When light conditions rapidly change, our eyes have to respond to this change in fractions of a second to maintain stable visual processing. This is necessary when, for example, we drive through a forest and thus move through alternating stretches of shadows and clear sunlight. In situations like these, a corrective mechanism is required. Such a corrective 'gain control' mechanism exists in the fruit fly Drosophila melanogaster, where it acts directly downstream of the photoreceptors.