Vision in the Equine

The horse is a prey animal, relying on their senses to assess their environment. Prey species are designed for scanning the environment compared to picking out sharp details. By scanning larger areas, prey is safer from a surprise attack from a predator. Horses use vision to orientate themselves, detect motion and distance, and evaluate the consistency of the environment. The equine eye is eight times larger than human eyes placed on the sides of their head. The position of the eyes on the horse’s face accounts for differences in how horses see, dictate visual range, peripheral motion detection and depth perception. A horses’ ability to see depth is limited because their eyes are set so far apart. From most angles, horses cannot get a left-eye and right-eye view of the same object in one glance. Unlike humans, the horse is able to see images to the left and right at the same time due to the eyes being at the side of the head.

The eye of the horse
The eye of the horse set to the side of the face

The cornea is the surface of the clear part of the eye and the colored part of the eye is the iris. Eyelids are a thin ring of two layers of muscles that relax and contract to open and close the eye. The third eyelid is a pink membrane that moves over the eye from the inside corner to the outside corner. In combination with the upper and lower eyelid, all three function to protect the eye. Eyelids have tear glands that keep the eye moist. The nasolacrimal duct drains the tears to a small opening just inside the nostril. An unusual feature of the equine eye is the corpora nigra. It is a knobby structure that juts out from the top of the iris and functions to shade the pupil from glare.

Parts of the horses eye
Parts of the horses eye

The retina is the major determinant of vision. The horse has a rectangular pupil shape which extends the area of visual perception. The size of the pupil determines the amount of light allowed into the back part of the eye. Pupils contract under bright light and enlarge in low light. Compared to other animals, horses have lots of cells in their retinas and have fairly good vision. Horses are thought to have vision somewhere in the range of 20/30 to 20/60. Cones are required for bright light (day light) vision, and rods are required for dim light vision. The horse’s ratio of rods to cones is approximately 20:1, compared to people who have a ratio of 9:1.  Thus, equine retinas are designed to detect motion even in low-lighting conditions. The retina also contains cones, or cells that sense color. Humans have three types of cones, which sense red, yellow-green, and blue light. Horses can see only two of the visible wavelengths in the light spectrum because they have only blue-sensitive cone cells and yellow-sensitive cone cells. Thus, they see blue, green, and variations of the two colors, but do not see red or shades of red.

Visible spectrum horses vs humans
Visible Sprectum Between Humans and Horses

The horse's night vision is superior to humans due to the tapetum lucidum, which enhances visual sensitivity under low light conditions. When light enters the eye, it triggers a photoreceptor on the retina, which is reflected by the tapetum to trigger additional receptors. As a result, the horse is able to see at lower light levels but the reflections also blur images by reducing the resolution. Because light is magnified, the pupil must constrict more during sunlight to protect the eye. This may cause the horse difficulty in identifying details and smaller objects and moving ones may trigger the flight response. Horses’ eyes are not adept at making a quick transition between bright and dark locations. This explains why horses are sometimes reluctant to enter dark places, such as a unfamiliar building, stall or trailer.

Horses are equipped to be highly aware of peripheral motion. The motion of anything causes the horse to register what it is (a survival mechanism for the prey animal). Anything that moves, regardless of what it is, may cause the horse to initiate the flight response before a predator begins to approach. Horses use two-forms of vision, monocular and binocular. Monocular vision allows the horse to see on both sides of his head, meaning the left eye and the right eye work independently and see different views. Each eye sees across an arc of approximately 200–210 degrees around the body at one time. The monocular fields straight in front of the horse’s face overlap slightly resulting in a  binocular field between 65 and 80 degrees. The binocular field is responsible for depth perception. A horse’s depth perception is considerably less than for humans. Because of their vision capabilities allowing for a panoramic view, it is impossible to sneak up on a horse.

Binocular and monovular vision field in horses
Binocular and Monocular Field of Vision in a Horse

The blind area is directly behind the animal’s head and body and has an arc of approximately 20 degrees. The front blind spot is directly under the head, immediately in front of the forehead and below the body in front of the horse’s face. A horse can’t see what they eat (grass they graze or grain in the bucket), fingers that stroke their muzzle or even the bit they accept in their mouths.  If a person raises a hand suddenly, they appear to the horse to come out of nowhere. Standing directly behind a horse, the animal can’t see you, thus when surprised from behind, a horse may kick you. For safety reasons, it is  always best to make sure the horse knows you are there.

All the horse has to do is change its head position and they can increase their visual horizons. If a horse lowers its head and moves it slightly from side to side, a horse can easily scan a 360-degree horizontal periphery. To focus on distant objects, a horse raises its head. When the horses head is held perpendicular to the ground, a horse’s visual field is lowered,  focusing less on distant objects and more on the immediate ground in front of the horse.

Horses vision with head up

When a horse raises its head, they use binocular vision to focus on distant objects

Horses vision with flexed poll

A horse with the head held vertically, the binocular focus will be on objects near its feet

Compared to humans, horses vision has the following characteristics:

  • Horses can detect the appearance of objects within an almost fully encompassing circle and are able to identify objects within most but not all of their panoramic field of view.
  • The horse may startle when an object passes from the field of vision from one eye to the other eye. For example, you take your dog with on a trail ride. The dog falls behind and jogs to catch up. The horse recognizes the dog on the left but when the dog moves over to the right side the horse jumps unexpectantly.
  • Horses are less able to distinguish details and contrast colors
  • Horses can see longer distances and can view the horizon and ground at the same time
  • Horses are easily blinded by bright light but can see better in dim light
  • Horses recognize patterns or outlines. For example, a horse is trail-ridden down a gravel road with no problem. The next time the horse is ridden down the gravel road there are garbage cans out by each drive and the horse spooks at the garbage can. This is most likely due to the garbage can being out of place. It was not there before and now it is a new item to be assessed.
  • Horses detect movement quickly. For example, you and a horse are trail riding, when a deer quite a way down the trail runs across the trail. The horse sees the movement of the deer and may react by trying to turn the other direction, or even taking off at a run.