This research project involve the study of the human Electroretinogram (ERG) which is an electrical signal that originates in the retina and can be measured non-invasively using electrodes placed on or close to the cornea. The ERG signal recorded at the cornea is initiated by light absorption which leads to electrical activity in the photoreceptors (of which there are four main types L-, M- and S-cones and rods) and in their post-receptoral pathways. Light absorption in the distinct photoreceptor types may lead to different ERG responses caused either by differences between the photoreceptors themselves or between their post-receptoral pathways. A complete characterisation of contributions of the different photoreceptor types to the ERG may therefore give more detailed insight into the functional integrity of the human retina. Such a description can be obtained by isolating the responses of a single photoreceptor type.
Current state-of-the-art LED stimulators now allow precise control of differently coloured light stimuli and have an advantage over more traditional CRT based stimulation systems in that they can better isolate the activity of rod and L-, M- and S-cone receptor populations. This precision, coupled with our knowledge of cone and rod spectral characteristics, enables a precise description and control of photoreceptor excitation. Theoretically, any desired combination of photoreceptor excitation modulation can be achieved, including conditions in which the activity in only one photoreceptor type is modulated (i.e. silent substitution). In this manner the response of one photoreceptor type can be isolated without changing the state of adaptation. This technique can and has been used to study the contribution of signals originating in the different photoreceptor types to the human ERG.