Seeing the world in all its beautiful glory is made possible by a type of nerve cells on the retina known as cones. There are about 6 million of these cones here that allow us to see the world in all its colorful hues. These work together with 120 million rods, which provide black and white vision.
Structure of Eye Cones
These light-sensitive cones are mostly concentrated into a portion of the eye's retina known as the fovea, which enables small details to come into sharp focus in bright light. These powerful little receptors get their name from their cone-like shape.
Tubular-shaped rods are the counterpart to the cones. They are located on the outside area of the retina. These are 500 to 1000 times more responsive to light than cones, making them ideal for providing vision in dim conditions.
Types of Cones
Located on each of the two retinas are actually three different types of cones:
Function of Cones
These cones contain photopigments, known as opsin amino acids, that are sensitive to different wavelengths of visible light. Fact is, each of the different colors of the rainbow have a different wavelength. Our cones are able to capture these various frequencies thanks to these color-sensitive photopigments.
Our eyes can actually perceive light frequencies as short as 380 nanometers and as long as 700 nanometers. Although these cones mainly respond to light in their own color zone, there is overlap between these. Each is actually able to respond to a variety of wavelengths.
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Color Vision
Color vision brings the world to life. In bright sunshine it's all about the cones.
It works this way. Light bouncing off a yellow flower, for example, would stimulate both the red and green cones in your eyes. This signal would then run from the optic nerve to the brain, which interprets the type of signal coming in based on its strength. In this case, it would peg it as yellow.
On the other hand, in dim light, just the rods work. Since these are unable to see color, any object would only appear in shades of grey.
But, when it's not entirely dark, such as around dusk or twilight, both rods and cones are able to work and you see some colors, as well as shades of grey.
The red cones, also known as L-cones, are stimulated by long-wavelength light. The green cones, dubbed M-cones, respond to medium-wavelength light. The blue cones, called S-cones, are stimulated by short-wavelength light.
Acuity
It's the cones packed into the eye's fovea that actually give us our ability to pick up fine details such as small letters. In the fovea cone density is almost 200-fold of anywhere else in the retina.
In this region, which is located in a pit, light rays are subject to minimal scattering and distortion. Meanwhile, rods drop off precipitously here. This is also the region with the sharpest vision.
The trichromatic vision theory explains how cones are responsible for how we view colors. All of our color vision comes down to three different types of cones, which are activated by one single wavelength of light, but in different amounts.
Short-absorbing wavelength S-cones responsible for seeing blue can mix with medium M-cones responsible for green and long wavelength L-cones for red. The proportion of the light recognized by each of these three cone types is interpreted by the brain and determines the color you see.
Problems With Eye Cones
Not everybody necessarily sees colors the same way. Color vision is tested with the Ishihara color palettes—a series of dots of different hues. This test, which identifies color issues, was named for Japanese ophthalmologist Shinobu Ishihara and includes numbers embedded in each of a set of circular images. The idea is to detect if you are unable to see certain colors.
Unfortunately, eye cones do not always function properly. Here are some conditions that can occur when they don't.
Color Blindness
If you are being tested with the Ishihara exam and can’t pick out some of the numbers amid the different shades of dots, it means that the color frequency isn’t registering because some of your cones aren’t functioning properly. You likely have some sort of color blindness.
The term color blindness is a bit of a misnomer, however. In most cases, this does not mean that you see the world as strictly black and white. In fact, most colors come through as clearly as they do for anyone else. It is just certain colors that you may be unable to detect.
It may be that some of the cones in your eyes have been damaged. The most common type of color blindness, red-green color blindness, tends to be present at birth or inherited. This affects up to 8% of males but just 0.5% of females. With this type of color blindness, shades of red and green are hard to distinguish and may appear brownish instead.
Cone-Rod Dystrophies
There is a group of malfunctioning gene-related, inherited disorders known as dystrophies that can affect both cones and rods. By mid-adulthood these result in legal blindness. Those with these dystrophies may experience the follow symptoms:
Blue Cone Monochromacy
One cone-related disorder, blue cone monochromacy, is also inherited. This mainly affects males. With this condition, while the blue cones function perfectly normally, neither the red nor the green cones work properly.
Those with this condition have signs such as:
While there is no cure for this condition it can be aided with specially-colored contact lenses or glasses. Also, low-vision aids may assist here.
Tetrachromacy
Some among us actually have an extra cone, giving them super color vision. It is believed that approximately 12% of women have this capability. This may enable them to see 100 times more colors than the rest of the population.