Large phase differences between L-cone- and M-cone-driven electroretinograms in retinitis pigmentosa

PURPOSE. TO Study the dynamics and interactions of the signals originating in the long- (L-) and middle (M)-wavelength-sensitive cone pathways in pati ents with retinitis pigmentosa (RP). METHODS. Twenty-six patients with RP and 29 normal subjects participated in the study. Electroretinographic (ERG) responses were measured to stimuli t hat modulated exclusively the L- or the M-cones or the two simultaneously ( both in-phase and in counter-phase) with varying ratios of L-to M-cone cont rasts. S-cones were not modulated. RESULTS. The data of the normal subjects and of the patients can be describ ed by a model in which the amplitudes and the phases of the signals origina ting in the L- and M-cones are vector summed. In the RP patients, there was a general reduction in ERG sensitivity. The L-cone-driven ERG response was significantly delayed, whereas the M-cone- driven ERG response was phase a dvanced. CONCLUSIONS. Large dynamic differences between L and M-cone-driven ERGs can be detected in RP. As a result, the interaction between the L and M-cone s ystems, when modulated simultaneously at 30 Hz, is subtractive in RP patien ts and additive in normal subjects. Our data show that the use of only a st andard white flicker ERG might lead to a misinterpretation of the mechanism s involved in retinal disorders, because the phases of different cone-drive n responses are not considered.