Fig. 5. Electroretinogram of a rat over an 8 log-unit intensity range. The brightest flash elicits a sharp, initial, negative a-wave followed by a larger, positive b-wave. As the stimulus is made dimmer, the a-wave is diminished first, and then the b-wave is reduced, until only a slow, negative scotopic threshold response (STR) is seen for the dimmest flashes. The a-wave reflects activity of rod photoreceptors, the b-wave of bipolar cells, and the STR of amacrine cells. Aspartate was injected into the vitreous of the eye. Aspartate blocks transmission at the photoreceptor synapse, so that bipolar and amacrine cells can no longer respond to light. Because the b-wave is no longer present after aspartate, the a-wave is larger. After aspartate, the a-wave is the initial and leading edge of PIII.