Effect of spatial frequency of stimulus on focal macular ERGs in monkeys
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To determine the effect of the spatial frequency of a small grating stimulus centered on the macula on the focal macular ERGs (fmacERGs) of monkeys.
fmacERGs were recorded from eight eyes of four adult monkeys (Macaca fuscata). The spatial frequency of the stimulus was changed from 0.25 to 8 cycles/degree. The luminance of the light bars was 10 cd/m2, and the contrast was 95%. The stimulus was flashed on and off with an on duration of 100 ms and an off duration of 150 ms (4 Hz). The stimulus was centered on the fovea and subtended 12.7° at the cornea. The luminance of the steady light-adapting background was 3.5 cd/m2. The location of the stimulus on the retina was monitored throughout the recordings. The effects of the spatial frequency of the stimulus on the amplitudes and implicit times of the a-waves, b-waves, and oscillatory potentials (OPs) were determined. fmacERGs were also recorded following intravitreal tetrodotoxin (TTX).
The amplitudes of the a- and b-waves did not change with changes in the spatial frequency of the stimulus. The OPs, on the other hand, responded best to the lowest spatial frequency, and the OPs after the first two were attenuated at intermediate and higher frequencies (Wilcoxon signed-rank test: P < 0.05). TTX reduced all OP wavelets in monkeys.
The OPs of the photopic macular ERGs are affected by the spatial frequency of the stimulus and are reduced by TTX, consistent with their being generated by inner retinal neurons.
KeywordsAmacrine cells Focal macular electroretinogram Inner retinal neural neurons Spatial frequency Tetrodotoxin (TTX)
The authors thank Prof. Duco Hamasaki, Bascom Palmer Eye Institute, University of Miami School of Medicine, Miami, FL, USA, and Prof. Geoffrey B Arden, IOVS volunteer editor, Henry Wellcome Laboratories, Applied Vision Research Centre, Department of Optometry and Visual Science, City University, London, UK, for editing and reviewing the manuscript.
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