The cells that give you color vision
Deep within the human eye’s retina reside light-sensitive (photoreceptor) cells called cone cells or ‘cones’. They paint our world in color by giving us color perception and light-intensity sensitivity. Cone cells are named for the distinctive cone-like shape of their outer segment, which is part of the cell containing the light-absorbing molecules known as photopigments (1).
Each human eye has approximately six to seven million of these cells. Cones are mostly concentrated within the central retina (macula), in the area called fovea centralis, which is a little indent in the retina. This region of the retina contains very densely packed cones, making it the place of the sharpest vision (2).
Where the magic happens
The most important parts of cone cells are the outer segment, the inner segment, the cell body, and the synaptic terminal (1).
It all starts in the outer segment, which faces toward the back of the eye, where the light enters. However, it is the part of the cell shaped like a cone that contains numerous stacked layers of membrane-bound structures (known as discs) which contain photopigments (3). These pigments absorb light initiating a chain of chemical reactions that results in an electrical signal (4).
Next is the inner segment, which contains the machinery for protein synthesis. It continuously produces new photopigments to replace those bleached by light in the outer segment (5).
Below is the cell body where the cell’s nucleus is. It contains the genetic information of the cell. The body is connected to the synaptic terminal, a communication interface between the cone and the nerve cells. When an electrical signal is generated in the cone cell, it travels to the synaptic terminal, where it triggers the release of neurotransmitters and passes the signals to the brain (1, 6).
How do we see color and shape?
Cones are photoreceptive, meaning they respond to incoming light and transform it into a signal the brain can interpret. They work only in bright light; hence we cannot see color very well in dark places. Human eyes contain three types of cone cells (S-, M-, L-cones), each sensitive to a different color wavelength.
The S-cones, sensitive to short wavelengths, give us the perception of blue light, M-cones (medium-wavelength cones) give us green, and L-cones reacting to long wavelengths make us see red light. This triad gives us what is called trichromatic vision. Our brain can combine red, green, and blue to create all known colors (7–9).
Cone cells also allow us to see fine details, a quality known as visual acuity (10). This function results from the high concentration of cone cells in the fovea centralis. When we look directly at an object, the light falls on the fovea, stimulating many closely packed cone cells. Each cone cell sends a separate signal to the brain, contributing to a detailed perception of the object (11).
The number of cones matters
Lack or dysfunction of one or more types of cone cells in the retina results in color blindness, also known as color vision deficiency. In this condition, the eyes cannot distinguish certain colors or shades of colors properly (12). You can see a simulation of that condition below.
The most common form of color blindness involves difficulties distinguishing between red and green due to an anomaly in the M-cones or L-cones. In the image below, you can see a simulation of green blindness (deuteranopia) and red blindness (protanopia), where a person cannot see one color at all. There are also types where the person can see the color, but it is weaker.
A less common form of color blindness is difficulty in distinguishing between blue and yellow, which involves the S-cones. The least common form is total color blindness or monochromacy, where one sees no color, only shades of gray, caused due to two types of cones being simultaneously dysfunctional (13).
On the other hand, there is a rare phenomenon of tetrachromacy, where an individual has four types of cone cells instead of the typical three. In humans, it is believed to occur almost exclusively in women. A tetrachromat can theoretically see 100 million colors, a hundred times more than a person with normal vision (14).
Recognizing and appreciating the labs working in this space:
- Department of Pharmacology, Case Western Reserve University, Cleveland, https://case.edu/medicine/pharmacology/research/jastrzebska-lab
- Albert Eye Research Institute, Duke University Medical Center, Durham, NC 27710, USA. https://dukeeyecenter.duke.edu/
- Eye Research Institute, Oakland University, Rochester, MI, USA, https://oakland.edu/eri/
- Department of Ophthalmology & Visual Sciences, University of British Columbia, Vancouver, BC, Canada, https://ophthalmology.med.ubc.ca/
- Department of Ophthalmology and Jules Stein Eye Institute, Department of Neurobiology, David Geffen School of Medicine at UCLA, University of California, Los Angeles, CA, USA https://www.uclahealth.org/locations/ucla-stein-eye-institute
- Human Vision Laboratory of the University of Western Australia https://www.uwa.edu.au/Research/Perception
References
- Mustafi D, Engel AH, Palczewski K. Structure of cone photoreceptors. Progress in Retinal and Eye Research. 2009;28(4):289–302.
