page_title: Photoreceptor cell
section_title: Humans
hierarchical_section_title: Photoreceptor cell / Humans
caption_attribution_description: English: Illustration of the distribution of cone cells in the fovea of a individual with normal color vision (left), and a color blind (protanopic) retina. Note that the center of the fovea holds very few blue-sensitive cones.
context_page_description: A photoreceptor cell is a specialized type of neuroepithelial cell found in the retina that is capable of visual phototransduction. The great biological importance of photoreceptors is that they convert light into signals that can stimulate biological processes. To be more specific, photoreceptor proteins in the cell absorb photons, triggering a change in the cell's membrane potential.
  There are currently three known types of photoreceptor cells in mammalian eyes: rods, cones, and intrinsically photosensitive retinal ganglion cells. The two classic photoreceptor cells are rods and cones, each contributing information used by the visual system to form a representation of the visual world, sight. The rods are narrower than the cones and distributed differently across the retina, but the chemical process in each that supports phototransduction is similar. A third class of mammalian photoreceptor cell was discovered during the 1990s: the intrinsically photosensitive retinal ganglion cells. These cells do not contribute to sight directly, but are thought to support circadian rhythms and pupillary reflex.
  There are major functional differences between the rods and cones.
context_section_description: The human retina has approximately 6 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) lies the fovea (or fovea centralis), which contains only cone cells; and is the region capable of producing the highest visual acuity or highest resolution.  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.
caption: Illustration of the distribution of cone cells in the fovea of an individual with normal color vision (left), and a color blind (protanopic) retina. Note that the center of the fovea holds very few blue-sensitive cones.
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