41. Most birds have retinas with four spectral types of cone cells that are believed to mediate tetrachromatic color vision. 42. Cone cells and rods are connected through intermediate cells in the retina to nerve fibres of the optic nerve.43. One proposed explanation for pentachromacy is a retina containing five distinct types of cone cells with differing absorption spectra. 44. Very few people see no color at all, a condition called monochromasy, in which there are no cone cells . 45. In contrast, nightjars have a limited number of cone cells , either lacking or having only a few oil droplets. 46. In vertebrates with three types of cone cells , at low light intensities the rod cells may contribute to color vision. 47. Individuals with the congenital form of this condition show complete absence of cone cell activity via electroretinography at high light levels. 48. Central vision is relatively weak in the dark ( scotopic vision ) since cone cells lack sensitivity at low light levels. 49. Red light only slowly depletes the rhodopsin stores in the rods, and instead is viewed by the red sensitive cone cells . 50. Steinberg determined that the cone cells renew themselves daily by shedding used-up parts that are then absorbed by the pigmented layer.
