Abstract
Originally discovered in the context of attempts to understand circadian and diurnal variations in mammalian physiology and behaviour there is growing interest in the possibility that melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs) may also contribute to more conventional vision. This reflects discovery in both primates and rodents that some types of ipRGC project to the dorsal lateral geniculate nucleus (dLGN) and that ipRGCs can signal to other cells within the retina. This raises the question of how ipRGCs and their intrinsic light response contribute to vision. We have addressed this problem using transgenic mice and recording visual responses at the level of the retina (by electroretinography) and the dLGN (by extracellular electrophysiology) and visually evoked behaviours. Our attempts to isolate the influence of melanopsin have employed melanopsin knockout and rodless+coneless mice, but we have concentrated our efforts on paradigms in which metamers are used to selectively manipulate melanopsin activity in visually intact animals. Our data provide support for the idea that ipRGCsfulfil at least two functions: 1.) they provide an independent measure of illuminance by which the state of retinal (and perhaps central) circuits are set; 2.) they contribute to the illuminance code in the dLGN.