Fig. 2. Left. Dark-adapted ERG demonstrating the oscillatory potentials riding the ascending limb of the b-wave. Right. By selective filtering, the slower components of the ERG, including the b-wave, can be eliminated leaving only the fast-frequency components, the a-wave, and the oscillatory potentials. (Carr RE, Siegel IM: Visual Electrodiagnostic Testing. Baltimore, Williams & Wilkins, 1982)