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Susan M. Menees, Michael Bach; Normal variability of reversal- and onset-VEPs and their amplitude measurement. Journal of Vision 2005;5(8):191. doi: 10.1167/5.8.191.
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© ARVO (1962-2015); The Authors (2016-present)
Question: The ISCEV VEP-standard suggests quantifying amplitude of pattern reversal-VEPs as the N75-P100 excursion; some laboratories use the P100-N170 deflection. We asked which of the two, or their average, is more suitable; we used intraindividual and interindividual variability as quality measures. Methods: We recorded pattern-reversal and pattern-onset VEPs from 22 normal eyes of 11 subjects. Check size was 0.25° and 1.0°, luminance 45 cd/m2, contrast 97% for reversal and 80% for on-off. Binocular runs, later discarded, initiated a block design balancing eyes, stimuli, and repetitions. One channel, Oz vs. Fpz, was recorded using the EP2000 “Freiburg Evoked Potentials” system. Results: Automatic algorithms evaluated all traces. Traces to onset-offset stimulation varied sizably in amplitude and shape between individuals. In some individuals the C1-C2-C3 complex was not well defined. Traces show that onset-offset stimulation produces poor consistency of response between subjects. Traces to reversal stimulation were markedly more similar between subjects. The N75-P100-N170 complex was always well defined. There was sizable variation between subjects with respect to amplitude, and the relation of the N75-P100 vs. P100-N170 amplitude; in some subjects a large N75-P100 compensated a small P100-N170 and vice-versa. CV of repeated runs was 7.25±0.82 for N75-P100, 8.50±1.26 for P100-N170, and 6.07±0.94 for the N75-P100-N170 average; mean amplitude across subjects was 14.34±2.07. Inter-ocular CV was very low, always in the range of the test-retest CV. Interpretation: It is important to recognize norm-variants. The higher variability of on-off is well known, as is the high interocular similarity. The results suggest that for reversal, the average of N75-P100 and P100-N170 amplitudes reduces population variability. Assuming on average similar pathologic effects on the two components, the N75-P100-N170 average may be a more sensitive and specific measure of VEP magnitude.
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