Posted on August 23rd, 2016
An eye witness is one who has seen something important. A horse is more likely to be an ear witness. We look at something unexpected by turning our eyes and our heads. A horse listens to something unexpected by turning his ears and his head.
Mammalian predators typically have their eyes in front of their heads, with pupils that are either round or vertical slits. This adaptation gives the predator a focused binocular view of any chosen point, with poor peripheral vision. Prey species, in contrast, typically have their eyes on the sides of their heads, with pupils that are horizontally elongated. This adaptation makes it harder for a predator to get to them undetected. Horses, like other grazing animals, are prey species.
The horse’s eye is the largest of any land mammal1, about eight times as large as a human’s. Big eyes gather more light, and are handy for the horse who must see well in dim light.
The eye of the horse is positioned on the side of its head. This extends the total visual field to about 350 degrees. Such a field of view is very valuable for a prey animal. But it greatly reduces the overlapping binocular visual field, which is important for depth perception and which is critical for visual acuity — the ability to resolve details.
Field of view for a horse. [Not a great diagram, because no blind spot in front of the horse is shown, and the 350 degree field of view is reduced with excessive blindspot (the white area) behind.] The horse is unable to see the area behind it, or inches in front of his head.2
The immense field of view of the horse may underlie your horse’s apparent short attention span. The more a horse can see, the less it focuses. The more it can see, the more distracted it becomes. Blinders or blinkers are very useful for horses with important jobs, like racing or pulling carts and wagons. By narrowing the field of view to just what is in front, we eliminate the distractions. Of course, this heaps more responsibility on us to ensure that there are no dangers coming that the horse can now not see.
Depth perception benefits from binocular vision, but when the horse bobs its head up and down as it moves, close objects in the monocular visual fields will bob down and up against a more static horizon, allowing a horse to infer that the object is closer. Putting a martingale on a horse, or otherwise constraining its head movement, likely interferes with his vision.
With our eyes on the front of our heads, we can easily follow our finger when we place it on our nose, as in a Finger-to-Nose sobriety test. But with its eyes on the sides of its head, a horse’s blindspots are quite different. If you reach to scratch your horse’s nose, or brush his face, he may not see you coming. The blind area in front of the horse’s face makes them very different than us, and when you should remember this when interacting with them. The shaded area of the diagram below shows the horse’s blind spots.
The blind spot in front of a horse extends about 4 feet from his eyes. If your horse fails a finger-to-nose sobriety test, he may not be drunk.3
Rods, Cones, and Fovea
The rods in our eyes formed the earliest basis of vision in vertebrates. Rods are best for seeing in very low light, or for sensing differences in light level. But they don’t work as well as cones for detecting details of an image, and can’t detect color. The cones in our eyes work best in bright light, give us the details of an image, but are not as good as the rods at detecting small differences in contrast.
Horses and humans have both cones and rods, but horses have relatively more rods. That is, their night vision is far better than ours, and their ability to see differences in contrast is better, but their ability to detect color is not as good as ours. On a dark, moonless night in the woods, a horse can discriminate between shapes and negotiate the terrain. Any glowsticks on the night ride are for the benefit of the humans.
The rods and cones in a horse’s eye project onto ganglion cells which pass information on to the brain. A horse’s high sensitivity to low light and poor visual acuity is partly a result of having a very thin layer of ganglia.4 Also of help is the tapetum lucidum, a pigmented part of the eye which reflects light back onto the retina, allowing for greater absorption when it is dark. It is the tapetum lucidum that makes a horse’s eyes shine at night when you aim a flashlight at them or take a picture with a flash. Animals that are active at night have a tapetum lucidum; humans and other primates don’t.
In humans, there are three types of cones, which detect red, green, or blue. Horses have only two types of cones, which detect blue or green, but not red. The colors seen by horses are the basic plan for most mammals that can see colors. Like the birds and the bees, primates need to know when fruit is ripe, and so have evolved the ability to see red.
Red and green apples in the top row show how they are seen by a human. The same apples in the bottom row show how a horse would see them.5
The vision of the horse should be considered when designing jumps. Horses will find it easiest to see a jump when it is painted in two or more contrasting colors, such as blue and green or black and white, and if the colors contrast with the footing and surrounding landscape.
The eye of the horse does not adapt to sudden changes in lighting as quickly as a human’s eye. So entering a dark trailer or barn on a bright day can be scary. It is a good idea to turn on your interior trailer lights before loading, and turn on the lights over the aisle of the barn before bringing the boys in.
The visual acuity of the horse (20/33)6 is better than a dog (20/50), cat (20/75), or rat (20/300), but not quite as good as a human (20/20), and far worse than a bird of prey (20/4 or 20/5), which can see an ant crawling on the ground from the roof of a 10-story building. 20/33 means that what we can see from 33 feet your horse can only see from 20 feet. A horse must be closer to see the same details.
The human eye has a fovea — a specialized small part of the retina loaded only with cones. Our acuity is highest for images that land on the fovea. Horses don’t have a fovea, but instead have a “visual streak” — a linear area of the retina with a high concentration of ganglion cells. Horses who want to look at something can tilt or turn their head to cause light from some object to fall on the visual streak, improving their acuity. But the acuity of the visual streak is no match for the acuity of our fovea.
Images landing outside our fovea are what we see “out the corner of our eye”, and this peripheral vision is likely what a horse sees of images whose light lands outside their visual streak.
