Signal Pathways in the Electroretinogram

The electroretinogram (ERG) is an electrical potential of retinal origin elicited by a visual stimulus. The basis of the ERG is the same photochemical process that leads to the neural response of the retina and the visual system1: photon absorptions in the photoreceptoral opsin pigments2. In addition, it is a purely retinal response that reflects the retinal physiology. It is not surprising that the ERG received substantial attention because it is a potentially useful way to study the function of the retina and its disease-related changes. Indeed, it still is a very important tool in the clinic. In addition, there was hope that the ERG might be related to the visual responses of retinal neurones whose signals are transmitted to the brain for visual perception and motor responses. However, even though the ERG is known for nearly 150 years now, the similarities with visually relevant neuronal signals and with visual perception were disappointingly small. In the 1970s, Armington (1974) wrote in his book “The Electroretinogram” when discussing the link between ERGs and psychophysics (chapter 7): “The electroretinogram is unique because its components allow the experimenter to follow several separate retinal activities, while recording is performed with a minimum of discomfort to the subject. Furthermore, the subject may make verbal reports or judgements regarding the same stimulus, which was used to elicit the electroretinogram. It is thus possible to relate the visual appearance of a stimulus to the underlying physiological processes. The full potential for doing this in a sophisticated manner, however, has not been yet realized.” As a result, ERGs were mainly used in studies that had a clinical interest. It was treated as an epiphenomenon of the responses of the visual pathways without showing a direct relationship with them. In this chapter, I will argue that under well chosen circumstances, which can be created owing to modern stimulus and recording techniques, a correlation with the physiological properties of the major pathways projecting to the brain and the related visual performances can be obtained.