To determine whether anterior ischemic optic neuropathy and compressive optic neuropathy in humans alter the photopic flash ERG and to investigate the cellular origins of the waves that are affected by pharmacologic agents in primates. Photopic flash ERGs were recorded differentially, with DTL electrodes, between the two eyes of 22 patients with diagnosed optic neuropathy (n = 17, anterior ischemic optic neuropathy [AION]; n = 5, compressive optic neuropathy) and 25 age-matched control subjects and in 17 eyes of 13 monkeys (Macaca mulatta). The stimulus consisted of brief (<5 ms) red (lambda(max) = 660 nm) Ganzfeld flashes (energy range, 0.5-2.0 log td-s) delivered on a rod-saturating blue background of 3.7 log sc td (lambda(max) = 460 nm). An eye of the patient with ischemic changes at the disc was classified as symptomatic if it showed visual field defects with a mean deviation (MD) of P < 2%. Recordings in macaque monkeys were made before and after inner retinal blockade with tetrodotoxin (TTX) (1.2-2.1 microM; n = 7), TTX+N-methyl-d-aspartate (NMDA; 1.4-6.4 mM; n = 7), and cis-2, 3 piperidine dicarboxylic acid (PDA; 3.3-3.8 mM; n = 3). The PhNR amplitude was significantly reduced in both symptomatic (P = 3.4 x 10(-8)) and asymptomatic (P = 0.036) eyes of patients with AION or compressive optic neuropathy (P = 0.0054) compared with control subjects. The PhNR amplitude in the symptomatic eye showed a moderate correlation with field defects (P < 0.05) similar to previous findings in open-angle glaucoma. The a-wave also was reduced significantly in the symptomatic eye (P = 0.0002) of patients with AION. The i-wave, a positive wave on the trailing edge of the b-wave peaking around 50 ms, became more prominent in eyes in which the PhNR was significantly reduced. In monkeys, the PhNR was eliminated by TTX. The a-wave at the peak and later times was reduced by TTX, further reduced by NMDA, and eliminated after PDA in response to the red stimuli. PDA also eliminated the i-wave. PhNR amplitude is significantly reduced in eyes with open-angle glaucoma, AION, and compressive optic neuropathy. Experiments in primates indicate that this reduction reflects loss of a spike-driven contribution to the photopic ERG. There also are small spike-driven contributions to the a-wave elicited by full-field red stimuli. The i-wave, which becomes more prominent when the PhNR is reduced, has origins in the off-pathway distal to the ganglion cells.