Cone cells, or cones, are photoreceptor cells in the retinas of vertebrate eyes (e.g. the human eye). They respond differently to light of different wavelengths, and are thus responsible for color vision and function best in relatively bright light, as opposed to rod cells, which work better in dim light. Cone cells are densely packed in the fovea centralis, a 0.3 mm diameter rod-free area with very thin, densely packed cones which quickly reduce in number towards the periphery of the retina. There are about six to seven million cones in a human eye and are most concentrated towards the macula. The commonly cited figure of six million cone cells in the human eye was found by Osterberg in 1935. Oyster's textbook (1999) cites work by Curcio et al. (1990) indicating an average close to 4.5 million cone cells and 90 million rod cells in the human retina.
Cones are less sensitive to light than the rod cells in the retina (which support vision at low light levels), but allow the perception of color. They are also able to perceive finer detail and more rapid changes in images, because their response times to stimuli are faster than those of rods. Cones are normally one of the three types, each with different pigment, namely: S-cones, M-cones and L-cones. Each cone is therefore sensitive to visible wavelengths of light that correspond to short-wavelength, medium-wavelength and long-wavelength light. Because humans usually have three kinds of cones with different photopsins, which have different response curves and thus respond to variation in color in different ways, we have trichromatic vision. Being color blind can change this, and there have been some verified reports of people with four or more types of cones, giving them tetrachromatic vision. The three pigments responsible for detecting light have been shown to vary in their exact chemical composition due to genetic mutation; different individuals will have cones with different color sensitivity.
Normalized responsivity spectra of human cone cells, S, M, and L types
|Location||Retina of mammals|
|Anatomical terms of neuroanatomy|
Humans normally have three types of cones. The first responds the most to light of long wavelengths, peaking at about 560 nm; this type is sometimes designated L for long. The second type responds the most to light of medium-wavelength, peaking at 530 nm, and is abbreviated M for medium. The third type responds the most to short-wavelength light, peaking at 420 nm, and is designated S for short. The three types have peak wavelengths near 564–580 nm, 534–545 nm, and 420–440 nm, respectively, depending on the individual.
Cone cells are somewhat shorter than rods, but wider and tapered, and are much less numerous than rods in most parts of the retina, but greatly outnumber rods in the fovea. Structurally, cone cells have a cone-like shape at one end where a pigment filters incoming light, giving them their different response curves. They are typically 40–50 µm long, and their diameter varies from 0.5 to 4.0 µm, being smallest and most tightly packed at the center of the eye at the fovea. The S cone spacing is slightly larger than the others.
Photobleaching can be used to determine cone arrangement. This is done by exposing dark-adapted retina to a certain wavelength of light that paralyzes the particular type of cone sensitive to that wavelength for up to thirty minutes from being able to dark-adapt making it appear white in contrast to the grey dark-adapted cones when a picture of the retina is taken. The results illustrate that S cones are randomly placed and appear much less frequently than the M and L cones. The ratio of M and L cones varies greatly among different people with regular vision (e.g. values of 75.8% L with 20.0% M versus 50.6% L with 44.2% M in two male subjects).
Like rods, each cone cell has a synaptic terminal, an inner segment, and an outer segment as well as an interior nucleus and various mitochondria. The synaptic terminal forms a synapse with a neuron such as a bipolar cell. The inner and outer segments are connected by a cilium. The inner segment contains organelles and the cell's nucleus, while the outer segment, which is pointed toward the back of the eye, contains the light-absorbing materials.
Unlike rods, the outer segments of cones have invaginations of their cell membranes that create stacks of membranous disks. Photopigments exist as transmembrane proteins within these disks, which provide more surface area for light to affect the pigments. In cones, these disks are attached to the outer membrane, whereas they are pinched off and exist separately in rods. Neither rods nor cones divide, but their membranous disks wear out and are worn off at the end of the outer segment, to be consumed and recycled by phagocytic cells.
The difference in the signals received from the three cone types allows the brain to perceive a continuous range of colors, through the opponent process of color vision. (Rod cells have a peak sensitivity at 498 nm, roughly halfway between the peak sensitivities of the S and M cones.)
