Sometime during my earliest dealings with horses, I was told by a now-forgotten authority that horses see only in black and white. I never questioned this version of equine reality, and over the years I've encountered others who shared the same view that most animals--certainly dogs and also horses--inhabit a colorless world.
But how to explain those plentiful barn anecdotes that fly in the face of the black-and-white theory? There's the horse who shies away from orange cones but doesn't take a second look at similar objects in other colors. There's the barrel racer who's startled by red barrels but not blue-and-white ones or the jumper who spooks only at blue jumps. Observant owners sometimes recognize color as the recurring factor in their horses' behavioral quirks. With little true science to go on, these apparent expressions of color perception have been explained away as reactions to the shade, the shape or the placement of the object rather than the color itself, if not purely random outbursts of flightiness.
Yet the anatomy of the equine eye suggests that some color perception is possible, and in the last 25 years, a few behavioral studies have attempted to test color recognition in horses. Using color as the distinguishing characteristic to mark the rewarded choice, some studies determined that shades of red are visible to horses, while others found that blue, not red, is a recognizable color. The inconsistent results may have arisen from flaws in the studies' designs, causing the horses to respond to the darkness or brightness of the color, rather than to the color itself.
More recent research has examined equine vision in a new and more objective light by monitoring horses' physiological reactions to the range of colors. In addition, more carefully designed behavioral tests have produced convincing support for the physiological findings that suggest horses do possess color vision.
Eyeballs vary in shape and size throughout the animal kingdom, but the color-sensing process is the same among all mammals. Two types of photoreceptors operate in the eye: rods, which are responsible for seeing in darkness or dimly lit conditions, and cones, which are sensitive to color. The well-studied human eye is known to contain millions of cones grouped into three classes that react in different ways according to wavelengths in the light.
"Light is made up of a lot of different wavelengths, just as sound is made up of a lot of frequencies," explains color-vision researcher Jay Neitz, PhD, a professor in the department of cell biology, neurobiology and anatomy with the Medical College of Wisconsin. "We recognize different frequencies when we hear different pitches. Light frequencies--what we call wavelengths--work the same way."
When light passes through the pupil, it is directed toward the retina, which consists of several layers of nerve cells--including rods and cones--lining the back of the eyeball. Light stimulates the pigments in the photoreceptors, which encode the information about each wavelength and send a message to the brain. Although each cone class responds best to a small range of wavelengths, they all respond in some way.
"With each wavelength of light, each of the receptors reacts to a different degree, and certain receptors prefer one wavelength," says Brian Timney, PhD, a researcher of mammalian vision who is dean of social science at the University of Western Ontario. In the human eye, the cones register short wavelengths as blue, medium wavelengths as green and long wavelengths as red. Horses' eyes have just two types of cones, and until recently, the visual effect was not known.
The Equine Palette
To evaluate horse color vision, Neitz tested six anesthetized ponies by exposing their eyes to individual colors and measuring the neurological responses using an electroretinogram. The instrument, which has also been used to examine cone pigments in cattle, goats and sheep, shines a narrow band of light into each eye. "It's like taking all the colors of the rainbow and showing each of them, one at a time," says Neitz.
When a photoreceptor responds to a wavelength, it sends out a nerve signal, which the testing equipment senses and records. "[The electrode] is a very thin thread that sits on the cornea and picks up electrical signals like a little antenna," says Neitz. "Those signals are processed by the computer. Basically, we're measuring the amplitude of the signal in response to different colors of light."
With only two types of cones in their retinas, horses have more limited color perception than people. Neitz found that the ponies' eyes responded to blue and green but not to red. Using the computer data, he constructed an equine color wheel showing that the horse's version of green is different from ours. "They have cones like our blue-sensitive ones," says Neitz, "and they have a cone [class] that's similar but not identical to our green-sensitive ones. Those cones perceive more of a yellow color."