What is a tapetum lucidum?

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by Kristin Berkery

The equine eye is an amazing structure that works in concert with the brain to take the many elements being viewed, like detail, distance, and color, and construct a complete image. The horse is particularly interesting for its wide field of vision and ability to see in the dark better than humans. Because the horse is a prey animal and its vision is a big factor in its behavior, it’s important for those who work with horses to have a basic understanding of the equine eye.

The horse has the largest eye of all land mammals and a nearly 360-degree field of vision, thanks to the placement of its eyes on the sides of its head. This feature is common to prey animals as it allows them to watch for danger from all sides, and the horse’s long head helps it to be on the lookout even while grazing.

Its main blind spots are a small area directly behind it and directly in front of its nose. This affects the horse’s performance under saddle in a couple of ways: A jumper can’t see the jump directly in front of it when it takes off, so it must rely on the last image it saw before jumping; and walking backwards is not a natural thing for horses, even though backing for the judge is usually required at shows.

Horses have a rectangular “streak” on their retinas with a high concentration of ganglion cells that allow them to focus on an object. Anything viewed above or below this area tends to be much less clear, so horses will often move their heads up and down to view an object clearly. That’s why a horse may suddenly spook at an object on the trail that’s been there all along: While walking, the horse’s bobbing head can suddenly focus on an object and the details become clear to the horse… in a way that surprises it.

Eyeshine

Eyeshine in a mare and foal, which is the result of camera flash reflection off the tapetum lucidum.

So what is a tapetum lucidum? Horses, dogs, and many nocturnal animals including cats and raccoons have a layer of tissue directly behind the retina called the tapetum lucidum that reflects visible light back through the retina, making more light visible to the animal. The horse can see better in the dark than we can, but the image tends to be grainy. The tapetum lucidum is also the structure that causes eyeshine, or the iridescent eye effect that animals sometimes have in photographs. (Listen to tapetum lucidum’s pronunciation.)

But can horses see color? The answer is yes, but there’s more to the story. There are two types of photoreceptor cells in the eye, cones and rods, that convert light into signals the brain can assemble into an image. Cones perceive color, so any animal with cones can see at least some color. (Rods are very light sensitive but cannot perceive color, which explains why it’s impossible to see color in very low light.) Horses are dichromats, which, like many other mammals, means they have two kinds of cones that perceive some, but not all, color. In clinical studies on horses taught to pick colors they can see, scientists learned that horses can see red and blue but not green, and orange and yellow probably look like variations of the color red.

In studies, horses have shown that they have the ability to see stereoscopically — in other words, they’re able to use both eyes in concert to perceive depth. But the placement of the eyes on the sides of the head limits them to a 60-degree view where they can use both eyes to focus on an object. Horses probably consider other factors when trying to judge distance, such as shadows and perspective (i.e. knowing that an object farther away will appear smaller).

To learn more about equine vision, as well as their evolution and behavior, check out The Nature of Horses by Stephen Budiansky.

About Kristin Berkery

Kristin is a freelance graphic designer and writer in Sacramento, California, with a life-long passion for horses. She designs websites and marketing materials for businesses. In her spare time she's active with her daughter and son. See Kristin's online portfolio and LinkedIn profile.