A cone cell or simply “cone” is a specialized light-sensitive cell found in the retina of the eye and used for discrimination of color and detail. In humans, three cone cell types are present, sensitized to different ranges of wavelength to provide a range of color perception. Different organisms have varying numbers of cones, sensitized to different areas of the spectrum, allowing animals like bees to perceive infrared light while humans cannot.
As the name suggests, part of the shape of a cone cell is indeed like that of a cone. The cone-shaped segment sits on top of the cell and reacts to different wavelengths of light, depending on the concentration of photopigment present in the structure. As this structure reacts, it sends a signal to the main body of the cone cell, and the input from scores of cone cells is used to create a complete picture for the brain. There are over four million cone cells in the eye, and they are especially densely packed in the fovea, a key area of the eye.
These cells are less sensitive to light than their fellow rod cells. Rod cells function in very low light, but do not respond to color, and have a rod-like structure. Organisms have varying concentrations of rod and cone cells depending on the environments where they evolved. Animals in need of good vision in low-light settings, like nocturnal animals, have many more rods, as these cells are more useful. Animals used to bright light and in need of color differentiation, like humans, tend to have an increased number of cone cells, although rods still outnumber cones.
Cones are sensitive to yellowish-green, blue-violet, and green light. When people perceive scenes in color, they do so with assistance from millions of these cells, each providing a small piece of the puzzle. When the cells sensitive to green light fire most strongly, for example, it is an indicator that the person is looking at something green, like a tree. Varying intensities of light provided by rods create shading and selective firing of yellow-green cones may reveal new growth, typically a lighter green.
People with disorders of color vision may have a cone cell problem, although there are other causes of colorblindness. Some people have no cones at all and cannot perceive color, while others have decreased numbers or defective cones, experiencing impaired color vision. When people are diagnosed with colorblindness, tests can be performed to learn more about why they have difficulties with color vision.