500x500For many years, it was assumed that rods and cones are the only photoreceptors capable of detecting light in the mammalian retina. However, research from several laboratories uncovered a third type of photoreceptor cell in the mammalian retina, called intrinsically photosensitive retinal ganglion cells (ipRGCs) that express their own photopigment called melanopsin. Our main goals are to understand how ipRGCs detect light and send light information to the brain to regulate physiology and behavior. We have shown that these cells target many visual centers in the brain including the circadian pacemaker and the area responsible for pupil constriction, among many others, and are critical for lights influence on circadian rhythms and pupil constriction. From this study, it was concluded that ipRGCs contribute only to non-image forming visual functions but not image formation. However, more recently we have found that ipRGCs are more abundant than previously appreciated and come in 5 different subtypes (M1–M5). Some of these subtypes target regions of the brain involved in image formation, which allows mice lacking rod and cone function to have rudimentary pattern vision. More recently, we have found that ipRGCs also mediate the negative effects of light on mood and learning and enhance the ability to detect contrast in an image. Many questions still remain about the function of these cells and the circuits that are critical for ipRGCs-mediated behaviors. We are continuing to explore them using a variety of techniques including mouse genetics, anatomy and viral circuit tracing, and animal behavior.