Horse vision: field of vision and visual acuity—what do we know?

1. Field of vision of a horse

To better understand how horses see, let's first take a look at the unique features of their eyes. Horses have the largest eyes of any land mammal. Their eyes are positioned very laterally, on the sides of their heads, and their pupils are horizontal.

All of this contributes to giving it a panoramic field of vision that is extremely wide horizontally. In fact, horses can see 340 degrees around them at once without moving their heads. Their blind spots are behind their hindquarters and under their noses.

By comparison, our eyes, located toward the front of our heads, give us a field of vision of only about 180 degrees. In addition, our pupils are round, which gives us a field of vision that is as high as it is wide.

 However, horses have limited vertical vision and cannot see well above their heads, so their field of vision is even more restricted when their heads are lowered. They also cannot see directly under their noses. This is why they sometimes have trouble grabbing treats you hold out to them, searching for them blindly with their whiskers.

Field of vision of a horse in the horizontal plane and in the vertical plane(source: FFE)

The line drawn here at the horse's eye determines its very limited vertical field of vision.

2. Visual quality of the horse

This very wide field of vision allows it to scan a large area at any given moment and detect the slightest movement. However, its monocular vision is fairly inaccurate and cannot distinguish depth. This corresponds to the areas seen by a single eye, i.e., the sides corresponding to most of the field of vision. The binocular field of vision is limited, especially if the horse lowers its head, but it is the only area where the horse can see precisely and in depth, allowing it to judge distances, for example.

Horses are therefore easily startled by objects on either side that cannot be seen by the rider. They can spot moving objects from a great distance but are unable to judge how far away they are, which can cause them to react disproportionately.

3. Colors seen by horses

The eye processes light, and therefore perceives colors and sees in the dark, through cells in the retina called cones and rods.

The cones enable color vision. For human perception, there are three types of cones, enabling the detection of reds, blues, and greens. Horses do not have "red" cones, which gives them a diminished perception of colors. This type of vision, called dichromatic vision, produces images that are slightly sepia-toned. Greens and reds are not very distinct.

Here is the range of colors perceived by humans compared to that of horses:

Comparison of the view of the same landscape by a human (top) and a horse (bottom). The differences in field of vision, sharpness, and color are clearly visible.(source: Haras de la Cense)

Color allows elements to be distinguished from their background. Unlike its rider, it is difficult for a horse to distinguish a stationary red or yellow object from the generally green background. However, it will notice it from a great distance if it is moving, usually before humans do, sometimes reacting violently.

Horses, on the other hand, have more rod cells than humans, which enable them to see in the dark. In addition, the retina has a reflective area, clearly visible during a fundus examination, which increases light intensity.

Horses therefore see better than humans in the dark. However, this does not prevent them from fearing the dark and being easily frightened in dim light.

In addition, horses have difficulty adjusting to sudden changes in light (entering/leaving a stable, in the forest, in a trailer, etc.).

Video simulating equine vision compared to human vision:

https://www.facebook.com/Haras.LaCense.Fanpage/videos/1135019499918691

4. Changes in vision during 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 better understand your horse's difficulties:

Corneal ulcer scars: presence of an opaque spot in the field of vision, like a smudge on glasses.

Synechiae (sequelae of uveitis): severely restricted vision when light levels decrease because the pupil cannot dilate.

Cataract: a very blurred area covering a large part of the field of vision, becoming increasingly opaque and spreading as it progresses.

One-eyed horses: impaired field of vision, but above all, loss of depth perception. Greater difficulty in estimating distances.

Apart from changes in or loss of vision, eye diseases often cause intense pain.

Pauline Cantet, equine veterinarian.