Frequency Analysis of OPs in the Slow-sequence MfERG in Primates: Comparison of Experimental Glaucoma and Pharmacological Inner Retinal Blockade

Abstract

Abstract: : Purpose: To define the frequency spectrum of oscillatory potentials (OPs) present in the macaque photopic flash electroretinogram (ERG) measured at different retinal eccentricities by analyzing the slow-sequence multifocal ERG (mfERG), and to compare the effect of experimental glaucoma and pharmacological inner retinal blockade on these frequency components. Methods: Slow-sequence mfERGs were recorded differentially between the two eyes of anesthetized (Ketamine- 20-25 mg/kg.hr; and Xylazine- 0.8 mg/kg.hr) adult monkeys (Macaca mulatta) using DTL fiber electrodes. The multifocal stimulus display consisted of 103 equal-sized hexagons within 17^o of the fovea. The m-sequence was slowed (14 blank frames ~200 ms, interleaved between flashes) to produce waveforms similar to photopic full-field flash ERG. Recordings were made under control conditions, and either after inner retinal blockade with (TTX [1-2 µM; n=4], TTX+NMDA [1.4-6.4 mM; n=3], TTX+NMDA+PTX [0.1-0.4mM, n=2], GABA [37-55 mM; n=2]) or after laser-induced experimental glaucoma (n=7). An FFT was used to determine the power of the various frequency components of the OPs. Glaucomatous visual field defects were assessed using behavioral static perimetry. Results: The major OPs in the slow-sequence mfERG that were affected by inner retinal damage or blockade occurred between 100-200 Hz. Under control conditions these OPs were larger in the foveal and temporal retina than in nasal retina. Pharmacological blockade of inner retina resulted in a decrease in 100-200 Hz power at all retinal locations with the greatest effect for the foveal and temporal regions. In experimental glaucoma, the power at 100-200 Hz in the summed mfERG decreased with progression of glaucoma, with an abrupt decrease at an early stage of glaucoma (MD<-5dB) followed by a gradual decrease with further progression of field defects (r=0.7 for foveal OPs, r=0.6 for temporal OPs). This decrease in power at early stage of glaucoma was similar to the effect of TTX alone but the further decrease in the power was more similar to effect when inner retinal nonspiking activity was also blocked. Conclusions: The high frequency OPs (100-200 Hz) of the slow-sequence mfERG are reduced in experimental glaucoma when field defects are mild, with progressive reduction with increasing field defects. The OPs from primate retina contain contributions from both spiking and nonspiking activity of inner retinal neurons.

Keywords: electroretinography: non-clinical • retina: proximal(bipolar, amacrine, and gangli • visual fields