The multifocal visual evoked potential and cone-isolating stimuli: Implications for L- to M-cone ratios and normalization

Abstract

Multifocal visual evoked potentials (mfVEP) were recorded with a pattern-reversing display that modulated only the long wavelength-sensitive (L) cones or only the middle wavelength-sensitive (M) cones. Outside the central 5.8 degrees (radius), the ratio of the amplitudes of the mfVEP responses to L- and M-cone modulation varied across the six subjects, ranging from 1.1 to 1.7. The responses from the central 1 degrees (radius) showed a substantially lower ratio, ranging from 0.8 to 1.1 (average of 0.9). The variation among individuals outside the central fovea is probably due to differences in the ratio of the L/M cone input to both magno- and parvocellular pathways. The substantially lower ratios for the central responses is consistent with an L/M cone ratio closer to 1.0 in the central 1 degrees and/or an adjustment in the gain of the L- versus M-cone contributions to the central parvocellular pathways. Taking into consideration evidence from other techniques, we believe it is unlikely that most individuals have a L/M cone ratio of 1.0 in the fovea. Instead, it appears that there is a change in gain before the mfVEP is generated in area 17.

Cone selective adaptation influences L- and M-cone driven signals in electroretinography and psychophysics, by Kremers, Stepien, Scholl & Saito

“…Our data show that, when the chromatic channel mediates flicker detection, the L-/M-cone sensitivity ratios are around unity for all trichromatic observers and for all adaptation conditions used in the present study. This confirms previous proposals (Hood et al, 2002;Kremers et al, 2000;Pokorny, Smith & Wesner, 1991) that a mechanism is present in the chromatic channel that compensates for individual differences in L-and M-cone densities. From the present data, we can extend the presence of the compensatory mechanism to different adaptation conditions.…”

“…These ratios are probably related to the number of L-and M-cones that are stimulated (Brainard et al, 2000;Kremers et al, 2000). However, when the psychophysical sensitivities are mediated by the chromatic channel, the L-/M-cone sensitivity ratio is around unity for all subjects with normal color vision (Hood et al, 2002;Krauskopf, 2000;Kremers et al, 2000).…”

Flicker ERGs Representing Chromaticity and Luminance Signals

“…The results of other experiments can also be explained on the basis of this idea. A change in cone weightings in multifocal ERGs and multifocal visual evoked potentials has been described [25][26][27] indicating that the L/M ratio can depend on the stimulus and recording technique. As mentioned in the introduction, we found previously that a change in chromatic adaptation changed the L/M ratio in the ERG measured at 30 Hz and in luminance-mediated psychophysical flicker detection thresholds of L-and M-cone isolating stimuli, 23 from which it can be concluded that the spectral sensitivities of the psychophysical luminance channel and the high-frequency flicker electroretinography are similarly altered by chromatic adaptation and therefore share postreceptoral adaptation mechanisms.…”

“…24 Other observations have shown that a ratio close to unity can also occur in the multifocal ERG and the multifocal visual evoked potential. [25][26][27] It was found that the L/M ratio in psychophysical flicker detection thresholds is approximately unity when the red-green chromatic channel underlies flicker detection. 19,28 -30 Furthermore, the 12-Hz ERGs showed less interindividual variability and were less strongly influenced by chromatic adaptation.…”

Topography of the long- to middle-wavelength sensitive cone ratio in the human retina assessed with a wide-field color multifocal electroretinogram

“…For example, one could speculate that unlike the L and M cones in the central retina those in the far periphery express or turnover mRNA unequally. The electroretinogram (ERG) has been an effective tool for investigating cone-derived signals in the retina (Hood et al, 2002; Murray et al, 2004) and has recently been proven to accurately predict relative differences in L:M cone ratio across individual in a direct comparison with results in the central retina from adaptive optics imaging (Hofer et al, 2005). However, the putative cone ratio changes that are suggested by the mRNA results are most dramatic in the far periphery beyond 40° of eccentricity with the percentage of cones approaching 100% L near the ora serrata.…”