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S Viswanathan, S Demirel; Naso-temporal Variations in the Human Multifocal Electroretinogram (mERG) . Invest. Ophthalmol. Vis. Sci. 2002;43(13):2139.
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Purpose: Recent studies indicate that naso-temporal (NT) variations in the mERG of monkeys originate from inner-retinal activity (Hood et al., 1999, Frishman et al., 2000, Hood et al., 2001). Similar variations have been reported in the human mERG to lower stimulus contrasts (Hood et al., 1999). The purpose of the present study is to determine the recording conditions that can best accentuate these topographical variations in humans. Methods: mERGs were recorded with a VERIS system, differentially between DTL electrodes across the corneas of the two eyes from 3 normal humans. In a few experiments mERGs were recorded with Burial-Allen (BA) electrodes. The stimuli consisted of 103-scaled hexagons that subtended 37X31 deg at 48 cm. The mean luminance ranged from 50 to 200 cd/sq m and contrast from 12.5 to 100%. The monitor frame rate was 75Hz and in most of the experiments the M-sequence exponent was 15 with 1 frame per M-step. The low and high cut off frequencies of the amplifier were set at 3 and 100Hz. Only the first slice of the first order kernel was analyzed. Results: mERGs obtained with DTL electrodes showed prominent NT variations that were mainly in the relative heights of two positive potentials (P1 and P2). P1 (with peak times around 26 ms) had maximum amplitudes in the nasal field and decreased in size towards the temporal field at locations that were equidistant from fixation. At the same locations, P2 (with peak times around 34 ms) decreased in size from the temporal to nasal field. Consequently the amplitude ratios of P2 to P1 were larger in the temporal field. At all mean luminances the NT differences in P2/P1 were larger for lower stimulus contrasts (0.8 at 100%, 1.5 at 50%, 2 at 25%). For any given contrast there was no systematic variation of NT differences in P2/P1 as a function of mean luminance. NT variations were reduced when successive stimulus presentations were separated by at least 93 ms and nearly eliminated when separated by at least 186 ms. For standard stimulus presentation when the DTL electrode on the tested eye was restricted to the central cornea in the shape of a loop the NT variations were considerable reduced. NT variations were also considerably reduced with BA bipolar recordings. The speculum and to a much smaller extent the corneal electrode revealed small NT variations when referenced to a DTL electrode on the fellow eye indicating that the variations usually cancelled out in the bipolar recording condition. Conclusion: Human mERG responses contain large naso-temporal variations that are accentuated at low contrasts and are best observed when recorded differentially between DTL electrodes across the two eyes.
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