Which wave is the second positive wave on a scotopic or photopic ERG?

• A. a wave
• B. c wave
• C. b wave
• D. d wave

Answer: (C)

In an ERG trace, the retina’s response to light unfolds in a sequence of waveforms that reflect different cell layers. The a-wave is the initial negative deflection coming from photoreceptors. Right after that, the first prominent positive deflection—the b-wave—arises from inner retinal activity, mainly ON-bipolar (and Müller) cells driven by the photoreceptors. If the stimulus is sustained long enough, you can see a slower, additional positive deflection that follows—the c-wave—generated by the retinal pigment epithelium and its interaction with photoreceptors. A small positive response at light offset is the d-wave.

So, the second positive deflection in a typical scotopic or photopic ERG, after the a-wave and the first positive b-wave, is the c-wave.