Evidence for two distinct nonlinear components in the human pattern ERG.
We obtained the electroretinogram (PERG) in three observers to a 4.6 c/deg grating pattern with a sinusoidal luminance profile which was modulated in time simultaneously with two sinusoidal temporal frequencies (f1 and f2), or at a single frequency (either f1 or f2). Input temporal frequency ranged from 2.4 to 7.5 Hz. A pattern modulated in the counterphase mode with a single frequency produces a response containing only even harmonics of that frequency. However, when the pattern is counterphase modulated with both f1 and f2, the PERG contains second order response frequencies corresponding to intermodulation components (sums and differences) between the two fundamental frequencies. Such frequencies do not exist in the stimulus--they can only be generated by nonlinear neural interactions. Our results provide evidence for at least two nonsaturating nonlinear response mechanisms in the human retina.