Beyond night vision: the expanding role of rod photoreceptors in bright light

According to the standard view, most species including humans possess a "duplex retina", with a rod system dedicated to low light (night) vision and a cone system dedicated to daylight vision. This separation of photon detection into a rod and cone regime is attributed to the low sensitivity of cones in dim light and saturation of rods in brighter light. However, mounting evidence gained from in vitro and in vivo studies in several species have demonstrated that specific mechanisms enable rod photoreceptors to significantly contribute to vision in bright and even very bright light. In this review we aim to elaborate on this revised framework for the duplex retina, and we propose rods should be considered to be tuned to "low contrast" rather than to "low ambient luminance". Importantly, saturation of rod photoreceptors at higher light levels has been an assumption in research studies as well as clinical tests, and consideration of an updated role of rod photoreceptors may warrant reinterpretation of past and future results.