Articles

Kimberly A. Foushee
NCSU Extension Horse Husbandry

By gaining a better understanding of how horses see, one can learn to work safely around them. A horse’s behavior is related to its vision or lack of vision. Understanding the equine sense of vision allows us to easier predict an animal’s behavior.

The mechanism of the eye consists of the cornea (1), a transparent outer covering which admits light. The sclera (6) or white portion is the supportive wall of the eye. The iris (2) is the colored portion that opens and closes the pupil according to the amount of available light. The pupil (3) is the horizontal slit that allows light to enter the center of the eye. The lens (4) allows light to pass through and be focused on the retina (5). The image formed by the lens on the retina is carried to the brain via the optic nerve (7).

Humans focus by adjusting the shape of the lens; however, the ciliary muscles of the horse are weak and unable to produce a thinning or thickening of the lens for proper focusing. Therefore, the horse’s lens is almost stationary.

The human retina has a smooth concave surface, but the horse has a ramp retina which is irregular and inconsistent in its concave appearance. Some areas of the horse’s retina are closer to the cornea than others. The ability to focus at varying distance depends upon the position of the horse’s head. Raising or lowering the head is necessary to admit light or the image into the ramp retina in the proper place to focus. The horse must raise its head to see items at a distance and subsequently, lower its head to see close objects.

Equine eyes are located on the corner of the head. This provides the horse with a wide field of vision. The horse can see 300 degrees around its body. However, the horse cannot see directly behind its hindquarters. In addition to a rear blind spot, the horse also cannot see two to four feet in front of its face due to the placement of the eyes on the head. Subsequently, the horse cannot see the food it eats.

The horse also uses binocular and monocular vision. With binocular vision, the horse uses both eyes to focus on one object, similar to the way humans see. Monocular vision is used when a horse looks to the side or rear. Each eye sees a different image and each image is transmitted to the brain at the same time. The horse is unable to use both monocular and binocular vision at the same time.

Several factors come into play when explaining why a horse shies at some objects and not others. Like any animal, they can be conditioned to accept new experiences, but one must always be prepared for a spooky reaction at anytime. Horses are capable of seeing movement immediately; however, most research agrees that horses are color blind and see only varying shades of grays, including varying degrees of shadow. This hinders the horse’s ability to recognize even familiar objects. Similar to humans, the horse also needs time to allow his eyes to adjust to darkness, whether it is a dark stall, barn or trailer. In addition, the ability to see three dimensionally with binocular vision and having no depth perception with monocular vision, greatly influences the horse’s behavior. The majority of the seeing the horse does is with monocular vision, so most of the time objects appear flat to the horse and seem to be the same distance away. So when that object moves it appears as if it is leaping out. Another factor, which encourages shying, is the wide placement of the horse’s eyes. An illusion of movement is created when an item in the horse’s field of vision passes from one eye to the other. The wider width between the eyes makes the object appear to jump from side to side. This is also true when an object moves from binocular to monocular vision.

Understanding the anatomy of the equine eye and how it affects the animal’s ability to see and therefore reason, enables the handler to develop effective handling techniques to ensure the safety of both.