The electroretinogram (ERG) is an electrophysiological signal evoked by light stimuli that can be recorded non-invasively. The ERG originates in the activity of retinal cells. It is often used in clinical routine, where the ERG is usually measured in response to flashes. The retina includes neural networks (visual pathways) that forward visual information to the lateral geniculate nucleus in the brain. They are responsible for visual perception. Until recently, researchers had been unable to prove a connection between the activity of these visual pathways and the ERG. Now, however, researchers at the Laboratory for Retinal Physiology have demonstrated that the ERG shows a clear connection to the activities of these visual pathways under certain stimulus and recording conditions. This made it possible for researchers to investigate important physiological properties of visual pathways in humans using non-invasive methods (thereby reducing the number of experiments conducted on animals). This research is now conducted also in healthy control subjects and in patients with retinal diseases.
ERGs evoked by physiologically relevant stimuli
Our research focuses on ERG responses to stimuli that have not as yet been investigated and that bring new insights into visual processing in the retina.
The general routine, followed in ophthalmological clinics, involves the measuring and analyzing of ERGs triggered by flashes of light. Flashes of light have the disadvantage, however, that they are extremely unnatural and outside the normal physiological range of operation of the retina. This may reduce the effectiveness of the ERG in delivering information about retinal function. We analyze ERG responses to stimuli within the normal physiological range of the retina and that can be easily analyzed. Examples are stimuli with different temporal profiles, such as sine-waves, sawtooth stimuli and white noise. In addition, the influence of modulating backgrounds on the ERGs is investigate.
Silent substitution
We derive ERGs from stimulation conditions in which only one type of photoreceptor cell is stimulated. Traditionally, individual types of photoreceptor cells are isolated by using strong chromatic adaptation to desensitize the non-required type before flashing a light that predominantly stimulates the photoreceptor type of interest. This method has disadvantages: as the stimuli cannot be quantified exactly, and the retina is subjected to a visually extreme situation, it means that the responses only give an indication of the retina’s physiology. We use a method based on spectral compensation. The sensitivity of the photoreceptor cells to differently colored stimuli is calculated exactly and adjusted to ensure that only one type of cell reacts to the stimulus. These methods allow adaptation to be investigated as an independent factor. The method may also be combined with other stimuli, as described in the previous section.
Research into retinal vision pathways
Visual information is already processed in the retina. Information is forwarded to various parallel visual pathways. We discovered that the activities of the two most important visual pathways are reflected in the ERG under certain stimulus conditions. We investigate the temporal, spatial and other physiological properties of these visual pathways in the ERG.
Pathological changes in ERG
The techniques described in the previous sections are also used in patients with various retinal diseases at various stages of the disease. The results are compared with each other and with those obtained from normal control subjects of the same age and gender. The aim is to improve diagnosis, monitoring of the disease progression and of the effects of therapies. We also hope to gain an improved understanding of underlying pathophysiological mechanisms.