Electroretinograms evoked by sinusoidal excitation of human cones.

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Abstract
The amplitude and phase of the fundamental Fourier component of the human electroretinogram (e.r.g.) were recorded with a synchronous detection method under conditions in which each of the 3 species of cones can be assumed most sensitive in turn. Weber-Fechner behavior is well established at, or more distal in the retina than, the source of these voltages. Results over the frequency range 7-50 Hz exclude a diffusion model of human flicker perception. The e.r.g. phase vs. frequency plot found with a red test differs from that obtained with a green. The shapes of the e.r.g. field sensitivity action spectra agree with those of the subject''s IIj (.mu.) (j = 3, 4 and 5) mechanisms of Stiles and with in situ measurements of the absorbance spectra of human cone pigments. Threshold phase with each test was independent of background wave-length but, consistent with the results in 3, the phase of the response to the red test (25 Hz) differed significantly from that to the green. If these differences resulted from the absorption of test photons of different colors at different points along the outer segment (independent of cone spectral sensitivity), they would be as clear on dichromats as on trichromats. Results on a protanope are inconsistent with this prediction. Differences in phase are due to the different kinetics of different cone mechanisms and that the e.r.g. studied here are evoked by exiting only the most sensitive cone mechanism, even though dark-adaptation studies prove that at e.r.g. threshold the test is well above psychophysical threshold for all 3 cone species. If this inference is correct, studies of sensitivity across the retina suggest that the spatial distribution of long-, middle- and short-wave-sensitive cones in the human retina differ remarkably.