There are two types of light-sensing cells in the retina of vertebrates: cones and rods. The main function of cones and rods is to convert light into impulses to the optic nerve. Cones process color perception and details of an object, while rods are used for night and peripheral vision. Cones are named by their size and shape, and they are more prominent in humans and other diurnal animals compared to nocturnal animals. Because cones identify details of an object, they need plenty of light to be activated. This is called photopic vision, while rods mediate scotopic (dark) vision.
Anatomy and Physiology
Research studies have measured the spatial density of cones and rods in the adult human eye. Specifically, it has been estimated that there are 4.6 million cones and roughly 92 million rods (Curcio et al., 1990). The location of cones and rods differs as well. Cones are concentrated in the central portion of the retina, called the macula, while rods are found on the elliptical ring of the optic disc mainly on the lateral and superior portions. The macula also contains a depression called the fovea (or fovea centralis), which is a rod-free zone. This densely cone-packed region is roughly about 0.3 millimeters in diameter and contains the majority of cone cells. The number of cones drastically decreases in number as you move away from the fovea centralis and toward the periphery of the retina.
As light enters the eye, it is bent and refracted via the cornea and lens of the eye. This allows an image to be focused onto the retina where the cones are located. Light then activates the cones, which starts a chemical reaction that propagates an electrical impulse from the cone cells to the axons of the optic nerve. This signal continues through the brain until it ends at the primary visual cortex of the occipital lobe. Cones have a faster response time to light stimuli compared to rods, thus allowing details of objects to be viewed and/or perceived.
There are three types of cone cells in humans: S-cone, M-cone, and L-cone cells, with each containing a protein called photopsin. Slight conformational changes in photopsin will determine how light is absorbed in each of the three cone types. As light enters the eye, all three types of cones will be stimulated and absorb the light. However, only at specific wavelength ranges will the peak absorption take place, best activating the cone cells. For instance, each cone type responds best to a specific color range of visible white light, which ranges from 400 to 700 nanometers (nm). S-cone cells are activated by short wavelengths with peak stimulation from 420 to 440 nm, detecting violet to blue colors. M-cone cells are called medium-wavelength sensitive cells and have peak activation from 534 to 545 nm. Finally, L-cone cells are called long-wavelength sensitive cells and have peak stimulation from 564 to 580 nm. However, medium to long wavelengths of light will activate both M-cone and L-cone cells at the same time. The cone type that has more cells stimulated at a time will determine the color that is perceived by the person. For instance, if more M-cones are activated compared to L-cones, then the color yellow is perceived. However, if significantly more L-cones are stimulated than M-cone cells, then the color red is perceived. The more cone cells that are activated at one time will allow a continuous range of colors to be perceived by the human eye.
Diseases of the Cones
In rare cases, cone cells can degenerate, resulting in loss of color vision (not color blindness, as this is generally caused by a different mechanism) and loss of vision acuity, particularly in central vision. Often, this also causes an increase in sensitivity to light that is painful. Degeneration of the cones is a type of cone dystrophy, which is a general term used to describe a group of rare disorders that affect cone cells. Cone dystrophies have been classified into two groups: stationary and progressive disorders. Stationary cone dystrophies are usually present at birth (congenital disorder) or in early childhood and continue to be stable over the lifetime of the patient. The opposite is true for progressive cone dystrophies in which symptoms worsen over time. Cone dystrophies can be inherited or occur spontaneously without a specific cause for the disease. Symptoms in persons with cone dystrophies can vary between patients as does the progression and severity of the degeneration. To date, there are no known cures for cone dystrophies and treatments are provided to alleviate specific symptoms, such as wearing dark sunglasses in brightly lit areas and using magnifying devices to help improve clarity when reading.
Patricia A. Bloomquist and Jennifer L. Hellier
See also: Color Blindness; Color Perception; Cone Dystrophy; Fovea Centralis; Retina; Rods; Visual System
Curcio, Christine A., Kenneth R. Sloan, Robert E. Kalina, & Anita E. Hendrickson. (1990). Human photoreceptor topography. Journal of Comparative Neurology, 292(4), 497—523.
Kostic, Corinne, & Yvan Arsenijevic. (2016). Animal modeling for inherited central vision loss. Journal of Pathology, 238(2), 300—310. http://dx.doi.org/10.1002/path.4641
Roorda, Austin, & David R. Williams. (1999). The arrangement of the three cone classes in the living human eye. Nature, 397(6719), 520—522. http://dx.doi.org/10.1038/17383