create perceptions in ways that are very different from ours. Visual information travels from the eye to the brain in both species, of course. But the human brain sends back six times more neural information in the opposite direction, from the brain to the eye. This wiring boosts perceptual interpretation: lots of knowledge is melded with the human eye’s pictures of the outside world. So, who’s more objective in seeing reality, you or your horse? Hate to break the news, but it’s probably your horse. Equine brains should be less prone to illusions than human brains are.
Visual Acuity
Horses often give the impression of superb eyesight. Walking in an open field, a bird flicks a wing and they’ll raise their heads, point their ears, quiver their nostrils, and widen their eyes with what seems to be intense focus on the bird’s location. Some trainers refer to this as the look of an eagle, and it is indeed an impressive display of intelligence and sensitivity. However, the reason for it depends less on good vision than bad vision. Horses try to improve blurry views by raising their heads and enlarging their eyes. Their ears perk up to listen because they can’t see stationary details well. Their nostrils expand to optimize an excellent sense of smell.
Equine eyes are eight times larger than human eyes, larger than those of any other land mammal. But a horse’s acuity is considerably worse than ours. Acuity refers to the ability to make tiny discriminations in detail while focusing on something in the center of the visual field. Reading is a great example for humans—right now, your eyes are picking up tiny differences in the black marks on a page. You can see the difference between an “e” and a “c,” for example. The distinction is meaningful—witness the confusion if you misread that you have “cars” on both sides of your head.
By convention, normal human acuity is 20/20. What a person with normal vision can see from a distance of 20 feet is the same as what you see from a distance of 20 feet—if you have normal vision. But normal equine acuity ranges from 20/30 to 20/60.
Let’s consider the visually gifted (20/30) horse first. Details you can see from 30 feet away, a sharp-eyed horse can only see from 20 feet away. In other words, he has to be 50% closer to see the same details—he has half your acuity. What if your sweetie-pie is near the low end of normal equine acuity at 20/60? Details you make out from 60 feet away, he cannot see until approaching within 20 feet. That’s a 200% impairment compared to human vision!
Even the 50% deficiency is enough for any rider to consider. Imagine what a horse sees when the two of you approach a jump (figs. 3.1 A & B). For you, it’s clear, sharp, and bright. You’d be mighty nervous if it looked fuzzy and faded. But equestrians are often startled to see photographs constructed to show what a jump looks like to a horse. Even in sunshine, the horse’s view of a jump is blurry, hazy, dim, flat, vague—all the adjectives you’d rather not deliberate as you’re galloping 30 feet per second to a big oxer that could break your neck.
Beyond the normal range from 20/30 to 20/60, horses differ in individual acuity just as people do. Twenty-three percent of horses are near-sighted (they do not see details clearly until they get much closer than equine normal to an object). Forty-three percent of horses are far-sighted (able to see more clearly only as they get farther away). It stands to reason that slightly far-sighted horses excel in disciplines like jumping because the ability to drill down on fine points from a distance fuels their athleticism.
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3.1 A The rider sees a jump clearly on approach.
3.1 B The horse’s brain sees the same oncoming jump with less acuity and poorer focus.
Acuity for objects close to us worsens with age because the natural lens inside human and equine eyes hardens over time. If you’re over 50, you know what I’m talking about. The best acuity in horses occurs around age seven. Prior to that it’s not fully developed, and afterward it begins to decay. Breed makes a difference, too. Horses with long convex faces, like Standardbreds and Thoroughbreds, have better acuity than horses with short concave faces, like Arabians.
Focus
The human eye is superb at focusing on one detail of a scene. Muscles holding the flexible lens of the eye pull it into a more convex shape to focus on objects close up. You can feel these muscles working if you hold a finger up in front of your eyes. Focus on the finger then look past it (without moving your eyes) to a distant object. Go back and forth, getting the feel of the ciliary muscles that flex your lens. This ability—called accommodation—allows us to inspect objects while working on them with our hands. Humans excel at visual accommodation.
Horses do not. Their ciliary muscles are too weak to pull the lens into a more convex shape for greater focus (fig. 3.2). So when you hold something near your horse’s face—a bridle or clippers—he can smell it but will have great trouble focusing his eyes on it. To show your horse something new, hold it near his nose for a good sniff or on his shoulder where he can feel it with his excellent sense of touch. If it must be seen, hold it out in the air several feet away, and wait a bit. What little focus horses can achieve happens slowly. That’s why sudden movements near their bodies can surprise them.
3.2 Like the human eye, the equine eye contains a pupil, iris, ciliary muscle, and lens. However, it is a bit more compressed, or flatter, in shape than the human eyeball.
Range of Vision
Ask a child to describe a horse’s eyes. One of the first things the child will mention is that they are very large and set on the sides of the head, unlike small human eyes that point forward. This simple observation generates profound differences in the ways humans and horses see. Eye position affects visual range, peripheral motion detection, and depth perception. Equine eyes can even move independently to scan one side of the world more intently than the other.
Human sight is accurate enough to decode tiny marks on a page, but only for a very small slice of the view. While reading, a few words in your central vision are truly clear; the rest are blurred. Stretch your arm out to the side, holding something like a pencil vertically in your hand. Look straight ahead. You won’t see the pencil in this position. You can’t even see your arm. Now move your arm slowly in a wide outstretched semi-circle toward the front, keeping your eyes focused on a distant point in front of you. No cheating! The pencil remains invisible until it reaches almost a 45-degree angle. Human vision is limited to roughly 45 degrees on either side of our noses, for a total of about 90 degrees (figs. 3.3 A & B).
By contrast, if we held a pencil straight out from the side of a horse’s head, it would be almost in the center of his vision. With eyes on the sides of his head, he catches a 340-degree view, almost four times greater than the range we see. Imagine what would happen if we humans had four times more vision to process every second of the day. We’d be edgy, too!
The horse’s visual range stretches from the outside of the nose all the way around to an imaginary line extending back from the hip. The last few degrees of angle near the hind legs and hips permit only very poor vision. When leading or riding your horse, the vehicle that you cannot see approaching from behind your shoulder is within his line of sight, but it’s not too clear. It’s coming toward him, often at a rate faster than he is moving. Green horses see this as a chase, and every fiber of their being says that the way to survive a chase is to run. Now!
3.3 A Human range of view is about 90 degrees, so the rider in this drawing only sees someone moving hay near a wheelbarrow.
