The b-wave of the electroretinogram (ERG) is believed to result largely from the light responses of ON bipolar neurons. One apparent inconsistency is that the b-wave is a phasic signal, more similar to ON responses of transient third-order neurons than to the ON bipolar light responses. However, when selective glutamate agonists or antagonists were used to isolate the b-wave, it was found to be tonic and resemble the light response waveform of the ON bipolar cell.