In a scotopic electroretinography, which wave is considered the slower third wave?

• A. A-wave
• B. B-wave
• C. C-wave
• D. D-wave

Answer: (C)

In a scotopic ERG, the sequence of waves maps to different retinal layers: the first negative deflection comes from photoreceptors, the following positive deflection mainly reflects ON-bipolar (and Müller) cell activity, and the slower, broader deflection that follows is the C-wave. The C-wave arises from the retinal pigment epithelium’s response to changes in the photoreceptor environment, which involves slower, subretinal processes like ion fluxes across the RPE and photoreceptor-RPE interactions. Because these RPE-driven changes unfold more slowly than the neural signals generating the a- and b-waves, this C-wave is the slower third wave. The D-wave is a later, condition-dependent response and isn’t the standard third wave in typical scotopic recordings.