Contribution of post-receptoral cells to the a-wave of the human photopic electroretinogram

ERGs were recorded to red flashes (0.01–50 ph cd s m2) presented against a steady background (2000 sc td) or 0–300 ms after its suppression. The cone a-wave was altered in form and increased in amplitude in the dark. Peak amplitudes were doubled when the dark period was 50–100 ms and also when it was 150–200 ms. Measurement of the a-wave at fixed times showed that amplitude increase occurred at times later than 6–8 ms. The a-wave receives a significant negative-signal contribution from two post-receptoral mechanisms. These are adapted by weak backgrounds and recover their sensitivity extremely rapidly in the dark. The cone photocurrent alone contributes 40–70% of peak amplitude depending on stimulus intensity.