Duplex Retina
A duplex retina describes a retina that consists of both Rod cells and Cone cells and has separate (but often overlapping) visual systems for Scotopic Vision (low light) and Photopic Vision (bright light). Duplex retinas therefore increase the range of light intensities that can be viewed. Humans – and most vertebrates – have a duplex retina. In contrast, simplex retinas (pure rod and pure cone retinas) have only rods or only cones. Some abyssal fish have a rod-only simplex retina.
The human duplex retina can also be divided into two parts spatially, with the central retina (Macula) predominantly containing cones and the peripheral retina predominantly containing rods. In total, there are about seven million cones and a hundred million rods in each retina.