The mysterious a‐wave

Abstract

Abstract Purpose The electroretinogram (ERG) waveform usually consists of two major components, the a‐ and b‐waves. 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 the no a‐wave ERG was a feature of scotopic ERG only and try to explain why it occurs. Methods 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). Results Human photopic ERG waveform always included an a‐wave. With flash luminance, its peak time first increased, reached a maximal value and then decreased progressively and could be fitted to a fourth order polynomial function. 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 shortened following a linear model.On average, for each log‐unit of flash attenuation, the scotopic a‐wave shortened by more than 10 ms compared to 1.4 ms for the photopic one. Of interest, in rat scotopic ERGs evoked to the dimmest intensities the a‐wave appeared to be replaced by a group of short latency oscillations that were best seen in recordings where the low frequency cut‐off had been raised to 100 Hz. Conclusion The a‐b‐wave rule seems to apply only to photopic ERGs where an a‐wave was still identifiable in ERGs smaller than 1% of maximal amplitude. One wonders if the submicrovolt oscillations occurring prior to the onset of the b‐wave of rat scotopic ERGs evoked to dim flashes could represent remnant of the original a‐wave. Funded by CIHR