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Effect of Experimental Glaucoma in Primates on Oscillatory Potentials of the Slow-Sequence mfERG

From the College of Optometry, University of Houston, Houston, Texas.

PURPOSE. To determine the effect of experimental glaucoma in macaque monkeys on oscillatory potentials (OPs) in the slow-sequence multifocal electroretinogram (mfERG).

METHODS. Photopic slow-sequence mfERGs were recorded from anesthetized adult macaque monkeys and normal human subjects. The stimulus consisted of 103 equal-sized hexagons within 17° of the fovea. The m-sequence was slowed, with 14 blank frames, 200 ms, interleaved between flashes for monkeys and 7 blank frames, 100 ms, for humans, to produce waveforms similar to the photopic full-field flash ERG. Recordings were made under control conditions (24 monkey eyes, 7 human) and after laser-induced experimental glaucoma in monkeys (n = 8). A Fourier fast transform [FFT] was used to determine the frequency ranges of the major OPs. OP amplitudes were quantified by using root mean square (RMS) for two-frequency bands in five horizontal and four vertical locations. Visual field defects were assessed using behavioral static perimetry. Full-field photopic flash ERGs also were recorded.

RESULTS. OPs in two distinct frequency bands were discriminated in the monkey mfERG: fast OPs, with a peak frequency of 143 ± 20 Hz, and slow OPs, with a peak at 77 ± 8 Hz. There were similar findings in humans and with the flash ERG in monkeys. The fast OP RMS in monkey control eyes was significantly larger in temporal than nasal retina (P < 0.01) and in superior versus inferior retina (P < 0.05) as reported previously. The slow OP RMS was largest in the foveal region. Experimental glaucoma reduced fast OP RMS in all locations studied, even when visual field defects were moderate (MD = –5 to –10 dB; P < 0.05), whereas the slow OP RMS was reduced significantly primarily in the foveal region when field defects were severe (MD < –10 dB; P < 0.05). The fast OP RMS showed a moderate correlation with local visual field sensitivity and with local ganglion cell density (calculated from visual field sensitivity). For the slow OPs the correlation was much poorer. Consistent with previous studies, the photopic negative response (PhNR) amplitude was significantly reduced when the visual sensitivity was minimally affected.

CONCLUSIONS. OPs in the ERG of primates fall in two frequency bands: fast OPs with a peak frequency around 143 Hz and slow OPs, with a peak frequency around 77 Hz. The fast OPs, which rely more on the integrity of retinal ganglion cells and their axons than do the slow OPs, have potential utility for monitoring the progression of glaucoma and the effects of treatment.

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