- Jonas JB, Schneider U, Naumann GOH. Count and density of human retinal photoreceptors. Graefe’s Archive for Clinical and Experimental Ophthalmology. 1992;230(6):505–10.
- Spencer WJ, Lewis TR, Pearring JN, Arshavsky VY. Photoreceptor discs: built like ectosomes. Trends in cell biology. 2020;30(11):904–15.
- Goldberg AF, Moritz OL, Williams DS. Molecular basis for photoreceptor outer segment architecture. Prog Retin Eye Res. 2016;55:52–81.
- Baker SA, Kerov V. Photoreceptor inner and outer segments. Current topics in membranes. 72: Elsevier; 2013. p. 231–65.
- Zhang KX, Tan L, Pellegrini M, Zipursky SL, McEwen JM. Rapid changes in the translatome during the conversion of growth cones to synaptic terminals. Cell reports. 2016;14(5):1258–71.
- Deeb S. The molecular basis of variation in human color vision. Clinical genetics. 2005;67(5):369–77.
- Kremers J, Baraas RC, Marshall NJ. Human color vision: Springer; 2016.
- Zhang F, Kurokawa K, Bernucci MT, Jung HW, Lassoued A, Crowell JA, et al. Revealing how color vision phenotype and genotype manifest in individual cone cells. Investigative ophthalmology & visual science. 2021;62(2):8-.
- Westheimer G. Visual acuity: Information theory, retinal image structure and resolution thresholds. Progress in Retinal and Eye Research. 2009;28(3):178–86.
- Pirson M, Ie A, Langer E. Seeing what we know, knowing what we see: Challenging the limits of visual acuity. Journal of Adult Development. 2012;19:59–65.
- Wong B. Color blindness. Nat Methods. 2011;8(6):441.
- Kulshrestha R, editor Review of Color Blindness Removal Methods using Image Processing2013.
- Jordan G, Mollon J. Tetrachromacy: The mysterious case of extra-ordinary color vision. Current Opinion in Behavioral Sciences. 2019;30:130–4.
About the author:
DR. AGA SZMITKOWSKA
Content Editor The League of Extraordinary Celltypes, Sci-Illustrate Stories
Aga did her Ph.D. in Biochemistry at the CEITEC/Masaryk University in Brno, Czech Republic, where she was a part of the Laboratory of Genomics and Proteomics of Plant Systems. She is a passionate public speaker and science communicator. After graduation, she became a freelance content coordinator and strategist in a start-up environment focused on lifestyle and longevity.
About the artist:
NELLY AGHEKYAN
Contributing Artist The League of Extraordinary Celltypes, Sci-Illustrate Stories
Nelli Aghekyan, did Bachelor’s and Master’s degrees in Architecture in Armenia; after studying architecture and interior design for 6 years, she concentrated on her drawing skills and continued her path in the illustration world. She works mainly on children’s book illustrations, some of her books are now being published. Currently living in Italy, she works as a full-time freelance artist, collaborating with different companies and clients.
About the animator:
DR. EMANUELE PETRETTO
Animator The League of Extraordinary Celltypes, Sci-Illustrate Stories
Dr. Petretto received his Ph.D. in Biochemistry at the University of Fribourg, Switzerland, focusing on the behaviour of matter at nanoscopic scales and the stability of colloidal systems. Using molecular dynamics simulations, he explored the delicate interaction among particles, interfaces, and solvents.
Currently, he is fully pursuing another delicate interaction: the intricate interplay between art and science. Through data visualisation, motion design, and games, he wants to show the wonders of the complexity surrounding us.
About the series:
The League of Extraordinary Celltypes
The team at Sci-Illustrate and Endosymbiont bring to you an exciting series where we dive deep into the wondrous cell types that make our bodies tick ❤.