All of the receptors contain the protein photopsin, with variations in its conformation causing differences in the optimum wavelengths absorbed.
The color yellow, for example, is perceived when the L cones are stimulated slightly more than the M cones, and the color red is perceived when the L cones are stimulated significantly more than the M cones. Similarly, blue and violet hues are perceived when the S receptor is stimulated more. Cones are most sensitive to light at wavelengths around 420 nm. However, the lens and cornea of the human eye are increasingly absorptive to shorter wavelengths, and this sets the short wavelength limit of human-visible light to approximately 380 nm, which is therefore called 'ultraviolet' light. People with aphakia, a condition where the eye lacks a lens, sometimes report the ability to see into the ultraviolet range. At moderate to bright light levels where the cones function, the eye is more sensitive to yellowish-green light than other colors because this stimulates the two most common (M and L) of the three kinds of cones almost equally. At lower light levels, where only the rod cells function, the sensitivity is greatest at a blueish-green wavelength.
Cones also tend to possess a significantly elevated visual acuity because each cone cell has a lone connection to the optic nerve, therefore, the cones have an easier time telling that two stimuli are isolated. Separate connectivity is established in the inner plexiform layer so that each connection is parallel.
The response of cone cells to light is also directionally nonuniform, peaking at a direction that receives light from the center of the pupil; this effect is known as the Stiles–Crawford effect.
Sensitivity to a prolonged stimulation tends to decline over time, leading to neural adaptation. An interesting effect occurs when staring at a particular color for a minute or so. Such action leads to an exhaustion of the cone cells that respond to that color – resulting in the afterimage. This vivid color aftereffect can last for a minute or more.
One of the diseases related to cone cells present in retina is retinoblastoma. Retinoblastoma is a rare cancer of the retina, caused by the mutation of both copies of retinoblastoma genes (RB1). Most cases of retinoblastoma occur during early childhood. One or both eyes may be affected. The protein encoded by RB1 regulates a signal transduction pathway while controlling the cell cycle progression as normally. Retinoblastoma seems to originate in cone precursor cells present in the retina that consist of natural signalling networks which restrict cell death and promote cell survival after losing the RB1, or having both the RB1 copies mutated. It has been found that TRβ2 which is a transcription factor specifically affiliated with cones is essential for rapid reproduction and existence of the retinoblastoma cell. A drug that can be useful in the treatment of this disease is MDM2 (murine double minute 2) gene. Knockdown studies have shown that the MDM2 gene silences ARF-induced apoptosis in retinoblastoma cells and that MDM2 is necessary for the survival of cone cells. It is unclear at this point why the retinoblastoma in humans is sensitive to RB1 inactivation.
The pupil may appear white or have white spots. A white glow in the eye is often seen in photographs taken with a flash, instead of the typical "red eye" from the flash, and the pupil may appear white or distorted. Other symptoms can include crossed eyes, double vision, eyes that do not align, eye pain and redness, poor vision or differing iris colors in each eye. If the cancer has spread, bone pain and other symptoms may occur.
Achromatopsia (ACHM), also known as total color blindness, is a medical syndrome that exhibits symptoms relating to at least five conditions. The term may refer to acquired conditions such as cerebral achromatopsia, but it typically refers to an autosomal recessive congenital color vision condition, the inability to perceive color and to achieve satisfactory visual acuity at high light levels (typically exterior daylight). The syndrome is also present in an incomplete form which is more properly defined as dyschromatopsia. It is estimated to affect 1 in 30,000 live births worldwide.
There is some discussion as to whether achromats can see color or not. As illustrated in The Island of the Colorblind by Oliver Sacks, some achromats cannot see color, only black, white, and shades of grey. With five different genes currently known to cause similar symptoms, it may be that some do see marginal levels of color differentiation due to different gene characteristics. With such small sample sizes and low response rates, it is difficult to accurately diagnose the 'typical achromatic conditions'. If the light level during testing is optimized for them, they may achieve corrected visual acuity of 20/100 to 20/150 at lower light levels, regardless of the absence of color.
