Physiology, Color Perception


Article Author:
Nathaniel Pasmanter


Article Editor:
Sunil Munakomi


Editors In Chief:
Kranthi Sitammagari
Mayank Singhal


Managing Editors:
Avais Raja
Orawan Chaigasame
Carrie Smith
Abdul Waheed
Khalid Alsayouri
Trevor Nezwek
Radia Jamil
Patrick Le
Anoosh Zafar Gondal
Saad Nazir
William Gossman
Hassam Zulfiqar
Hussain Sajjad
Steve Bhimji
Muhammad Hashmi
John Shell
Matthew Varacallo
Heba Mahdy
Ahmad Malik
Sarosh Vaqar
Mark Pellegrini
James Hughes
Beata Beatty
Beenish Sohail
Nazia Sadiq
Hajira Basit
Phillip Hynes


Updated:
6/15/2019 6:25:38 PM

Introduction

In humans, the perception and ability to distinguish different colors is mediated by a variety of mechanisms in the retina as well as the brain. Understanding the physiologic basis of color vision is essential to detecting abnormalities and devising treatments. In this article, we will review the cellular and genetic mechanisms that underlie color perception and apply these mechanisms to characterizing defects in color vision and avenues for treatment.

Issues of Concern

Color vision deficiency can result from a variety of abnormalities, both systemic and specifically within the visual system. Defects in the genes responsible for visual transduction often lead to congenital color vision deficits. Any abnormalities of the retina, optic nerve, optic tract, and visual cortex can cause defects in color vision. As such, systemic diseases like diabetes can alter color vision, as can eye-specific diseases like glaucoma and cataracts.[1]

Cellular

Most mammalian retinae only contain two types of cones (dichromats), sensitive to short wavelengths (S cones, maximally sensitive near the blue end of the visual spectrum) and medium wavelengths (M cones, maximally sensitive to green wavelength light). Humans, as trichromats, have an additional L cone, which is sensitive to long wavelength light at the red end of the visual spectrum. Differential stimulation of these cones creates color axes between red-green and blue-yellow, enabling visualization of mixtures of colors that fall in the range of the visual spectrum. The blue-yellow color axis is somewhat of a misnomer, as it more specifically refers to the ability to differentiate blue from green as well as yellow from red.[2]

The first step of visual transduction is the light-mediated conformational change of 11-cis-retinal, which activates an associated opsin that acts as a G-protein coupled receptor. Each type of cone is associated with a different opsin, which has different genetic bases. S cone opsins are autosomally encoded on chromosome 7, while M and L cone opsins are located nearby on the X-chromosome and are nearly identical. M and L cone opsin genes are believed to undergo frequent homologous recombination, which underlies changes in normal spectral sensitivity that explain the variations in the severity of red-green color blindness. These defects are far more likely to affect karyotypic males than females as they are sex-linked. S cone opsin defects are far less common than M and L.[3]

Development

Studies indicate that infants have a functional color vision by two months of age. They are further able to discriminate between multiple hues independent of luminance and rod function, although their vision differs from that of adults.[4] The refinement and continued development of color vision are not well-understood, nor is the age of peak functioning, but likely depends on the strengthening of cone photoreceptor pathways through continued visual experience.[5]

Organ Systems Involved

Normal color perception is a function of the nervous system, dependent on visual transduction and relaying of information to the visual cortex.

Function

The evolution of trichromatic color vision is believed to have aided early primates in differentiating red, orange, and yellow fruit from leafy green foliage.[2] In modern-day humans, the color vision has a vital role in household tasks, driving, and many other interactions with the surrounding environment. Loss of functional color vision can impair these everyday tasks, including job performance, and may even preclude specific career choices.[6]

Mechanism

The eye forms images based on differences in the reflectance of light on external objects. Small perturbations, in contrast, are processed through a center-surround system, where surrounding background luminance is subtracted from the center signal, highlighting the local features of the central signal. This system allows for high sensitivity to light-dark contrast. Additionally, the presence of different types of cone photoreceptors in the retina, which is sensitive to different wavelengths of light, enables contrast of refracted light, providing the basis for visualization and separation of a spectrum of colors.[2]

