Your horse's eyes are unique. Knowing more about them helps you understand his reactions.
Wondering exactly what (and how) your horse sees with those big, expressive eyes? Very differently from the way you do--and the way he sees often affects the way he acts.
He can see almost 360 degrees around himself because his eyes are on the sides of his head. That's why he notices objects or movements behind him that you (with eyes on the front of your head) can't even see without turning around. But his "rear view" vision is less distinct than his vision from about his shoulder forward--so he naturally wants to skitter away from unfamiliar things behind him or turn to see them better.
He sees most things with one eye--monocular vision--instead of with both eyes simultaneously (binocular vision --the way you see the world--which he uses for just a small area in front of his head). That's why he may spook at something that he's already walked past and reacted to once: He's seeing it for the first time--with his other eye. (How to know whether he's looking with one eye or two? As he tries to focus with both eyes on something, he pricks both ears straight ahead.)
He has two ways to bring objects into focus--by using tiny muscles to change the shape of his eye's lens (which is the only way you can focus)--or by changing his head position to direct the image to a different part of his eye. He raises his head to focus on far-away objects (and may turn it slightly at the same time to bring one eye to bear) and lowers it to see closer objects. That's why you see his head going up and down as he tries to figure out some new object he's spotted. It's also why he raises his head on the approach to a fence, then lowers it as he gets closer and gathers himself to spring. If you restrict these natural head movements, he may shake his head, shy because he can't see the object clearly, or even stop because he can't see well enough to jump.
His extravagantly big eyeball (largest of any land mammal's) magnifies everything fifty percent larger than we perceive it. That enables him to see distant objects in clearer detail than we can (an advantage for a prey animal needing to spot predators far away).
He has a completely different method of depth perception. Because he can't always use two eyes (binocular vision is what enables you to to perceive depth), he first gauges the relative distance of objects by comparing how big they appear with how big he knows they are. He knows humans are a certain approximate size, for instance, so a human who looks small to his monocular vision is a greater distance away. (That's why, if he sees something with one eye that doesn't fit his idea of what's normal, he turns his head for a more accurate binocular fix.)
He sees much better at night than you--even better than your cat! In the dusk, though, you see better than he does, because... ...he sees some colors (yellow, green, blue; red is iffy)--but this color vision diminishes with decreasing light. That's why he may tend to bump into you, the gate, or his pasture mates if you're bringing in your horses around twilight: His color perception has dimmed, but his night vision hasn't quite kicked in yet.
Having eyes on the sides of his head gives him a small "blind spot" directly in front of his muzzle--just where you'd expect things to be most clearly in plain sight. So he's more comfortable if you approach him from a slight angle (near his shoulder) that keeps you in view. He may even back away from a head-on approach--or at least turn his head away to keep from getting you into his blind spot.
He has a second blind spot, too: about 6 feet directly behind his tail. Hearing something coming from that vulnerable angle, he may swing his body to one side so he can see what's approaching--or just kick in self-defense. For that reason, if his quarters are toward you as you approach his stall, keep the door closed and quietly but firmly push them over so he can see you before you go in.
That's how your horse sees normally. Here are signs he's not seeing well:
more frequent or more exaggerated spooking
side-to-side head movements (to try to view an object with one eye, then the other)
delayed reaction--he starts to go past or over something, then seems to see it (and maybe spooks)
"jigging" on uneven ground (a compensation for inability to see the footing well).
Special Protective Features
Sensory hairs around your horse's eyes trigger the "blink reflex" if he gets too close to underbrush or other possibly eye-damaging surfaces while grazing or drinking. So when you're grooming, even for shows, avoid trimming these hairs shorter than an inch.
Nigra (pronounced NYE-grah) bodies--those round shapes in each pupil--seem to serve as a built-in visor, shielding interior eye structures from excess glare.
The third eyelid is a lightning-fast flap that zips across from the inner corner to seal the eye shut against threat even before the lids can close. It's also the source of lubricating tears.
This article originally appeared in the March 2000 issue of Practical Horseman. For useful information on how to apply ointment safely to your horse's amazing eyes, see the October 2003 Practical Horseman.