So, You Think You Love Horses? Some Reflections on the Nature of Horses and Man

More Discussions by “The Accidental Horseman”

The Horse’s Eye

The Horse’s Eye

It might be natural to assume that a horse’s vision is much the same as our own. Vision is a paramount human sense, and we tend to believe our eyes when confronted by conflicting sensations. However, a horse’s eye is very different than that of a human in both design and function.

A very basic and obvious difference is the placement of the eyes on the face. Human eyes are placed forward on the face. Because of this we have very well developed binocular vision, accounted for by overlapping visual fields. If you look at a point in front of you and alternately cover and uncover each eye, you will see part of your visual field disappears. Our vision is most acute when we focus an object directly on the macular area of the retina. When we are seeing with only one eye we lose depth perception, but we may not be fully aware of it since the brain is good at organizing the environment based on our prior knowledge of it. One of our eyes is able to take in an arc about 60 degrees toward the nose and 100 degrees toward the temple.

The Horse’s Visual Fields

A horse’s eye is set on the sides of its head and between the two of them they are able to take in an amazing 285 degrees of the environment around them. They do experience a cone shaped blind area immediately in front of them for a distance of about 4 feet and a larger trailing blind area in behind them that never resolves. To see things in these blind areas a horse must swing its head much the same as a person would. The equine eye is huge and when you look at a horse’s eye you are aware of lids, a large pupil and the colored iris that is often, but not necessarily, brown. What you are not so much aware of is the sclera, or white, of the eye. The sclera is mostly totally hidden by the lids and although very obvious in humans, it is not so obvious in horses. Since the exposed cornea occupies nearly the entire visible area of the equine eye, for the most part there is little point to a horse moving its eyeball. About the only time that you will notice a horse doing this is when its head is restrained or when it is pulling its head away from a threat coming from the side. Horses generally move their heads, not their eyes. Horses do have a degree of binocular vision ahead of them at the point where their visual fields converge. All in all this takes in about 65 degrees of arc fanning outward from that point 4 feet in front of their head. Human’s most acute vision is in a circular macular region of the eye, and for a horse the most acute vision is in an area called the visual streak. This area is linear in a horse and constitutes a band of improved visual acuity running across a horse’s visual field. To bring this area to bear on an object, the horse moves its head so that the object falls within the region of the visual streak. A human’s visual acuity is superior to that of a horse, which is why you do not find horses doing high wire acts.

There are other areas where the two species differ. Our pupils readily react to light; a horse’s eye does not. Horses are at a distinct disadvantage when there is a sudden change in the intensity of light. In their natural environment, which is open grassland, sudden dramatic changes in light are uncommon. However, in man-made structures, and with things such as light switches, or in darkened horse trailers, horses are often challenged by being asked to do things when they have been blinded by a sudden change in the level of illumination. If you look inside of the anterior chamber of a horse's eye you will see a structure known as the corpora nigra just above the pupil. It is visible in the above photograph. The purpose of the corpora nigra is to shade the pupil from glare. Another difference is that the lenses of horses’ eyes do not accommodate as well as those in humans. There are small muscles on the inside of the eye which bend the lens and focus objects on the light sensitive retina. Horses use a different strategy for focusing objects. The surface of the eye itself is set at different focal lengths such that when a horse has its eyes down and is grazing, near objects are in focus, and when a horse raises its head, far objects are in focus. This happens without the need to wait for the much larger equine lens to accommodate. This ability affords the horse some additional milliseconds of reaction time that might be the difference between falling prey to a predator and a successful escape.

In certain situations you can see the whites of their eyes.

The final areas to mention are the nature of a horse’s color vision and night vision. Animal behaviorists and comparative anatomists have studied these questions. A horse’s night vision is far superior to a human’s. Ride a horse at night and you will quickly appreciate this fact, but watch out for those low limbs. Although their night vision is superior, their color vision is not. Humans have very rich trichromatic color vision because we have three types of color-sensing cone cells in our eyes. Horses, dogs and cats have dichromatic color vision, along with some people who we call color blind. Horses cannot distinguish the colors red or orange. Their world is full of blues, yellows, and grays. To them, the other colors we see are just a slightly different shades of these colors.

It would be possible to design an optical system that would simulate for a human the visual experience of a horse, and I suspect that it would be both informative and somewhat disorienting for us. However, I do not know if anyone has taken the time and expense to do this. Our failure to appreciate the nature of a horse’s vision often leads to problems. Those dressage horses with an exaggerated tuck to their necks look rather grand in the ring but have been known to collide with each other while being schooled in a group because they are basically looking down at the ground and cannot see what is directly in front of them. There have been occasions when while trail riding I have had to warn my horse off from walking directly into a tree that was in his blind spot. Show riders have learned to allow their horses freedom of head movement so that they are capable of seeing the jump. Course designers have begun to take the nature of the horse’s color vision into account by designing jumps whose color characteristics contrast in such a way that the horse can clearly see them.

The differences between human and equine eyes were developed over millennia of natural selection. Humans are primates and developed as tree dwelling creatures. One false move and a primate took a tumble. Good color vision aided finding food. Superior vision was the key to survival in that world. For horses, visual needs were different. Having a huge visual field allowed them to detect approaching predators. Since predators are equally active night and day and since horses cannot really hide, good night vision was equally important. Since horses are specialized to eat green grass, color vision was not critical. The differences in vision between horses and men are all accounted for by the survival needs of the species. It would be wrong to say one is somehow better or worse. Both were well adapted to their natural environment. When working with horses we need to appreciate how they experience the world and understand that their experience is not the same as ours.