Cones relay visual information through parvocellular layers to the lateral geniculate nucleus (LGN) of the thalamus. Neurons in the LGN process the magnitude of contributions from opponent cone signals and continue to relay the signal to the primary visual cortex V1. The mechanisms of color perception beyond the LGN are not as well characterized, but fMRI studies suggest that there are additional separation and processing of both color luminance and color contrast in V1 as well as additional extrastriate visual areas.[7] fMRI studies of the pathways involved in color perception and processing indicate a potential top-down mechanism in the interaction of the temporoparietal cortex with visual areas V2/3. The relationship may reflect how the language processing center of the left hemisphere aids in color discrimination. Additionally, it may explain why the right visual field has superior color discrimination over the left.[8] fMRI of the brain also suggests that V4, the ventral occipitotemporal cortex, appears to play a significant role in color processing, and lesions to this area are associated with achromatopsia or dyschromatopsia.[9]

A variety of additional complex mechanisms influences color perception. The hue, saturation, and brightness of both center and surround areas greatly influence the perception of the central color, such that the same stimulus presented on different backgrounds may be perceived as different colors entirely. Individual differences in color processing also mediate how these factors influence color perception.[10]

Chromatic visual perception appears to be subject to a high degree of neural plasticity; this seems to be mediated by average background luminance and equilibrating average chromatic stimuli (perception of the normal background color is adjusted to yield equal contributions of opposing color axes). The capability of the visual system to adjust this equilibrium point based on different external environments enables greater color contrast.[11]

All colors are attainable by the process of additive or subtractive mixing processes within the primary three colors.

Related Testing

The most common method of diagnosing color vision defects is with ‘pseudoisochromatic’ plate tests, such as Ishihara plates, due to the ease of access and use. However, these tests often fail to differentiate different types of dyschromatopsia. Another diagnostic tool, ordering tests, requires patients to sort different disks based on color progression. Compared to plate tests, these tests are typically more sensitive at detecting color vision abnormalities, as well as better at determining specific diagnoses. Additional diagnostic methods of interest include matching tests, which are attractive due to the possibility of administering and scoring them on a computer. Genetic tests, the examination of retinal morphology (including fundoscopy and OCT), and electrophysiological tests (such as ERG) can also aid in the diagnosis and characterization of color vision defects.[1]

Pathophysiology

Abnormal color vision typically subdivides into congenital and acquired forms. Congenital disorders of color vision occur due to defects in genes encoding cone opsins, genes that encode the expression of cone opsins, and genes for proteins involved in phototransduction (such as PDE and CNG channel subunits). Such defects have a wide range of possible outcomes and may feature different rates of progression, retinal degeneration, and loss of visual acuity. Guanylate cyclase deficiencies can lead to Leber congenital amaurosis and severe vision loss early in life, while rod dystrophies leading to retinitis pigmentosa may cause late color vision abnormalities as degenerating rods affect cone structure and function. Additional causes of color vision defects include fundus albipunctatus and Stargardt disease. Acquired color vision deficiency, or dyschromatopsia, can occur due to any injury or disruption to visual pathways.[1]

Clinical Significance

Defects affecting specific cone opsin types have associated prefixes – protan for L cones, deutan for M cones, and Tritan for S cones. For example, if L cone opsin is present but abnormal, the patient would be classified as a ‘protanomalous trichromat,’ while complete loss of L cone opsin would yield a ‘protanope dichromat.’ Such dichromats would be unable to distinguish red and green only see functional combinations of two hues, while anomalous trichromacy can range in severity from essentially normal color vision to functional dichromacy.[3]

Diseases that primarily affect the cone-rich macula, such as age-related macular degeneration (AMD), may demonstrate early losses in color discrimination before detectable changes in retinal morphology or visual acuity. The loss of color discrimination can affect both the red-green and blue-yellow axes, and severity appears to correlate with an increased quantity of drusen and reticular pseudodrusen in the retina.[12]

Glaucoma and ocular hypertension, diseases that can result in compression of the optic nerve, may also cause color vision defects. Color perception may suffer alteration before other functional or structural changes are detectable and appear to primarily affect the blue-yellow axis.[13]

