Cone signal contributions to electrograms in dichromats and trichromats

PURPOSE. TO find out how the different tone types contribute to the electro retinogram (ERG) by quantifying the contribution of the signal pathways ori ginating in the long (L-) and the middle (M-) wavelength-sensitive cones to the total ERG response amplitude and phase. METHODS. ERG response amplitudes and phases were measured to cone-isolating stimuli and to different combinations of L- and M-cone modulation. Conditi ons were chosen to exclude any contribution of the short wavelength-sensiti ve (S-) cones. The sensitivity of the ERG to the L and the M cones was defi ned as the cone contrast gain. RESULTS. In the present paper, a model is provided that describes the ERG c ontrast gains and ERG thresholds in dichromats and color normal trichromats . For the X-chromosome-linked dichromats, the contrast gains of only one co ne type (either the L or the M cones) sufficed to describe the ERG threshol ds for all stimulus conditions. Data suggest that the M-cone contrast gains of protanopes are larger than the L-cone contrast gains of deuteranopes. T he response thresholds of the trichromats are modeled by assuming a vector summation of signals originating in the L and the M cones. Their L- and M-c one contrast gains are close to a linear interpolation of the data obtained from the dichromats. Nearly all trichromats had larger L- than M-cone cont rast gains. Data from a large population of trichromats were examined to st udy the individual variations in cone weightings and in the phases of the c one pathway responses. CONCLUSIONS. The data strongly suggest that the missing cone type in dichro mats is replaced by the remaining cone type. The mean L-cone to M-cone weig hting ratio in trichromats was found to be approximately 4:1. But there is a substantial interindividual variability between trichromats. The response phases of the L- and the M-cone pathways can be reliably quantified using the response phases to the cone-isolating stimuli or using a vector additio n of L- and M-cone signals.