Color is an integral part of life. It tells us if a piece of fruit is ripe, if the sky is about to rain, or if the grass and trees are healthy. However, since we all have unique life experiences, it begs the question, “can one person perceive color differently than someone else?” For the most part, we all have the same anatomical and physiological means to see color, but do our brains interpret it differently? For decades, color perception was thought to be based on the amount of light available to illuminate an object. More recently, however, research has shown that color perception is controlled more by the brain than by how many cones in the retina are activated by light.
Anatomy and Physiology
There are two types of light-sensing cells in the retina of vertebrates: cones and rods. The main function of cones and rods is to convert light into impulses to the optic nerve. Three types of cones process color vision—a spectrum of colors based on the primary colors of red, blue, and yellow—as well as details of an object. Rods, however, are used for night and peripheral vision. Cones are named by their size and shape, and they are more prominent in humans and other diurnal animals compared to nocturnal animals.
Research studies have measured the spatial density of cones and rods in the adult human eye. Specifically, it has been estimated that there are 4.6 million cones and roughly 92 million rods (Curcio et al., 1990). The location of cones and rods in the retina differs as well. Specifically, cones are concentrated in the central portion of the retina, called the macula, while rods are found on the elliptical ring of the optic disc, mainly on the lateral and superior portions. The three types of cones are named by the length of light waves that activate them. Thus, there are the short wavelengths (S-cones) that perceive blue colors; medium wavelengths (M-cones) that perceive yellow; and, long wavelengths (L-cones) that perceive red colors.
As light hits an object, the color an individual perceives is actually the color of light being reflected by the object’s surface. Ratios of S-, M-, and L-cones will be activated based on the amount and wavelengths of light that enter the eye. As more cones are activated, the eye is able to differentiate more hues and vibrancy of a color. Additionally, color perception requires visual experience so that the brain can interpret what the person is seeing. This is called visual-experience-dependent neural plasticity, which was originally thought to occur only in preadult brains. For example, when a child is learning colors, the brain is wired so that the color yellow is associated with the word “yellow.” Thus, plasticity allows the color sensory experience to act as a guide so that the brain does not need to hard-wire all the necessary connections for a single hue and brightness of the color yellow. Since light is a spectrum, there are hundreds of ways yellow can be seen. This plasticity allows the brain to remodel the neural connections and produce adaptive adjustments as the person’s experiences change.
Within the past decade, neuroscientists have shown that between individuals the ratio of L- to M-cones in the retina is highly variable, meaning that some people have more M-cones than L-cones and vice versa. For our previous example of perceiving the color yellow, we would expect that a person with fewer M-cones would not be able to perceive yellow or not as many hues of yellow because there simply is not enough M-cone activation. On the other hand, a person with more M-cones would be able to perceive all hues of yellow. But Roorda and Williams (1999) showed that there was no difference in individuals perceiving or seeing the same color and hue of yellow. Thus, it is hypothesized that the visual cortex is able to use a plastic normalization mechanism along with previous experience to compensate for individual differences in cone ratios, which ultimately allows color perception to be uniform.
Jennifer L. Hellier
See also: Color Blindness; Cone Dystrophy; Cones; Fovea Centralis; Retina; Rods; Visual System
Curcio, Christine A., Kenneth R. Sloan, Robert E. Kalina, & Anita E. Hendrickson. (1990). Human photoreceptor topography. Journal of Comparative Neurology, 292(4), 497—523.
Neitz, Jay, Joseph Carroll, Yasuki Yamauchi, Maureen Neitz, & David R. Williams. (2002). Color perception is mediated by a plastic neural mechanism that is adjustable in adults. Neuron, 35, 783—792.
Roorda, Austin, & David R. Williams. (1999). The arrangement of the three cone classes in the living human eye. Nature, 397(6719), 520—522. http://dx.doi.org/10.1038/17383