This study is devoted to a quantitative characterization of the functional properties of the photopic electroretinogram (ERG) in the normal human eye. During this analysis, the relationship between the conventional flash ERG and the response to more complex input patterns is investigated.
In order to meet these goals, a mathematical model is developed which simulates the behavior of the photopic system. First, the flash ERG is expressed as the summation of five simple shapes, thus parameterizing the complex waveform. The theoretical development of a model based on these parameters is then presented. This model consists of five similar branches, one to account for the behavior of each component of the ERG. Instaneous model output is calculated on a digital computer by implementing the following discrete elements for each branch: (1) an element D which delays each input sample as a function of its energy, (2) a linear element G, whose output g represents the amplitude memory of the system, (3) a static nonlinearity Y, which acts upon g to determine the instantaneous amplitude gain factor y, (4) a model operator which multiplies y by the delayed instantaneous input intensity in order to compute the amplitude of the component response to each input sample, and (5) a shaping filter H whose impulse response is determined by the component being calculated and by the previous exposure to light.
This model structure is then used to successfully simulate the response to various light patterns, including flashes, steps, and sinusoids. In this manner, the functional properties of the ERG can be analyzed under different conditions of light adaptation.