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M.-L. Garon, A. Polosa, E. Zimak, P. Lachapelle; The No A-Wave ERG Paradigm: Hiding or Absent?. Invest. Ophthalmol. Vis. Sci. 2009;50(13):2176.
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The electroretinogram (ERG) waveform usually consists of two major components, the a- and b-waves, each representing the electrical manifestation of the orderly sequence of events triggered by the ERG stimulus. Exceptions to the above rule are scotopic ERGs evoked to dim flashes, where only b-waves are recorded. The aim of this study was to examine if: 1- the no a-wave ERG was a feature of scotopic ERG only and, 2- test the claim that it might be masked by the b-wave or absent.
Photopic (background: 30 cd.s.m-2; flash:-1.81 to 2.84 log cd.s.m-2 in 0.2 log unit steps) and scotopic (-5.01 to -0.96 log cd.s.m-2 in 0.4 log unit steps) ERG luminance-response functions were obtained from normal human subjects (n=30) and Long-Evans (n=8) and Sprague-Dawley (n=8) rats (scotopic ERGs only).
Human photopic ERG waveform always included an a-wave, irrespective of intensity. With flash luminance, its peak time first increased, reached a maximal value and then decreased progressively and could be fitted to the following fourth order polynomial function [y=15.54+(-1.650)x+(-0.6329)x2+0.2664x3+(-0.02811)x4; R2=0.9981]. In contrast, scotopic a-waves appeared only in ERGs evoked to flashes equal to or brighter than -3.01 log cd.s.m-2. With increasing intensity, the peak time of the a-wave shortens following a linear model [y=9.173x+13.82, R2=0.9454]. On average, for each log-unit of flash attenuation, the timing of the scotopic a-wave shortens by more than 10 ms compared to 1.4 ms for the photopic one. In rats, the a-wave peak time luminance-response function could be fitted to a sigmoidal curve [Long-Evans: y= 7.377 + 15.623 / (1+10^((0.5452-x))), Max: 23.0 ms, R2=0.9370; Sprague-Dawley: y= 9.033 + 20.387 / (1+10^((-0.2997-x))), Max:30.4 ms, R2=0.9916].
The a-b-wave rule seems to apply only to photopic ERGs where an a-wave could still be easily identified even in responses less than 1% of maximal amplitude. Solving the scotopic LR function of human and rats for the no a-wave ERGs point to a group of submicrovolt, positive, oscillations occurring prior to the onset of the b-wave, that could represent remnant of the original a-wave.
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