But how could scientists be so sure of this fact? Maybe our own “blue” color cone picks up on one shade of blue, while the blue cone of horses and dogs picks up on an entirely different one.
I asked the color vision researcher Joseph Carroll, who has studied this phenomenon in a number of animals. It turns out that we know a lot about color vision in humans, and, by proxy, in dogs and horses—thanks to researchers who wanted to understand the phenomenon of red-green color blindness, the inability of some people to detect the color red. One of the many high-tech tools that research has spawned is the electroretinograph, or ERG, which Carroll has used to understand color vision in animals. The device records the electrical activity in the eye rather the way an EKG records the electrical activity of the heart. “You flash different colors of lights and see how the eye responds. If you do this over and over, you can deduce the color sensitivity,” Carroll explained.
It turns out that the horse’s sensitivity to color is quite like that of people who lack the third color cone and are red-green color-blind. To illustrate the range of colors horses see, Carroll took a photograph of two children wearing red clothing mounted on horseback. He took a second photograph of a horse walking in a paddock next to a barn with a green field in the background. Then he altered the colors in the photograph to simulate what a horse might see.
He also created a color wheel for horses. The green grass a horse sees is still “green,” but not the rich, vital green most humans enjoy. Instead, the color is washed out. There are no reds. Blue is there, but lacks richness. From our perspective, horses see a washed-out world.
That missing vitality means that the horse has a limited ability to perceive fine detail. In Carroll’s photographs comparing a scene as observed by the eyes of a human with the same scene as observed by the eyes of a horse, to our eye a brightly colored halter stands out on the head of the white horse. From the horse’s point of view, that halter is much more dully colored and hence less obvious.
Gerald Jacobs, an expert in the evolution of color vision, believes that the ability of animals to detect color may have begun to evolve as long as 540 million years ago, when the first animals appeared in the ocean. “Color vision was probably inevitable,” he told me. “Color vision is the way you resolve space.”
If you have poor vision, it’s easy to understand what Jacobs means. Take your glasses off and you will still perceive objects—not by perceiving sharp outlines but by perceiving color differences. You can navigate a room by following those differences. Now imagine that you are a horse. You’ll have fewer color cues.
Horses, therefore, have more difficulty perceiving objects than we do. Imagine a red British public bus in the distance, behind a wall of green foliage. Although you can only detect bits and pieces of that red bus, in your mind you will “see” the red bus, because your brain will fill in the missing pieces for you. “The brain is a creativity machine that seeks out coherent patterns in an often confusing welter” of signals, as the neuroscientist Eric Kandel has pointed out.
For a horse, the red will not be obvious. He may not have enough information to perceive the bus. Imagine a few apples on a faraway, well-weathered picnic table. You see them and ride over for a snack. Your horse may not know they are apples until he gets close enough to detect the aroma. On the other hand, if he is used to seeing a tasty pile of apples on the picnic table, his mental creativity machine may put the pieces of the picture together and he’ll head right over. This is why it’s very important for horses to be allowed to experience new things in familiar areas. They need to examine novelty in order to assemble their own mental pictures.
This is particularly true for distant objects. Imagine a rider wearing a red jacket standing against a field of green grass. We can clearly see this person. The horse might not—until the person moves. At that point, he may well startle and bolt.
Sadly, although vision researchers can tell us about the colors a horse can detect, they can’t tell us what the horse perceives, or how the horse puts the bits and pieces of visual information together into a mental picture of the world around him. But we do know that the horse is brilliant at assimilating novelty: once he’s looked closely at something, he’s likely to come to terms with it.
We know a great deal about this visual learning process in the human brain, and something about this process in the brains of dogs and even cats. But, as Gerald Jacobs warned, how the horse brain processes visual information remains a mystery, other than that the horse probably does not detect the color red.
That raised another question for me.
“Then what,” I asked, “about the New York City carriage horse who stopped at the red light?”
Several years ago, a Central Park carriage horse named Oreo, frightened by an unexpected loud noise, had stampeded down one of the city’s major thoroughfares, Ninth Avenue. Several people had tried to stop him, but Oreo was too frightened, having by then run into a silver BMW, torn the car’s bumper off, and decimated his own carriage, still clattering behind him.
Then the horse came to an intersection. The light turned red. Oreo stopped, standing with vehicles waiting for the light to change.
“If horses are red-green color-blind, how did he know to stop?” I asked.
Jacobs explained that we can’t know exactly why Oreo stopped at the red light, but it’s possible the horse was aware of the position of the red light versus that of the green light. Horses may not distinguish the color red from the color green, but they can use all kinds of other visual cues in order to navigate.
He may also, Jacobs said, have seen a color difference in the two lights. That’s because “green” lights in traffic signals are no longer true greens. Of course, the horse may simply have stopped because other vehicles had stopped at the light, but he may also have perceived a real color difference: “If the red and the green lights were perfectly red and perfectly green, and balanced in luminance [brightness], color-blind drivers would not be able to see the difference at all. What highway engineers did was add a lot of short-wavelength light to the green. It has a bluish cast, so it makes the discrimination one that could be detected by color-blind drivers who, like Oreo, only have two color cones.”