Numerous other disorders, both specific to the visual system and global, have been associated with color vision abnormalities. Retinal tears and detachments may lead to loss of color vision, as can disorders affecting the optic nerve, including hereditary optic neuropathy, optic neuritis, and optic nerve compression, Many patients with diabetic retinopathy exhibit deficiencies in color vision as well. Multiple drugs have implications in dyschromatopsia, including PDE5 inhibitors, chloroquine, ethambutol, and digoxin, as well as environmental effects such as UV exposure, industrial chemicals, and hypoxia.[1]

Gene therapy is a promising avenue of treatment for many congenital retinal dystrophies. Many genetic disorders present in non-human animals that are analogous to conditions seen in humans. Translational therapies for gene replacement, typically mediated by recombinant adeno-associated virus (rAAV), have been successful in treating multiple congenital defects affecting color vision. RPE65 genetic defects, which affect the ability of the retinal pigment epithelium to recycle retinyl esters and can cause Leber congenital amaurosis, was successfully treated with gene therapy in dog models. The treatment has since been extended to humans, yielding the first FDA-approved gene therapy treatment for inherited retinal dystrophy.[14] Other promising treatments include gene therapy for CNGB3 congenital achromatopsia countering the absence of a CNG channel subunit in cones, which has been successful in dogs,[15] and gene therapy for red-green color blindness in squirrel monkeys.[16]


Interested in Participating?

We are looking for contributors to author, edit, and peer review our vast library of review articles and multiple choice questions. In as little as 2-3 hours you can make a significant contribution to your specialty. In return for a small amount of your time, you will receive free access to all content and you will be published as an author or editor in eBooks, apps, online CME/CE activities, and an online Learning Management System for students, teachers, and program directors that allows access to review materials in over 500 specialties.

Improve Content - Become an Author or Editor

This is an academic project designed to provide inexpensive peer-reviewed Apps, eBooks, and very soon an online CME/CE system to help students identify weaknesses and improve knowledge. We would like you to consider being an author or editor. Please click here to learn more. Thank you for you for your interest, the StatPearls Publishing Editorial Team.

Physiology, Color Perception - Questions

Take a quiz of the questions on this article.

Take Quiz
An 82-year-old female presents to the office for an annual checkup. She denies any complaints except for occasional blurry vision and mild difficulty seeing at night. She occasionally takes ibuprofen for arthritis, and levothyroxine for hypothyroidism, but denies any additional medications or medical diagnoses. On physical exam, her vital signs are normal, but a fundoscopic examination of both eyes shows the presence of small amounts of drusen in both maculae. Which of the following would be a likely additional finding in this patient?



Click Your Answer Below


Would you like to access teaching points and more information on this topic?

Improve Content - Become an Author or Editor and get free access to the entire database, free eBooks, as well as free CME/CE as it becomes available. If interested, please click on "Sign Up" to register.

Purchase- Want immediate access to questions, answers, and teaching points? They can be purchased above at Apps and eBooks.


Sign Up
A 73-year old female presents to the emergency department with complaints of chest pain and difficulty breathing. Her EKG demonstrates a likely left bundle branch block, and a chest x-ray shows cardiomegaly and thin, parallel linear opacities on the lung periphery. After stabilizing the patient, a daily medication based on her constellation of symptoms is prescribed. On follow-up one month later, the patient notes significant improvement of her initial symptoms but states that she has since experienced blurred vision and worse color vision. Which of the following medications was most likely given to this patient that caused these side effects?



Click Your Answer Below


Would you like to access teaching points and more information on this topic?

Improve Content - Become an Author or Editor and get free access to the entire database, free eBooks, as well as free CME/CE as it becomes available. If interested, please click on "Sign Up" to register.

Purchase- Want immediate access to questions, answers, and teaching points? They can be purchased above at Apps and eBooks.