One common trait is hemeralopia or blindness in full sun. In patients with achromatopsia, the cone system and fibres carrying color information remain intact. This indicates that the mechanism used to construct colors is defective.Chromatic adaptation
Chromatic adaptation is the human visual system’s ability to adjust to changes in illumination in order to preserve the appearance of object colors. It is responsible for the stable appearance of object colors despite the wide variation of light which might be reflected from an object and observed by our eyes. A chromatic adaptation transform (CAT) function emulates this important aspect of color perception in color appearance models.
An object may be viewed under various conditions. For example, it may be illuminated by sunlight, the light of a fire, or a harsh electric light. In all of these situations, human vision perceives that the object has the same color: an apple always appears red, whether viewed at night or during the day (unless it is green). On the other hand, a camera with no adjustment for light may register the apple as having varying color. This feature of the visual system is called chromatic adaptation, or color constancy; when the correction occurs in a camera it is referred to as white balance.
Though the human visual system generally does maintain constant perceived color under different lighting, there are situations where the relative brightness of two different stimuli will appear reversed at different illuminance levels. For example, the bright yellow petals of flowers will appear dark compared to the green leaves in dim light while the opposite is true during the day. This is known as the Purkinje effect, and arises because the peak sensitivity of the human eye shifts toward the blue end of the spectrum at lower light levels.Color
Color (American English), or colour (Commonwealth English), is the characteristic of human visual perception described through color categories, with names such as red, orange, yellow, green, blue, or purple. This perception of color derives from the stimulation of cone cells in the human eye by electromagnetic radiation in the visible spectrum. Color categories and physical specifications of color are associated with objects through the wavelength of the light that is reflected from them. This reflection is governed by the object's physical properties such as light absorption, emission spectra, etc.
By defining a color space, colors can be identified numerically by coordinates, which in 1931 were also named in global agreement with internationally agreed color names like mentioned above (red, orange, etc.) by the International Commission on Illumination. The RGB color space for instance is a color space corresponding to human trichromacy and to the three cone cell types that respond to three bands of light: long wavelengths, peaking near 564–580 nm (red); medium-wavelength, peaking near 534–545 nm (green); and short-wavelength light, near 420–440 nm (blue). There may also be more than three color dimensions in other color spaces, such as in the CMYK color model, wherein one of the dimensions relates to a color's colorfulness).
The photo-receptivity of the "eyes" of other species also varies considerably from that of humans and so results in correspondingly different color perceptions that cannot readily be compared to one another. Honeybees and bumblebees for instance have trichromatic color vision sensitive to ultraviolet but is insensitive to red. Papilio butterflies possess six types of photoreceptors and may have pentachromatic vision. The most complex color vision system in the animal kingdom has been found in stomatopods (such as the mantis shrimp) with up to 12 spectral receptor types thought to work as multiple dichromatic units.The science of color is sometimes called chromatics, colorimetry, or simply color science. It includes the study of the perception of color by the human eye and brain, the origin of color in materials, color theory in art, and the physics of electromagnetic radiation in the visible range (that is, what is commonly referred to simply as light).Double cone (biology)
Double cones (DCs), known as twin cones when the two members are the same, are two cone cells (colour detecting photoreceptors) joined together that may also be coupled optically/electrically. They are the most common type of cone cells in fish, reptiles and birds, and are present in most vertebrates, though they have been noted as absent in placental mammals (including humans), elasmobranches and catfish. There are many gap junctions between the cells of fish double cones. Their function, if they have any unique function compared to single cones, is largely unknown; proposed uses include achromatic (non-colour vision) tasks such as detecting luminance, motion and polarization vision. Some double cones have members with identical visual pigments (twin cones), while others have members with different cone types (members have a different spectral sensitivity). Behavioural research on the reef dwelling triggerfish Rhinecanthus aculeatus has provided evidence that individual members of double cones can act as independent channels of colour information.In a book about vision in fishes, James Bowmaker writes that double cones tend to be sensitive to longer wavelengths of light than single cones. He also states that the single cones are usually smaller than the individual members of the double cones.GNAT2
Guanine nucleotide-binding protein G(t) subunit alpha-2 is a protein that in humans is encoded by the GNAT2 gene.HCN3
Potassium/sodium hyperpolarization-activated cyclic nucleotide-gated channel 3 is a protein that in humans is encoded by the HCN3 gene.Impossible color
Impossible colors or forbidden colors are supposed colors that cannot be perceived in normal seeing of light that is a combination of various intensities of the various frequencies of visible light, but are reported to be seen in special circumstances.Line of purples
In color theory, the line of purples or purple boundary is the locus on the edge of the chromaticity diagram formed between extreme spectral red and violet. Except for these endpoints of the Line, colors on the Line are non-spectral, i. e., no monochromatic light source can generate them; rather, every color on the Line is a mixture in a ratio, unique to that color, of fully saturated red and fully saturated violet, these two spectral colors being the endpoints of visibility on the spectrum of pure hues. Colors on the Line and spectral colors are the only ones that are fully saturated in the sense that, for any point on the Line, no other possible color being a mixture of red and violet is more saturated than it.
