The oscillatory potentials (OPs) of the electroretinogram (ERG), first demonstrated by Cobb and Morton,¹¹ and subsequently named by Yonemura et al.,⁶⁵ identify the low-voltage, high-frequency components, sometimes referred to as ERG wavelets, which are often seen indenting the rising phase of the b-wave. This is best exemplified with the ERG tracings shown in figure 43.1A, where two OPs (identified as 2 and 3 in the figure) are seen riding on the ascending limb of the b-wave. Although the true oscillatory nature of the OPs was previously questioned,¹² what singles them out is really the difference in frequency domain between the OPs and the other components of the ERG. This prompted Dawson and Stewart¹² to suggest the term fast retinal potential as a more appropriate descriptor of these low-voltage, high-frequency ERG components. As we will see, the OPs are not oscillations in the true sense of the word and are most probably not generated by the same retinal structure or pathway. In this chapter, I will focus mostly on oscillatory potentials (and therefore on the ERG) evoked to full-field (Ganzfeld) brief flashes of white light delivered in light- and dark-adapted conditions because this remains, to date, the most universally used method to elicit clinical electroretinograms. It should be noted, however, that OPs can be obtained by using long flashes (i.e., >100ms) to separate ON-OPs and OFF-OPs²³ or colored flashes.⁵⁴ Finally, OPs can also be evoked to localized spots of light, either projected directly on the retina,⁴⁶ such as in the technique that is used to obtain focal ERGs, or viewed on a screen, as is the case with multifocal ERG techniques (mfERG).⁶⁴

Figure 43.1.  

Representative broadband photopic ERGs (1- to 1000-Hz bandwidth; column A) and oscillatory potentials (100- to 1000-Hz bandwidth; column B) recorded simultaneously from the two eyes of a normal human subject. C shows the fast Fourier transform obtained from the two broadband ERG responses. Note the three major power peaks at approximately 35, 72, and 132Hz, the latter component representing the oscillatory potential contribution to the ERG potential. The signals were recorded with DTL fiber electrodes (30 deniers/10 filaments in OD and 27 deniers/7 filaments in OS for comparison). Calibration: vertical, 40µV (A) and 10µV (B); horizontal, 20ms (A, B) and hertz (C). Vertical arrows identify flash onset. Tracings in A and B are preceded by a 20-ms prestimulus baseline.