1. The horse's field of vision
To better understand a horse's vision, let's first take a look at the particularities of its eye. The horse's eye is the largest of all land mammals. It is located very laterally, on the sides of the head, and its pupil is horizontal.
All this contributes to a panoramic field of vision, extremely wide in a horizontal plane. Without moving his head, the horse can see 340 degrees around him at the same time. His blind spots are behind his rump and under his nose.
By comparison, our forward-facing eyes give us a field of vision of only about 180 degrees. What's more, our pupils are round, giving us a field of vision as high as it is wide.
On the other hand, a horse's vision is not very extensive in a vertical plane, and it sees poorly above its head, so its field of vision is all the more restricted the lower the head. Nor can he see directly under his nose. That's why he sometimes has trouble grasping the treats you hand him, looking for them blindly with his vibrissae.
The horse's field of vision in the horizontal and vertical planes(source : FFE)
The line drawn here at eye level determines the horse's very limited vertical field of vision.
2. Visual quality of the horse
This very wide field of vision enables it to scan a large area at all times, and detect the slightest movement. On the other hand, monocular vision is not very precise, and cannot distinguish relief. This corresponds to the areas seen by a single eye, i.e. on the sides corresponding to most of the field of vision. The binocular vision zone is restricted, especially if the horse lowers its head, but it is the only zone where the horse can see precisely and in relief, enabling it to grasp distances, for example.
The horse is therefore easily surprised by elements located to the side, which are not perceived by the rider. In fact, it can spot moving elements a long way off without being able to estimate their distance, which can lead to disproportionate reactions.
3. Colors seen by the horse
The processing of light by the eye, and therefore color perception or vision in the dark, is carried out by retinal cells called cones and rods.
The cones enable color vision. For human perception, there are 3 types of cones, enabling red, blue and green to be detected. Horses don't have any "red" cones, so their color perception is attenuated. This so-called dichromatic vision results in somewhat sepia-toned images. Greens and reds are not very distinct.
Here is the panel of colors perceived by man compared to that of the horse:
Comparison of human (above) and horse (below) vision of the same landscape. The differences in visual field, sharpness and color are clearly visible.(source: Haras de la Cense)
Color distinguishes elements from their background. Unlike its rider, it's difficult for a horse to distinguish a stationary red or yellow object from its generally green background. However, if he's moving, he'll notice it from a distance, usually ahead of the human, sometimes reacting suddenly.
Horses, on the other hand, have more rod cells than humans, which enable them to see in the dark. What's more, the retina has a reflective zone, clearly visible in a fundus examination, which increases light intensity.
Horses can therefore see better than humans in the dark. However, this doesn't prevent them from fearing the dark and becoming easily frightened in the twilight.
What's more, horses adapt very poorly to sudden changes in light conditions (going in and out of a stable, in the forest, in the van...).
Video simulation of equine vision compared to human vision:
https://www.facebook.com/Haras.LaCense.Fanpage/videos/1135019499918691
4. Changes in vision due to ophthalmological problems in horses
We always fear the consequences of eye diseases in horses, but what impact do they really have on vision?
Here are a few examples to help you understand your horse's difficulties:
Corneal ulcer scars: presence of an opaque spot in the field of vision, like a spot on glasses.
Synechia (after-effects of uveitis) : severely restricted vision in dim light, as the pupil cannot dilate.
Cataract: a very blurred area extending over a large part of the field of vision, opacifying and expanding as it progresses.
One-eyed horses: field of vision impaired, but above all, no longer able to perceive relief. Greater difficulty in estimating distances.
In addition to changes or loss of vision, eye pathologies often cause intense pain.
Pauline Cantet, equine veterinarian.
