Factors influencing threshold of the fundamental electrical response to sinusoidal excitation of human photoreceptors.

The amplitude and phase of the fundamental Fourier component of the electroretinogram (e.r.g.) in response to sinusoidally modulated light were studied in the range 7-50 Hz. Sensitivity was best at the lowest frequency. The threshold-frequency relationship divided into two parts. A weak steady background depressed sensitivity of the low, but increased sensitivity of the high, frequency component. At 8 Hz a small test spot was 0.7 log10 units more effective on the most sensitive part of the retina than on the optic disk. On the fovea, it was 0.1-0.2 log10 units less effective than on the disk. The fovea was 0.7 log10 units more sensitive to 25 Hz than the blind spot. Psychophysical and e.r.g. dark-adaptation curves were similar, but the former was 10(4) times more sensitive than the latter. Four sets of experiments examined the possibility that the Fourier component of the e.r.g. response at the modulation frequency of 8 Hz during the 'rod' phase of the e.r.g. dark-adaptation curve arose from excitation of rods alone. The only hint of a possible cone contribution was a very small but systematic increase in phase delay with increase in background wave number found while measuring the field sensitivity action spectrum. No suggestion was found that the fundamental Fourier component of threshold e.r.g. responses at the modulation frequency of 25 Hz was influenced by photons absorbed in rods.