A PROXIMAL RETINAL COMPONENT IN THE PRIMATE PHOTOPIC ERG A-WAVE

Purpose. The monkey photopic ERG was studied during administration of glutamate analogs to determine whether the photopic a-wave derives exc lusively from photoreceptors. Methods. Monkey photopic ERGs were elici ted using 200-msec flashes or 30-mu sec xenon photostrobe flashes on a steady light-adapting background of 40 cd/m(2) (3.3 log scotopic trol and). Intravitreal injections of APB, PDA, or both were given to block transmission to depolarizing and hyperpolarizing second-order retinal neurons, respectively. Results. After injecting PDA to block light re sponses of horizontal cells and hyperpolarizing bipolar cells, part of the photopic a-wave was eliminated. The PDA-sensitive component, pres umed to be due to activity postsynaptic to cones, was responsible for the photopic a-wave threshold and dominated the response over the init ial 1 to 1.5 log units of intensity. For brighter stimuli, this compon ent made a constant contribution to the photopic a-wave. A non-PDA-sen sitive contribution to the a-wave, presumed to originate directly from cones, was first evident 1 to 1.5 log units above photopic a-wave thr eshold. It progressively dominated the a-wave at higher intensities, p articularly at early time points after the flash. Injecting PDA almost eliminated the photopic a-wave elicited with bright xenon photostrobe flashes that are commonly used for human clinical ERG diagnostic test ing, indicating that this a-cave may contain significant postreceptora l activity. Conclusion. The primate photopic ERG a-wave derives, in pa rt, from retinal activity postsynaptic to cone photoreceptors, particu larly for stimuli near the photopic ERG threshold that are typically u sed for human clinical studies.