Vision is a complex process that involves not only the eye focusing and detecting light that may vary in intensity, but also the brain interpreting what the eye is sensing. Since the horse's food is essentially immobile and not particularly challenging to identify or capture, much of the horse's visual adaptations as a prey animal appear to be defensive and devoted to ensuring the a horse’s survival by more easily identifying potential threats.

Light passes through the eye and lands on the back layer called the retina. The retina contains rods and cones that are the cells which detect light. Rods sense light in black and white, cones sense colors. The ratio in horses and humans of rods to cones is approximately 20:1. However, in horses the distribution of cones is different. In humans, cones are packed together much more centrally in the retina. In horses they are much more widely distributed over the retina but also clustered near a “visual streak” which runs horizontally side to side in the retina. For horses who tend to live on open plains, this streak pattern conforms to the line as earth and sky meet, enhancing their ability to scan the horizon for any movement.

In low light and in the dark most of the vision is done by rods. As the midday sun floods the back of the eye, the highly sensitive rods are overwhelmed and the cones pick up the primary role in vision.

Horses have a number of adaptations to aid vision in both dim light and bright light. Their eyes are among the largest in domestic animals. They can dilate their pupil 6 times larger than humans. Their horizontal slit like pupil can dilate into a nearly round configuration at night. Behind the retina is a reflective layer called the tapetum. The ability to reflect the light gives the receptors a second chance to capture the light. Just shine a light at a horse at night and the reflection you see is the light bouncing of the tapetum. People do not have this.

At the same time the horse has adaptations to deal with bright light as well. The slit like pupil can be closed tight to fend off the light. On the top of the pupil you will see what looks like a small piece of tissue hanging down. This is called the corpora nigrans. It is thought that this acts as an internal visor blocking glare and thus enhancing daytime vision. In a manner similar to sun glasses parts of the lens contains yellow pigment which filters out bright light

Other adaptations reflect the need for a wide field of view and ability to maintain vision while in motion. Horses need to be vigilant for threats all around them. To aid them their eyes have a evolved to be widely set. They can see in both a monocular way (with one eye) and binocular way (with two eyes). Their widely set eyes can see 190 degrees in the horizontal direction and 170 degrees in the vertical direction. This gives them a range of over 350 degrees with small blind spots that can be overcome by shifting the head slightly. The degree of binocular (two-eyed vision) overlap is 55-65 degrees. It rivals or exceeds that of dogs and horses use both monocular and binocular clues to estimate depth, although the binocular depth perception is 5 times better than for one eye.

As the horse moves it needs to maintain its sight and vigilance for any threat. To keep its vision from bouncing you can watch as the eyes, head and neck go through a coordinated rhythmical motion to stabilize the eyes while a horse trots and canters. Horses can’t wipe their eyes but must keep them free from debris. So beyond simply blinking their eyelids, horses have a third eyelid that sweeps across the cornea removing any debris as needed.

At 20:30 to 20:60 the horse's visual acuity, the ability to see details of an object, is among the best of the domestic mammals and better than that of many people. From 6 feet away horses can detect small differences in depth, which approximates the ability of a cat.

Their color vision is another story. Cones are cells that sense colors. Horses have dichromatic color vision with two cone types: blue sensitive cones, and yellow sensitive cones. Humans in contrast have three types of cones blue, green, and red. Thus horses are similar in many ways to men who are color blind having trouble distinguishing between red and green. In contrast horses have a less accurate perception of other colors. Blue and orange may be indistinguishable to a horse. . It is probable that their color sense results in colors appearing as washed-out pastels. Because many predators evolved coat colors which closely match the background in terms of color and visual texture, color may be a relatively poor way for the horse to "break the camouflage" of a predator. Instead, objects can be differentiated from their surroundings if they are suitably different in any one of five different aspects: brightness, motion, texture, depth, and color.

Clinically it is difficult to recognize anything but the most massive visual impediments in horses. This is not because the horse is unusually resistant to disorders which impair vision in other species, but because of the crudeness of the tools veterinarians currently have available to determine how well a particular horse can see.

New treatments available for horse owners:

For clients interested in keeping a mare out of heat an extended release formulation of Altrenogest – the active ingredient in Regumate - is now available. This formulation has been shown to be more reliable than other forms of injectable heat suppression and avoids the need for daily administration of medication.

Snake bites can be devastating and costly. I have treated two this year and many cases over the years. Now a vaccine offering protection received conditional approval. Owners of horses in the Saratoga hills, East San Jose and Milpitas foothills and south Santa Clara County areas in particular should consider vaccination.
 

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