An “electronegative” or “negative” ERG is the term used when a normal dark-adapted a-wave is followed a markedly lower amplitude b-wave (the waveform is dominated by the negative-going a-wave). It commonly occurs in post-receptoral rod system dysfunction e.g. congenital stationary night blindness. The term has recently applied to a rod-cone dystrophy (RP) as the dark-adapted bright flash b-wave was smaller than the a-wave (1).

In the "photopic hill" phenomenon, the photopic b-wave, derived from synchronised cone On- and Off- bipolar cell (BPC) signals with rod function suppressed by light adaptation, reaches a maximum amplitude with increasing stimulus strength but then reduces in amplitude even though the a-wave amplitude continues to increase. However, this phenomenon is a physiological property of cones and can be observed under dark adaptation in clinical disorders where rod photoreceptor function is lost, such as vitamin A deficiency or RDH5-retinopathy (fundus albipunctatus). Thus, dark-adapted ERGs in a cone isolated retina can mimic a negative ERG despite different underlying cellular origins. The same phenomenon, a scotopic b-wave smaller than the a-wave, can also occur in patents with severe RP when rod function is lost and all remaining ERG signals, under both dark and light adapted conditions, arise in residual cones. The dark-adapted a-wave is also profoundly subnormal due to loss of rod photoreceptor function. A dark-adapted red flash ERG in such a patient shows detectable dark-adapted cone responses, but no detectable rod system responses.

As the term negative ERG usually implies dysfunction involving On-BPCs with preserved rod photoreceptor derived a-wave, confusion can arise if the term is used when rod photoreceptor function is largely lost. The term “pseudo-negative” ERG has been suggested to prevent such confusion in relation to the implied underlying pathophysiological mechanisms when the signals arise in dark adapted cones. The electroretinographic features that enable that pathophysiology to be correctly identified will be demonstrated.

Electrophysiology recording requires precision and attention to detail. The ISCEV stimulus descriptions are unambiguous (DA 0.01, LA 3.0 etc.). Waveform terminology should be equally precise.