* * *
And now, with a foundation for understanding how horses and humans see, we can return to Brian Timney and his research. After watching his horses watch the hot air balloons seven miles away, Timney decided to find out just how well horses could see when relying on acuity rather than color. In humans, acuity—our sharpness of vision—is central to our perception of the world. Our own acuity is tested when we read an eye chart: when we get to the line that we can’t read, the optometrist tells us just how bad our vision is. Good human eyesight is 20/20. My own vision was 20/one zillion until I had eye surgery. Things that others saw in the distance simply did not exist for me. I was in a national park once when a number of people stood and watched a grizzly bear climb on a not-that-distant cliff. I saw nothing.
But how do you figure something like that out for a horse, since they can’t tell us what they’re seeing? Timney applied time-honored techniques for assessing the acuity of human infants, but added a few twists. First he built two hinged swinging doors into a wall-like structure. Behind each door were trays with treats. To get the treats, the horse had to learn to push the swinging door with his muzzle. As any barn manager knows, horses learn such things easily.
Next, Timney kicked things up a notch. The horses learned that there were no treats behind the door painted solid gray. There was a treat behind the door painted in very wide black and white stripes. The horses also learned this easily, so he knew the horses were “reading” the stripes.
Then he began progressively narrowing the width of the stripes. This gradually increased the difficulty of detecting which door was striped and which was not. In a sense, the horses were now reading a version of an optometrist’s eye chart.
To ensure that the test measured acuity at a distanc
e, Timney had the horses stand back a bit more than six feet. A two-meter-long wall separated the path leading to one door from the path leading to the other door, so that the horses had to make their choices from a distance.
The horses were pretty good at the game. At first, when the stripes were quite wide, their success level was nearly 100 percent. But as the stripes narrowed in width, getting closer and closer to solid gray, the horses had more and more difficulty choosing. When they made correct choices only half the time, Timney knew they were just guessing. The very narrow stripes were probably appearing to the horse to be no different from the solid gray.
This very cool experiment tells us a lot. First, it reveals that horses make good research subjects, as they were willing to stand and learn in order to get a paycheck. It also tells us that although horses have vision that’s less than 20/20, they do see with more clarity than most mammals other than primates. Timney has conducted similar experiments with cats, monkeys, camels, and even bumblebees. He’s found that the horse’s long-distance acuity is superior to that of most other animals.
“Horses have a visual acuity that’s about two-thirds as good as human acuity,” he told me. “That’s actually pretty good. Certainly, when they saw those hot air balloons they were picking up something that was pretty small. A cat could have had no hope of seeing something that distant.” This tells us that not only does a horse have relatively good vision compared to a lot of other mammals, but that when a rider is on a horse’s back and they’re going for the jump, the horse will be able to see the jump relatively well.
Timney also wanted to know whether a horse can perceive depth. Was the horse’s world simply flat and two-dimensional? Probably not, or horses would have difficulty jumping over fences. But what kinds of depth-related concepts do they have?
He used the Ponzo illusion to figure this out. This is a two-dimensional visual deception that fools most of us—so much so that even after it’s explained, we still have trouble seeing what’s really there.
Draw two lines of equal length on a piece of paper, stacked one on top of the other, so that they make a kind of column. You’ll easily see those lines as equal in length.
But then put those two equal-length lines in context. If you place them on top of a drawing of a set of receding railroad tracks, you will perceive the top line as longer than the bottom line. Your eye will detect the data correctly, but when your brain assembles the data into a picture, your brain will misinform you.
This phenomenon is so strong and consistent that even when we know for a fact that the two lines are exactly the same length, we will continue to misperceive their relationship. The neuroscientist Kandel explains the phenomenon this way: “Vision is not simply a window onto the world, but truly a creation of the brain.” In other words, the human brain assembles a concept of depth—even when it’s not really there. When Renaissance artists finally realized this strange truth—that the brain automatically “sees” three-dimensional perspective even in flat, two-dimensional art—Western art completely changed.
Timney discovered that horses do the same thing. He showed his horses two separate sets of lines stacked on top of each other. One set of lines was of equal length, as in the experiment with people. The other set showed a line on top that was indeed longer than the lower line. The horses learned to go to the set of lines with the longer line on top. Then he presented his horses with the Ponzo illusion. He showed them two sets of two lines—but both sets contained lines of equal length. One set of lines was placed in a drawing with trees and other landscape items not drawn in perspective. The second set of lines was placed on top of the receding railroad tracks, as in the human test.
“Overwhelmingly, they chose to go to the one that seemed as though the line was longer at the top,” he told me. “They were susceptible to the illusion.”
