The spectral response curve (amplitude versus wavelength) of the R2of the early receptor potential (ERP) was studied in normal, protan, and deutan subjects. The R2amplitude peaked at 520nm in most normal subjects. The R2at long wavelengths was smaller than normal in protans and larger than normal in deutans when the maximum amplitudes were normalized to 100 at the peak. The ratio of the R2amplitude at 460 nm to that at 600 nm clearly differed between protans and deutans. The ERP and the rapid off-response, which is mainly due to the cessation of the late receptor potential, were recorded in the same subjects. The ratio of the sensitivity of the rapid off-response at 500 nm to that at 600 nm was correlated with the ratio of the R2amplitude at 460 nm to that at 600nm (correlation coefficient, 0.823, p<0.001). This study, in conjunction with our previous study, indicates that the abnormality is in the outer segments of the cones in protans and deutans.
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