Eye Movement or Head Movement
Horses can move their heads to improve their vision. Looking forward with a raised head, a horse has binocular vision for focusing on distant objects. With a vertical head, the horse has a binocular focus on objects in front of its feet. Horses participating in show jumping will have vertical heads; race horses will carry their heads horizontally. A horse trudging through the woods and surprising a deer will suddenly raise its neck and shift its head to a more horizontal position, to place its binocular vision on the deer. Once he understands what it has startled, he’ll settle and return to his trudging, with a vertical head.
A horse has two ways to get an object of concern into his visual streak: move his head, or move his eye. Ocular muscles attached to the eye allow it to move within the horse’s skull, but it doesn’t roll easily or often: because the horse has no fovea and such a wide field of view, there is rarely any benefit to the horse in moving its eye. You many never see it move, and will have to guess at what it is looking at.
Head movement is far more noticeable to us than movement of the horse’s eye, and may be the more common means of bringing things into focus. But eye movement does occur, and the proof is that we sometimes see the white of a horse’s eye, sometimes don’t. A horse, can not only move its eyes, but it can move them independently, in the same way that it can move its ears independently.
In the human eye, the fovea is packed with cones — no rods — with one cone for each ganglion. The result is high acuity trichromatic color vision, and precision stereo depth perception. But the field of view provided by a stationary fovea is extremely small. So primates have developed eyes that are constantly on the move, jumping around in the socket to take in things of interest. Our sense of this, however, is a construction of reality, rather than what we are actually seeing. By looking from A to B and back, we feel that we are seeing both A and B. But at any given time, our fovea might be admiring C, and the vividness of A and B are recalled, and assembled by the brain to give us a picture of what’s out there.
In the horse looking at the horizon, nearly every point is just as clear as every other point. No mental construction effort is needed. And because points A and B are both actually being seen at the exact same time, any movement of A or B is immediately detected. In a human, we would not see such movement unless our fovea was directed at it.
Neither umwelt is “better”, though the horse could not have survived without his, and we could not have survived without ours.
Like goats, deer, and other grazing animals, horses have horizontal pupils. As the horse lowers its head to graze, or raises its head, the pupil remains parallel to the ground, a trick called cyclovergence. This elongation increases their field of view of the horizon by increasing the light entering from forward and rear, at the expense of seeing areas above. This also improves the quality of the image of the ground surface, making it easier for them to pick their way over rough surfaces.
Sleeping with One Eye Open
Birds and aquatic mammals are the only taxonomic groups known to exhibit unihemispheric slow-wave (deep or non-REM) sleep — sleep on one side of the brain at a time. How can swifts fly non-stop for months7? I believe that this can be accomplished by sleeping on one side of their brain, then the other. When in flight, a bird sleeping on just one side of their brain still has another hemisphere to handle the details of flight, and even though it appears that sleeping birds do a lot of gliding and soaring, flight is tricky business. At a roost, a chicken often engages in unihemispheric sleep, with the open eye generally facing toward the greatest source of danger. For instance, when four chickens are lined up on a perch, those on the ends of the line are likely to direct their open eye away from the group.8
As I write this, I don’t know if horses ever fully close both eyes at the same time when sleeping. All of the horses I’ve seen sleeping seem to have at least one eye partly open, even when dreaming.9 Search YouTube for “horse sleeping” and see if you can find any.
Sleeping with One Eye Open.10
I think it is possible that one side of a horse’s brain can be sleeping, while the other side is awake — that they can exhibit unihemispheric slow-wave sleep. This would be extremely valuable for horses in the wild, who regularly sleep while standing.
This is an excerpt from a recent draft of “Horse Play”, a book I’m writing about how humans can have more fun with horses. The first edition of the book should be completed sometime in 2017, and initially published for the Kindle. I would be grateful for constructive comments on this piece.
1 Soemmerring DW. A comment on the horizontal sections of eyes in man and animals. Anderson SR, Munk O, eds. Schepelern HD, transl. Copenhagen: Bogtrykkeriet Forum; 1971.; Knill LM, Eagleton RD, Harver E (1977). “Physical optics of the equine eye”. Am J Vet Res. 38 (6): 735–737. PMID 879572.
2 Image source: Banks, M., W. Sprague, G. Love, J. Parnell, J. Schmoll. “Horses and Sheep and their Amazing Eye Movements”. University of California Television (UCTV) https://www.youtube.com/watch?v=LOoe2XVaFrU
3 Image source: “Equine vision”. WIkipedia. https://en.wikipedia.org/wiki/Equine_vision
4 Prince, J.H., Diesem, C.D., Eglitis, I., Ruskell, G.L. 1960. Anatomy and Histology of the Eye and Orbit in Domestic Animals. Charles C. Thomas. Springfield, IL, pp 130-139.
5 Photo source: “Equine vision” in Wikipedia. https://en.wikipedia.org/wiki/Equine_vision
6 Sources differ on the exact acuity. Some suggest it is as poor as 20/60, while others suggest it is as good as 20/30.
7 Lockley, R. M. (1969). Non-stop flight and migration in the common swift Apus apus. Ostrich, 40(S1), 265-269.; Liechti, F., Witvliet, W., Weber, R., & Bächler, E. (2013). First evidence of a 200-day non-stop flight in a bird. Nature Communications, 4.
8 Rattenborg, N. C., Lima, S. L., & Amlaner, C. J. (1999). Facultative control of avian unihemispheric sleep under the risk of predation. Behavioural brain research, 105(2), 163-172.
9 For example, see “My horse running in her sleep” https://www.youtube.com/watch?v=hymzSlzmvGE
10 Photo source: “The Cutetest Loud Snoring Horses ever” https://www.youtube.com/watch?v=w90nF1x2Rko