The photopic negative response of the macaque electroretinogram: Reductionby experimental glaucoma

PURPOSE. TO investigate the photopic flash electroretinograms (ERGs) of mac aque monkeys in which visual field defects developed as a consequence of ex perimental glaucoma. METHODS. Unilateral experimental glaucoma was induced in 10 monkeys by argo n laser treatment of the trabecular meshwork. Visual field sensitivity was assessed behaviorally by static perimetry. Photopic ERGs were recorded to b rief- (less than or equal to 5 msec) and long-duration (200 msec) red ganzf eld hashes on a rod-suppressing blue-adapting background. Electroretinogram s were recorded in four other monkeys, after intravitreal injection of tetr odotoxin (TTX; 3.8-8 mu M) to suppress action potentials of retinal ganglio n and amacrine cells, and in six normal adult human subjects. RESULTS. Experimental glaucoma removed a cornea-negative response, the phot opic-negative response (PhNR), from the ERG. The PhNR in control eyes was m aximal approximately 60 msec after a brief flash, 100 msec after onset: and 115 msec after offset of the long-duration stimulus. The PhNR in experimen tal eyes was greatly reduced when the mean deviation of the visual field se nsitivity was as little as -6 dB. As visual sensitivity declined further, t he PhNR was reduced only slightly more. The a and b-waves were unchanged, e ven when sensitivity decreased by more than 16 dB. Tetrodotoxin also select ively reduced the PhNR. The PhNR was observed in normal human ERGs. CONCLUSIONS. The cornea-negative PhNR of the photopic ERG depends on spikin g activity and is reduced in experimental glaucoma when visual sensitivity losses are still mild. The PhNR most likely arises from retinal ganglion ce lls and their axons, but its slow timing raises the possibility that it cou ld be mediated by glia. Regardless of the mechanism of its generation, the PhNR holds promise as an indicator of retinal function in early glaucomatou s optic neuropathy.