Flicker electroretinograms: a systems analytic approach.

We observed multiple maxima and minima in the electroretinogram (ERG) first harmonics which varied as a function of adaptation level, mean illuminance, and modulation depth. Based on differences in response characteristics we identified a minimum of 3 parallel mechanisms operating at 3 frequency regions within the ERG: a low frequency region (less than 10 Hz), a midfrequency region (centered near 20 Hz), and a high frequency region (centered near 40 Hz). The low frequency region was observed both in dark- and light-adapted conditions, was basically linear, and showed nonlinear behavior only at high contrasts in either light or dark adaptation. It may be modeled as a low pass filter. The midfrequency region was clearly observed only above cone threshold, was nonlinear at low contrasts, and may be modeled as a linear filter followed by an essential nonlinearity. The high frequency region was observed under high levels of light adaptation and also was nonlinear at low contrast. It may be modeled also as a linear filter followed by an essential nonlinearity.