Assessment of the Absolute Excitatory Level of the Retina by Flicker ERG

Electroretinography (ERG) is important for functional diagnostics of the retina. Types of information about retinal function obtainable by ERG differ depending on recording conditions, e.g., a combination of light stimulus and adaptation. In terms of stimulation, single-flash and flicker stimuli are frequently used because response properties have been well investigated, allowing an assessment of fundamental retinal functionality; for example, how photoreceptors and bipolar cells, including signal transmission between them, are affected under pathological conditions. Usually, ERGs are recorded with a nonzero lower cutoff frequency of amplifiers to avoid certain artifacts, and additionally, responses are averaged over time so that non-event-related signals are cancelled out. However, the improved signal quality is associated with a loss of information. Especially in steady-state flicker ERG, information about the absolute baseline of recordings is missing because the prestimulus baseline is not included on the recording trace as well as because a zero response is obtained in all cases in which the signal baseline stays constant for a sufficient amount of time. In other words, it is impossible to tell from the conventional flicker ERG whether a zero signal is obtained under conditions of maximal or no excitation of the visual system. In this chapter, we describe a direct current ERG protocol (featuring a lower cutoff frequency of zero) with repetitive single flashes mimicking conventional flicker that contains a defined onset. Using this recording protocol, it is possible to assess not only the absolute excitatory level of the retina but also the development of steady-state responses from the single flash response.