The retina is the light-sensitive layer of tissue at the back of the eye that contains photoreceptor cells called rods and cones. Cones are responsible for color vision. There are three types of cones that each detect different wavelengths of light corresponding to red, green or blue. The brain combines the signals from the three cone types to produce the perception of a whole spectrum of colors.
The Retina and Photoreceptors
The retina lines the inside of the back two-thirds of the eye. It contains two main types of photoreceptor cells: rods and cones. Rods function in dim light and detect shades of gray. Cones function in bright light and are responsible for color vision. There are approximately 120 million rods and 6 million cones in the human retina.
Rods and cones have different distributions across the retina. Rods are concentrated around the periphery, while cones are found mostly in the central retina, in an area called the macula. At the very center of the macula is the fovea, which contains only cones and is responsible for sharp, central vision.
Cone Photoreceptors for Color Vision
There are three types of cones that each contain a different photopigment, which is a light-sensitive protein. The three photopigments have peak sensitivities corresponding to short (blue), medium (green), and long (red) wavelengths of visible light.
| Cone type | Peak sensitivity |
|---|---|
| Short wavelength (S) | 420 nm – blue |
| Medium wavelength (M) | 534 nm – green |
| Long wavelength (L) | 564 nm – red |
When light hits the retina, the photopigments in the cones absorb photons and trigger chemical reactions that generate electrical signals. The signals travel via the optic nerve to the visual cortex in the brain, which interprets them as color.
Trichromatic Color Vision
Having three different cone types with overlapping sensitivity ranges allows us to perceive the whole visible light spectrum. This is called trichromatic color vision. It works via a process called opponency, where signals from cones are processed by comparing responses of the different cone types.
For example, some neurons compare L and M cone responses and transmit a red-green opponent signal. Other neurons compare S with a combined L and M signal for a blue-yellow opponent signal. These opponent signals then travel to the visual cortex where they are further processed to produce the wide range of colors we can perceive.
Color Blindness
Sometimes cone photoreceptors are missing or do not function properly, leading to deficiencies in color vision or color blindness. The most common forms are:
- Red-green color blindness – caused by missing or defective L or M cones. Reds, oranges, and greens are difficult to distinguish.
- Blue-yellow color blindness – caused by missing or defective S cones. Blues and yellows are hard to tell apart.
- Total color blindness – missing or defective S, M, and L cones. Vision is essentially in black, white, and shades of gray.
Color blindness affects approximately 1 in 12 men and 1 in 200 women globally. There is no treatment currently available, but colored filters and other aids can help improve color discrimination.
Distribution of Cones in the Retina
The three types of cones are not distributed equally across the retina. Studies of cone distributions have found:
- The fovea has a very high density of cones but contains almost exclusively L and M cones. S cones are very rare in the fovea.
- Towards the retinal periphery, the total number of cones reduces but the proportion of S cones increases.
- The ratio of L to M cones varies significantly between individuals from as low as 1:2 up to 16:1.
- On average, the ratio of L to M cones is around 2:1 for Caucasian populations.
The higher proportion of L and M cones in the central retina accounts for our excellent perception of fine details and color when looking directly at objects. The predominance of S cones towards the edges enhances perception of movement and causes enhanced blue color perception in peripheral vision.
Adaptations for Color Vision
Several anatomical and physiological adaptations allow the retinal cones to detect color:
- The presence of the three distinct cone photopigments with different spectral sensitivities.
- The cones contain various colored oil droplets that act as filters to fine-tune sensitivity.
- The cones have large exposed outer segments packed with photopigment for efficient photon capture.
- The retina contains interconnected neurons that enable opponent processing.
- The visual cortex contains specialized color-selective neurons.
Together, these adaptations make human color vision very sophisticated compared to most mammals. However, some species like birds have an even more complex quadruple cone system, allowing them to see into the ultraviolet spectrum.
Conclusion
In summary, cone photoreceptors in the retina enable color vision. The three types of cones with peak sensitivities to blue, green and red wavelengths allow us to perceive the full visible spectrum through an opponent processing system. The central area of the retina is specialized for high acuity color vision. Defects in cone function can lead to various types of color blindness. The complex adaptations of the retinal cones and visual system allow humans to have excellent trichromatic color vision.
