Eye cones are one of two main types of cells in the retina that process light input. Along with rod cells, they comprise a group of specialized nerve cells called photoreceptors, which transform light into electrical impulses which are sent to the brain. Concentrated mainly in an area at the retina's center known as the fovea centralis, the human eye has roughly four to six million cones. Unlike the more sensitive rods, the cones require bright light to provide adequate stimulation for them to function properly, though they do adjust more quickly to shifts in light levels.
As their name indicates, eye cones have a conical shape at one end which starts narrow and widens as it goes toward the body of the cell. This is where light entering the cone is filtered. The cone cells are most densely populated in the fovea centralis, where they are packed tightly together, though a smaller number of them are also found further out in the retina.
The main purpose of the eye cones is to recognize colors. There are three types of cone cells in the human eye, each one processing information mainly from a certain segment of the light spectrum; this type of separated color processing is called trichromasy. L cones respond most to long wavelengths, so they mainly recognize colors from red to yellow. M cones are stimulated by medium wavelengths, corresponding to the color green. Blue and violet, which fall under short-wavelength light, are seen primarily by the S cones, which is the only type of the three to be located mainly outside the fovea.
Another role the eye cones play is to allow visual recognition of details. In order to see small details in high resolution, light that is hitting them must be focused on the fovea centralis. The eye will adjust continually to keep the light on the fovea so it can focus on the detail it is trying to view.
Certain genetic issues may affect the ability of the eye cones to recognize colors appropriately. Although true total colorblindness is rare, many people see certain colors more weakly than other people do, or cannot view certain parts of the color spectrum normally. These issues are known as anomalous trichromasy and dichromasy respectively. Men are significantly more likely to suffer from these types of colorblindness than women.