Sign Up

Physiology, Color Perception - References

References

Horwitz GD, What studies of macaque monkeys have told us about human color vision. Neuroscience. 2015 Jun 18;     [PubMed]
Rowe MH, Trichromatic color vision in primates. News in physiological sciences : an international journal of physiology produced jointly by the International Union of Physiological Sciences and the American Physiological Society. 2002 Jun;     [PubMed]
Lotto RB,Purves D, The empirical basis of color perception. Consciousness and cognition. 2002 Dec;     [PubMed]
Neitz J,Carroll J,Yamauchi Y,Neitz M,Williams DR, Color perception is mediated by a plastic neural mechanism that is adjustable in adults. Neuron. 2002 Aug 15;     [PubMed]
Neitz M,Neitz J, Molecular genetics of color vision and color vision defects. Archives of ophthalmology (Chicago, Ill. : 1960). 2000 May;     [PubMed]
Papaconstantinou D,Georgalas I,Kalantzis G,Karmiris E,Koutsandrea C,Diagourtas A,Ladas I,Georgopoulos G, Acquired color vision and visual field defects in patients with ocular hypertension and early glaucoma. Clinical ophthalmology (Auckland, N.Z.). 2009;     [PubMed]
Shapley R,Hawken M, Neural mechanisms for color perception in the primary visual cortex. Current opinion in neurobiology. 2002 Aug;     [PubMed]
Simunovic MP, Acquired color vision deficiency. Survey of ophthalmology. 2016 Mar-Apr;     [PubMed]
Ting Siok W,Kay P,Wang WS,Chan AH,Chen L,Luke KK,Hai Tan L, Language regions of brain are operative in color perception. Proceedings of the National Academy of Sciences of the United States of America. 2009 May 19;     [PubMed]
Vemala R,Sivaprasad S,Barbur JL, Detection of Early Loss of Color Vision in Age-Related Macular Degeneration - With Emphasis on Drusen and Reticular Pseudodrusen. Investigative ophthalmology     [PubMed]
Miraldi Utz V,Coussa RG,Antaki F,Traboulsi EI, Gene therapy for RPE65-related retinal disease. Ophthalmic genetics. 2018 Dec;     [PubMed]
Kom�romy AM,Alexander JJ,Rowlan JS,Garcia MM,Chiodo VA,Kaya A,Tanaka JC,Acland GM,Hauswirth WW,Aguirre GD, Gene therapy rescues cone function in congenital achromatopsia. Human molecular genetics. 2010 Jul 1;     [PubMed]
Mancuso K,Hauswirth WW,Li Q,Connor TB,Kuchenbecker JA,Mauck MC,Neitz J,Neitz M, Gene therapy for red-green colour blindness in adult primates. Nature. 2009 Oct 8;     [PubMed]
Steward JM,Cole BL, What do color vision defectives say about everyday tasks? Optometry and vision science : official publication of the American Academy of Optometry. 1989 May;     [PubMed]
Brown AM,Lindsey DT, Infant color vision and color preferences: a tribute to Davida Teller. Visual neuroscience. 2013 Nov;     [PubMed]
Sugita Y, Experience in early infancy is indispensable for color perception. Current biology : CB. 2004 Jul 27;     [PubMed]

Disclaimer

The intent of StatPearls is to provide practice questions and explanations to assist you in identifying and resolving knowledge deficits. These questions and explanations are not intended to be a source of the knowledge base of all of medicine, nor is it intended to be a board or certification review of PA-Hospital Medicine. The authors or editors do not warrant the information is complete or accurate. The reader is encouraged to verify each answer and explanation in several references. All drug indications and dosages should be verified before administration.

StatPearls offers the most comprehensive database of free multiple-choice questions with explanations and short review chapters ever developed. This system helps physicians, medical students, dentists, nurses, pharmacists, and allied health professionals identify education deficits and learn new concepts. StatPearls is not a board or certification review system for PA-Hospital Medicine, it is a learning system that you can use to help improve your knowledge base of medicine for life-long learning. StatPearls will help you identify your weaknesses so that when you are ready to study for a board or certification exam in PA-Hospital Medicine, you will already be prepared.

Our content is updated continuously through a multi-step peer review process that will help you be prepared and review for a thorough knowledge of PA-Hospital Medicine. When it is time for the PA-Hospital Medicine board and certification exam, you will already be ready. Besides online study quizzes, we also publish our peer-reviewed content in eBooks and mobile Apps. We also offer inexpensive CME/CE, so our content can be used to attain education credits while you study PA-Hospital Medicine.