finger with both eyes simultaneously. But even in a rearing position, Twinkletoes would have to be a contortionist to get his hoof in front of both eyes at the same time. As prey animals, horses are built for peripheral motion detection at the expense of depth perception. As predators, we’re built in reverse.
4.1 With two eyes from a stride away, the equine brain can sense a minimum of only 9 inches in depth from front to back. In contrast, the human brain can sense 1/8 inch in depth from front to back. We perceive differences in depth that the horse cannot see.
For horses in disciplines like dressage, reining, or pleasure, depth perception is not so critical. But consider cutting, barrel racing, or jumping. A horse needs to know how far away relevant objects are and how fast those distances are changing as he moves. A horse can improve depth perception by raising his head, dropping his withers, or lifting his nose, but this often complicates his task. In cutting, for instance, horses need to keep their eyes down on the cow and their heads low to make quick turns. In jumping, they need impulsion from their hindquarters to power off the ground and abdominal tuck to lift their legs. The physics of such movements require horses to maintain a round back for core strength, which often precludes the position of a high head.
The distinction between hunters and jumpers is also important here. Top jumpers are judged by the clearance and speed of their rounds over high, wide fences—fences that are often approached off sharp turns from short distances. Such horses are often selected as jumpers because their necks are set high on the withers, with head position proportionately higher. Those without that conformation are encouraged to approach jumps with their heads raised. If you watch a jumper approaching a fence, you’ll see his head lift in the last stride or two. This natural form provides both eyes with a brief view of the jump, so that the equine brain can determine its height and width. But the view is indeed brief—fractions of a second—and it’s late.
Occasionally, we hear that jumpers are aided in depth perception by wagging their heads back and forth on approach, to allow each eye a view of the jump. This suggestion does not hold up in terms of brain science. To compute distance, the brain requires a simultaneous view of the object with both eyes. Wiggling the head back and forth only interferes with centering the horse. It probably also prevents him from concentrating on other cues from the rider that are much more important.
Depth perception is easier for hunters. These horses are judged on the quiet beauty of their jumping form and are taught to maintain a long frame with hindquarters engaged, necks arched long, heads low, and faces nearer the vertical to form a strong topline. This position can be preserved over fences because hunters are given a long approach with which to see relatively low jumps without raising their heads. Good hunter riders encourage horses to look at a fence while rounding a distant corner. This supplies the horse with a better side view, a longer front view, and more time for the two eyes to send images of the jump to the brain. It’s still a good idea, of course, to allow any jumping horse some freedom in moving his head to improve his view.
In terms of width, only about half the area visible to two human eyes at the same time is visible to a horse’s two eyes at the same time (fig. 4.2). Stand about 30 feet back from an arena fence. Roughly 5 feet of that fence is clear and sharp to both of your eyes as you hold them still. From the same position, only half of that—about 2½ feet—is clear to both the horse’s eyes. And it is only that small portion visible to both eyes for which a brain can calculate depth by stereoacuity.
4.2 With two eyes focused straight ahead, the horse sees only half the width of the human view.
When you’re aiming a horse toward a fence, center him on the narrow middle portion that he can see with both eyes. Many early jumping errors occur when a rider does not steer the horse to the center of a jump. These problems are frequently blamed on the horse—he ran out, he refused, he chipped, he jumped in bad form. Well, that’s not because he’s a bad horse; it’s because the rider didn’t let him see the fence!
Visual Capture
Vision is our strongest human sense—it hogs more neural real estate than any other sensory system, filling almost one-third of the human brain. With such clout, it can override other senses, a neural ability known as visual capture.
Visual capture is responsible for the fact that ventriloquists can fool us. We believe that speech is emanating from a dummy’s mouth, even when the human speaker is standing right there. The visual movement of the fake mouth captures our attention and links it to the spoken words. Magicians use visual capture to hide actions of their hands in plain sight. Moviegoers gesture toward an actor’s mouth on the screen when asked where the sound is coming from—even when theater speakers are placed at the back or sides of the house.
Because we rely so heavily on sight, we suppose our horses do, too. Yet we’ve seen that many aspects of equine vision are worse than ours. The horse compensates for his visual weaknesses with good hearing and a fantastic sense of smell. We help him by encouraging reliance on these stronger senses. The next time your horse skitters away from a safe spot, think about what he might hear or smell that is imperceptible to you. In his world, it’s not all about vision.
Color Vision
Harley was a six-year-old red dun Quarter Horse who knew how to be led and longed. Nothing else. An adult horse who knows so little is not the safest animal. (Let me make a plea to backyard horse owners everywhere: Hire a trainer when the horse is young and small!) I was holding this overgrown baby in an indoor arena one day when snow slid off the roof. Many horses shy from this, but most don’t try to jump in my lap. Harley jumped a foot high and a couple feet forward off all four hooves and landed on the arch of my left foot.
The ensuing injury interfered with my ability to use a stirrup, so a friend suggested mountain bike shoes with rigid soles that would support my foot. Off I hobbled to the cycling store, where the only inexpensive pair pulsed on the shelf in blazing yellow with a chartreuse afterglow. To accommodate the padding around my wrapped foot, they were also two sizes too big. They were probably visible from the surface of the moon. But riding is more important to me than style—a good thing, because I looked absurd wearing these shoes coupled with some old sunburned half-chaps.
On Day One of the new fashion, every horse at the ranch stared in surprise at my feet. They knew me well; some I’d ridden daily for years. They’d seen me wear a variety of neon shirts in an effort to liven up the atmosphere. But this was clearly a strong visual sight for them. All day, as soon as I mounted, every horse turned to my stirrups and sniffed each shoe carefully. We joked that the horses were embarrassed, but in reality we had proven a feature of equine color vision.
What colors do horses see best? Yep, you guessed it: bright yellow with a chartreuse tint, just like my shoes! Horses also pick up bright turquoise and teal hues well. Hang a few jackets on your arena fence someday. Notice how the horses are much more aware of the bright yellow rain jacket than they are of the equally flappable bright purple or flame red versions. A horse who needs to pick up his feet over fences can be reminded to do so with the simple use of some bright yellow or turquoise rails. A young horse who is just learning to negotiate ground poles will excel with other colors—he needs to see the poles without feeling provoked by them.
Remember the eye’s rod and cone cells from chapter 3 (p. 42)? Where rods sense movement in dim light, cones pick up detail and color. Horses have many rods and few cones; humans have the opposite. So, all colors visible to horses—even the bright yellows—are faded in comparison to human perception of color. Most critically, horses do not have any cones that pick up red and green. Because of this a horse (like a dog, squirrel, or pig) cannot distinguish between red and
