Abstract

We have used the presence of a nonlinearity in the visual system to determine the temporal properties of the distal retina. If a sinusoidal luminance modulation is presented to the eye, the electroretinogram (ERG) responds both at the fundamental frequency of stimulation, and at higher harmonics of the stimulation frequency. We have used the presence of a nonlinear response at a difference frequency to probe the structure of the early visual system, prior to the nonlinear responding elements. A nonlinear element will respond to a sum of two frequencies both at the harmonics of each individual frequency and at sums and differences of the input frequencies. We presented pairs of sine waves with a fixed frequency difference of 8 Hz. Since the frequency of the difference remained constant, it is assumed that filtering of the difference frequency subsequent to the nonlinearity is identical across stimulus conditions. Frequency pairs presented were 12-20, 16-24, 24-32, 32-40, 40-48, 48-56, 56-64, and 64-72 Hz. We presented each frequency pair at a range of modulations (from 0.087 to 0.5) and measured the response of the eye at each modulation by Fourier analysis of the averaged ERG response. The retinal illuminance was 4000 td, and a broadband green LED was used as the light source (peak wavelength 565 nm). By using a criterion response voltage we can control for response amplitude nonlinearities (the 8-Hz response saturated at low modulations). The filter we obtained was low pass and essentially flat up to 40 Hz. This result is in contrast to the psychophysical bandpass filter measured at similar retinal illuminances.

© 1989 Optical Society of America