Visual Frequency: How the Eye and Brain Create Color
Color is not an inherent property of objects but a perception our brain builds by interpreting the frequencies of light captured by the eye.
Visual frequency is a property of light, which travels in waves. The frequency—the number of waves passing a point in a given time—is interpreted by our visual system as color. This process turns the physical phenomenon of light waves into the perception of sight.
The Spectrum of Light
Light is a form of electromagnetic radiation that exists across a wide range of frequencies known as the electromagnetic spectrum. This spectrum includes everything from low-frequency radio waves to high-frequency gamma rays. The portion human eyes can detect is a narrow band called visible light. Within this range, there is an inverse relationship between a wave’s frequency and its wavelength, the distance between two consecutive peaks.
The lowest frequencies of visible light have the longest wavelengths and are perceived as red. As the frequency increases and the wavelength decreases, the color changes through orange, yellow, green, and blue. The progression ends with the highest frequencies of visible light, which are seen as violet.
This progression of colors represents the range of visual frequencies our eyes can distinguish. When light contains a mix of all these frequencies, our brain perceives it as white light. A prism can split white light into a rainbow because it bends each frequency at a slightly different angle, separating them.
How the Human Eye Perceives Frequency
Seeing color begins when light hits the retina, a layer of tissue at the back of the eye with photoreceptor cells. These cells come in two types: rods and cones. Rods are sensitive to light intensity for vision in dim conditions but do not detect color. The perception of frequency, and therefore color, relies on the cone cells.
Humans have three types of cone cells: S-cones, M-cones, and L-cones. Each type contains a different light-sensitive pigment, or photopsin, tuned to a specific range of light frequencies. The S-cones are sensitive to short-wavelength light, perceived as blue. The M-cones are most sensitive to medium-wavelength light, corresponding to the green range of the spectrum.
The L-cones are sensitive to long-wavelength light, which we see as red. The combined input from these three cone types allows us to perceive the entire color spectrum. When light enters the eye, it stimulates each cone type to a different degree, and the brain interprets this unique combination of signals as a specific color.
From Eye to Brain: The Creation of Color
The cones do not “see” color; they act as transducers, converting light energy into electrical signals. These signals are sent through the optic nerve to the brain for processing. The trichromatic theory explains how the three cone types detect different wavelengths. The opponent-process theory describes how the brain then interprets this information.
In the brain’s visual cortex, specialized neurons receive the inputs from the cones and process them in an antagonistic manner. This system operates through three opponent channels: red versus green, blue versus yellow, and black versus white (which handles brightness). For example, certain neurons are excited by signals from L-cones (red) and inhibited by signals from M-cones (green). This opposition means we cannot perceive a color as being reddish-green.
Similarly, another set of neurons is excited by signals from S-cones (blue) and inhibited by the combined signals from L- and M-cones, which the brain interprets as yellow. The brain analyzes the relative strength of the signals in these opposing channels to create the full palette of hues we experience.
The Boundaries of Human Vision
Human vision is limited to the slice of the electromagnetic spectrum our cones can detect. Light with frequencies just above this range is called ultraviolet (UV), while light with frequencies just below is known as infrared (IR). We cannot see these frequencies because our photoreceptors lack the pigments to be stimulated by them.
While invisible to us, these frequencies are part of the visual world for other species. Bees, for example, perceive UV light, allowing them to see patterns on flower petals that guide them to nectar. Some birds also see in the UV spectrum, which can reveal patterns on the plumage of potential mates.
On the other end of the spectrum, some animals detect infrared radiation. Certain snakes, like pit vipers and pythons, have specialized pit organs that sense the heat emitted by their warm-blooded prey. This allows them to “see” the heat signature of an animal in complete darkness.
When Frequency Detection Fails
A failure in the frequency detection system can lead to color vision deficiency, or color blindness. This inherited condition results from a malfunction in one or more cone types. If a cone is absent or has an abnormal photopigment, the brain receives incorrect frequency information, altering color perception.
The most common form is red-green color deficiency, which is linked to the X chromosome and affects men more often than women. This condition arises from a problem with either the L-cones (protanomaly) or M-cones (deuteranomaly). If the M-cones are faulty, the brain has difficulty distinguishing between red and green signals, causing the colors to appear similar.
Less common is blue-yellow color deficiency, which results from issues with the S-cones. In rare cases, a condition called blue cone monochromacy occurs when both L and M cones are non-functional. This leads to very limited color vision.