Electroretinogram (ERG). Diagram of ERG demonstrating the combined response of rods and cones after bright flash stimulus. The electrical tracing of the ERG (yellow) overlies the retina and the respective cells of origin. The A wave is a negative deflection reflecting the initial hyperpolarization of the photoreceptors as a result of the phototransduction cascade. The B wave is positive deflection reflecting the depolarization of the bipolar cells resulting from the absence of the inhibitory neurotransmitter glutamate. Müller cells also contribute to the B wave. The contribution of ganglion cells is lost in the large B wave and requires different testing conditions.