This is stunning. A visual ability of which we humans are particularly proud, our ability to “read” depth on a piece of paper—turns out to be something that horses do, too. Moreover, the fact that horses and humans make the same error in perception provides yet another clue to our common evolutionary heritage. Our most recent shared ancestor may have had the same tendency. It made me wonder: What would horses make of Venetian Renaissance art, with its great advances in communicating depth? Would they be able to pick that painting out of a group of earlier paintings that don’t communicate depth?
It’s a silly question, asked lightly, but it is remarkable that horses can extrapolate information from a two-dimensional page clueing them in to facts about the three-dimensional world around them. If you think about what the horse is doing, it shows a pretty sophisticated mental capacity. Is their sense of perspective similar to our own?
I wouldn’t have thought so. And yet, Timney’s research shows that there may be more similarities than we expect. After all, life evolved in a three-dimensional world. It makes sense that brains evolved to somehow deal with that fact.
Excellent binocular vision is a terrific aid in depth perception, but there are other ways in which animals can perceive depth and distance. We humans have at least one other tool in our toolbox. Open one of your eyes and move your head: you’ll easily perceive which objects are near to you and which are far away. This is because of a phenomenon called “motion parallax.” Looking out the window of a speeding car with one eye closed, you will still know which objects are close to you and which objects are farther away.
Horses do this, too. They can do this when galloping or walking, but they can also do this just by moving their heads. This is one reason why it’s important to let a horse move his head when you’re riding. Holding a horse on a tight rein inhibits the horse’s ability to use motion parallax to perceive depth. For example, with one eye, a horse may have trouble assembling a mental picture of the world around him when the light is dappled, as it is under a canopy of trees. We can use our binocular vision to do this, but horses have limited binocular vision and thus more trouble with this task. This is yet another reason why the horse responds to any movement, any sparkling difference between light and shadow, and not just to specific objects.
* * *
We humans excel at color detection, compared to other mammals, but horses are much better than we are at detecting even the smallest movements in low light, as at dusk. This is because horses, compared to us, have a greater proportion of super-light-sensitive rods to every color cone. There seem to be wiring differences as well. Some rods in horse eyes send messages to the central nervous system at much greater speeds than do the rods in our own eyes.
On the other hand, our eyes adapt more quickly to changes in light levels. If you turn out the lights in a room at night, you’ll be able to see in a matter of seconds. The horse requires about a half hour to make the equivalent change. Because our primate ancestors lived in thickly forested areas, we needed this dark-to-light-and-back-again flexibility. Evolved to live on the open plain, Equus only required rods that changed during sunset or sunrise, or about the evolutionarily appropriate thirty minutes.
We often put horses in situations where they cannot see very well. When we lead a horse from bright sunlight into a dark horse trailer, for example, our own vision improves immediately. But the horse’s vision will not be good for about thirty minutes. To him it may seem as though we are leading him into a dark, dangerous cave.
Sometimes when a horse appears to be behaving badly, the problem is not the horse per se, but his difficulty in seeing. Outside of Edinburgh, Scotland, I once saw a trainer bring a “problem” horse out of the sunlight into a poorly lit arena. All alone, with only a stranger holding his lead line, the horse was standing very much on alert. Every muscle in his body was tense. His head was held very high and his nose was up in the air. He appeared to be trying to see something. His ears were pointed directly forward.
Following the direction of his eyes and ears, it was easy to see what frightened him. The far end of the arena was darker than where he was standing. At that end
was a solid wall that stood out visually since it was painted a bright, luminous white. This solid wall rose to about shoulder height. Behind that white wall people in black watch caps scurried back and forth. Their bodies were hidden. Only their black-capped heads were visible. To us, because our acuity is better and because we’ve had experience with watch caps, the wall looked like a wall and the people looked like people. Our brains created an appropriate picture.
The horse, a newcomer to the riding arena, was unable to put the mental picture together. He probably perceived something that his evolutionary heritage predisposed him to notice. Perhaps the picture the horse assembled from the data gathered by his eyes was not a picture of a white wall, but a picture of a faraway cliff. And upon that white cliff were black-pelted predators—wolves?—scurrying back and forth, biding their time until ready to pounce. No wonder the horse was nervous. The poor thing kept snorting, dancing back and forth, and raising his head, trying to get a better look.
The horse behaved like this only when alone in the arena, a trainer said. When other horses were with him, this horse was quite calm. As we know, horses travel in bands, and like other mammals who live in tightly organized groups—prairie dogs, for example—they often rely as much on the vision of their band members as on their own vision. That’s why horses in the open rarely all lie down at the same time. One has to remain on guard.
Possibly if the skittish horse had been with a longtime, trusted human companion, he might have been calmer, but sometimes horses may rely too much on human cues. The Australian neuroscientist Alison Harman, an experienced dressage rider, once saw two dressage horses, with noses tucked into their chests, canter toward each other and collide. Harman conducted research that revealed that horses with heads tucked in to their chests are traveling blind. The horses who collided were depending on their riders’ cues, and their riders were not paying attention.
The Horse Page 23