Why do mice have ultra-violet vision?

Murine vision has become a fascinating entity due to discoveries about the histology and physiology of its retina over the past decade. It has two varieties of cones, one serving the traditional green-yellow region of the vision spectrum and another serving the ultra-violet region, essentially invisible to man and many other mammal. This puts unusual constraints on the optical transmission of the murine eye, in particular its relatively large lens. Its ultra-violet vision appears to involve its upper much more than its lower visual field, providing a heuristic clue to its purpose. In addition behavioural evidence exists for colour vision in mice. On the other hand there is unequivocal evidence that many murine cones contain both cone photopigments, an unrealistic but not impossible arrangement for colour vision. A better understanding of how ultra-violet vision is interwoven into cone and rod vision and possible colour vision can be clarified by analysing the responses of single retinal neurons. This paper reviews the current information on this topic and provides new insights from single retinal ganglion cell recordings.