scaled, 103 hexagonal elements, subtending 84 degrees x 75 degrees of visual angle. Typically, the amplitude of the L-cone driven signal was greater than that for the M-cone driven one at all retinal eccentricities, but large differences were found among observers. These values correlated with L- to M-cone ratios obtained psychophysically in the same observers using 2 degrees (dia.) heterochromatic flicker photometry. Interestingly, the L-to M-cone driven amplitude ratios differed between the central and peripheral retina. For the central fovea (5 degrees dia.), the mean ratio was 1.4 +/- 0.6 (for the N1P1 component), whereas for the annular ring centered at 40 degrees in the periphery, it was 2.3 +/- 2.0. The mean P1 latency of the summed M-cone driven mfERG (28.0 +/- 2.6 ms) was significantly advanced relative to the L-cone driven signal (29.0 +/- 1.9 ms), but the mean N1 latencies were similar (15.6 +/- 1.7 ms and 16.2 +/- 1.3 ms, respectively). The P1 latency difference between the L- and M-cone driven waveforms was not found in the central 5 degrees (dia.) of the retina. However, it increased with retinal eccentricity. The regional differences in the amplitudes and latencies of the L- and M-cone driven mfERG signals can be related to variations in the L- to M-cone ratios and/or the receptor to bipolar gain factors that depend on eccentricity.