The purpose of our project is to develop, optimize and validate tests of fundamental visual functions that can be used as sensitive biomarkers to study functional changes in patients with hereditary retinal diseases. We have developed tests, which isolate different photoreceptors types (using the so-called silent substitution technique) and postreceptoral pathways because photoreceptor types (L-, M-, S-cones and rods) are often differently affected by these diseases. The working hypothesis is that methods that selectively probe the function of particular cell types and pathways will also be most sensitive to detect functional changes in these cells and pathways. The advantage of the silent substitution paradigm is that function of different photoreceptor types can be measured separately at identical states of retinal adaptation. Preliminary data have shown that tests can be successfully employed in studying changes in patients with hereditary retinal disease compared with healthy subjects. We want to be able to monitor disease progression and the effect of therapeutic intervention. We also hypothesize that these functional tests reliably reflect the visual performances that are the most relevant for daily life. Therefore, the proposed project aims to be adapted for the different patient groups and cell groups of interest.