Unlike spectral colors, which may be implemented, for example, by the nearly monochromatic light of a laser, with precision much finer than human chromaticity resolution, colors on the Line are more difficult to depict. The sensitivity of each type of human cone cell to both spectral red and spectral violet, being at the opposite endpoints of the Line and at the extremes of the visible spectrum, is very low. (See luminosity function) Therefore, common purple colors are not highly bright.
The line of purples, a theoretical boundary of chromaticity, is distinct from "purples", a more general denomination of colors, which also refers to less than fully saturated colors (see shades of purple and shades of pink for examples) that form the interior of a triangle between white and the line of purples in the CIE chromaticity diagram.List of distinct cell types in the adult human body
There are many different types of cell in the human body.Oil droplet
Oil droplets are found in the eyes of some animals, being located in the photoreceptor cells. They are especially common in the eyes of diurnal (active during the day) reptiles (e.g. lizards, turtles) and birds (see bird vision), though are present in other taxa such as lungfish. They are found in cone cells far more often than in rods, suggesting a role in colour vision. Occurrence in rod cells may imply that they have been modified from a cone cell ancestor. They occasionally occur in double cones/double rods. Some oil droplets are coloured, while others appear colourless. They are located in the cone inner segment, where they intercept and filter light before it can pass through to the cone outer segment where the visual pigment is.The adaptive advantage of oil droplets is not firmly established. Coloured oil droplets have a cost in that they reduce the amount of light available to the visual system. They also reduce the overlap in spectral sensitivity between different types of cone (e.g. short wavelength sensitive, medium wavelength sensitive etc.). This can be a benefit because it increases the number of colours that can be discriminated, and calculations by Vorobyev (2003) support the hypothesis that this is of net benefit.Ora serrata
The ora serrata is the serrated junction between the retina and the ciliary body. This junction marks the transition from the simple, non-photosensitive area of the ciliary body to the complex, multi-layered, photosensitive region of the retina. The pigmented layer is continuous over choroid, ciliary body and iris while the nervous layer terminates just before the ciliary body. This point is the ora serrata. In this region the pigmented epithelium of the retina transitions into the outer pigmented epithelium of the ciliary body and the inner portion of the retina transitions into the non-pigmented epithelium of the cilia. In animals in which the region does not have a serrated appearance, it is called the ora ciliaris retinae.Outer plexiform layer
The outer plexiform layer (external plexiform layer) is a layer of neuronal synapses in the retina of the eye. It consists of a dense network of synapses between dendrites of horizontal cells from the inner nuclear layer, and photoreceptor cell inner segments from the outer nuclear layer. It is much thinner than the inner plexiform layer, where amacrine cells synapse with retinal ganglion cells.The synapses in the outer plexiform layer are between the rod cell endings or cone cell branched foot plates and horizontal cells. Unlike in most systems, rod and cone cells release neurotransmitters when not receiving a light signal.Pars plana
The pars plana (Latin: flat portion) is part of the ciliary body in the uvea (or vascular tunic), the middle layer of the three layers that comprise the eye.
It is about 4 mm long, located near the junction of the iris and sclera, and is scalloped in appearance.
