Photoreceptor Cell - Humans

Humans

The human retina has approximately 5 million cones and 120 million rods. Signals from the rods and cones converge on ganglion and bipolar cells for preprocessing before they are sent to the lateral geniculate nucleus. At the "center" of the retina (the point directly behind the lens) is the fovea, which contains only cone photoreceptor cells; this is the region capable of producing the highest visual acuity. Across the rest of the retina, rods and cones are intermingled. No photoreceptors are found at the blind spot, the area where ganglion cell fibers are collected into the optic nerve and leave the eye.

The photoreceptor proteins in the three types of cones differ in their sensitivity to photons of different wavelengths (see graph). Since cones respond to both the wavelength and intensity of light, the cone's sensitivity to wavelength is measured in terms of its relative rate of response if the intensity of a stimulus is held fixed, while the wavelength is varied. From this, in turn, is inferred the absorbance. The graph normalizes the degree of absorbance on a hundred point scale. For example, the S cone's relative response peaks around 420 nm (nanometers, a measure of wavelength). This tells us that an S cone is more likely to absorb a photon at 420 nm than at any other wavelength. If light of a different wavelength to which it is less sensitive, say 480 nm, is increased in brightness appropriately, however, it will produce exactly the same response in the S cone. So, the colors of the curves are misleading. Cones cannot detect color by themselves; rather, color vision requires comparison of the signal across different cone types.