The a- and b-wave electroretinogram (ERG) represents the initial events of a fairly long series of potential changes in the retina and the retinal pigment epithelium (PE) that are evoked by light. When the dark-adapted eye is stimulated by turning on a continuous light, the fast a- and b-waves are followed by the slow c-wave of the ERG and by fast and slow (light peak) oscillations, which are still much slower (figure 42.1). If the light is turned off, a series of off-effects arise, including the “off c-wave,” the “off fast oscillation,” and the “off slow oscillation” (the dark trough). These off-potentials are of opposite polarity as compared with the corresponding on-potentials (figure 42.1). Whereas the a- and b-waves represent the photoreceptor potential^2,4 and interactions between the neural elements and the Müller cells in the inner retina^6,30,34 respectively, the slower potential changes reflect mainly PE changes in response to neuroretinal activity. It is of interest to study these PE responses clinically. The slow oscillations are generally investigated indirectly by means of the electro-oculogram (EOG).^3,14,16 For the c-wave of the ERG, however, a setup for corneal direct current (dc) recordings must be used. Such equipment allows us to record the fast and slow oscillations as well. This section will provide a brief background regarding the generation of the three PE potential changes mentioned above as well as a description of equipment for corneal dc recordings of such slow responses in patients.

Figure 42.1.  

Schematic representation of the light-induced changes in the voltage across the eye. In response to a prolonged light stimulus (upper curve), the ERG with the a-, b-, and c-waves is first elicited. At about 45 seconds the negative “fast oscillation” is maximal, and at about 12 minutes the positive “slow oscillation” (light peak, light rise) reaches its peak. After light adaptation when the light is turned off (lower curve), a series of off-effects occur. The “off-ERG” includes the h-wave or “off c-wave.” The fast oscillation is now positive, and the slow oscillation (dark trough) is negative. The response is to a large extent a mirror image of the response to light. (From Nilsson SEG.²⁵ Used by permission.)