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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 patients 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 that 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 contrasts. S-cones were not modulated.


RESULTS
The data of the normal subjects and of the patients can be described by a model in which the amplitudes and the phases of the signals originating 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 advanced.


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 systems, when modulated simultaneously at 30 Hz, is subtractive in RP patients and additive in normal subjects. Our data show that the use of only a standard white flicker ERG might lead to a misinterpretation of the mechanisms involved in retinal disorders, because the phases of different cone-driven responses are not considered.