The pars plana may not have a function in the post-fetal period, making this a good site of entry for ophthalmic surgery of the posterior segment of eyeball; this surgery is known as pars plana vitrectomy.Pentachromacy
Pentachromacy describes the capability and capacity for capturing, transmitting, processing, and perceiving five independent channels of color information through the primary visual system. Organisms with pentachromacy are termed pentachromats. For these organisms, it would take at least five differing ranges of wavelengths along the electromagnetic spectrum to reproduce their full visual spectrum. In comparison, a combination of red, green, and blue wavelengths of light are all that is necessary to simulate most of the common human trichromat visual spectrum.
One proposed explanation for pentachromacy is a retina containing five distinct types of cone cells with differing absorption spectra. In actuality the number of cone cell types may be greater than five as different types may be active at a specific intensity or range of intensities for a given wavelength of electromagnetic radiation.Photoreceptor cell
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 (visible electromagnetic radiation) 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 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 photosensitive 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. Rods are extremely sensitive, and can be triggered by a single photon. At very low light levels, visual experience is based solely on the rod signal. This explains why colors cannot be seen at low light levels: only one type of photoreceptor cell is active.
Cones require significantly brighter light (i.e., a larger number of photons) in order to produce a signal. In humans, there are three different types of cone cell, distinguished by their pattern of response to different wavelengths of light. Color experience is calculated from these three distinct signals, perhaps via an opponent process. The three types of cone cell respond (roughly) to light of short, medium, and long wavelengths. Note that, due to the principle of univariance, the firing of the cell depends upon only the number of photons absorbed. The different responses of the three types of cone cells are determined by the likelihoods that their respective photoreceptor proteins will absorb photons of different wavelengths. So, for example, an L cone cell contains a photoreceptor protein that more readily absorbs long wavelengths of light (i.e., more "red"). Light of a shorter wavelength can also produce the same response, but it must be much brighter to do so.
The human retina contains about 120 million rod cells, and 6 million cone cells. The number and ratio of rods to cones varies among species, dependent on whether an animal is primarily diurnal or nocturnal. Certain owls, such as the nocturnal tawny owl, have a tremendous number of rods in their retinae. In the human visual system, in addition to the photosensitive rods & cones, there are about 2.4 million to 3 million ganglion cells, with 1 to 2% of them being photosensitive. The axons of ganglion cells form the two optic nerves.
Photoreceptor cells are typically arranged in an irregular but approximately hexagonal grid, known as the retinal mosaic.
The pineal and parapineal glands are photoreceptive in non-mammalian vertebrates, but not in mammals. Birds have photoactive cerebrospinal fluid (CSF)-contacting neurons within the paraventricular organ that respond to light in the absence of input from the eyes or neurotransmitters. Invertebrate photoreceptors in organisms such as insects and molluscs are different in both their morphological organization and their underlying biochemical pathways. Described here are human photoreceptors.Retina bipolar cell
As a part of the retina, bipolar cells exist between photoreceptors (rod cells and cone cells) and ganglion cells. They act, directly or indirectly, to transmit signals from the photoreceptors to the ganglion cells.SLC32A1
Vesicular inhibitory amino acid transporter is a protein that in humans is encoded by the SLC32A1 gene.The protein encoded by this gene is an integral membrane protein involved in gamma-aminobutyric acid (GABA) and glycine uptake into synaptic vesicles. The encoded protein is a member of amino acid/polyamine transporter family II.Tetrachromacy
Tetrachromacy is the condition of possessing four independent channels for conveying color information, or possessing four types of cone cell in the eye. Organisms with tetrachromacy are called tetrachromats.
In tetrachromatic organisms, the sensory color space is four-dimensional, meaning that to match the sensory effect of arbitrarily chosen spectra of light within their visible spectrum requires mixtures of at least four primary colors.
Tetrachromacy is demonstrated among several species of bird, fish, amphibian, reptile, insect and some mammals. It was the normal condition of most mammals in the past; a genetic change made the majority of species of this class eventually lose two of their four cones.Thyroid hormone receptor beta
Thyroid hormone receptor beta (TR-beta) also known as nuclear receptor subfamily 1, group A, member 2 (NR1A2), is a nuclear receptor protein that in humans is encoded by the THRB gene.