Understanding Equine Vision

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Originally published on TheHorse.com

Editor's Note: This article is part of TheHorse.com's ongoing coverage of topics presented at the 2012 American Association of Equine Practitioners' Focus on Ophthalmology conference, held Sept. 6-8 in Raleigh, N.C.

To a rider, a deer walking across a field might seem like a scenic bonus, but the horse he or she is riding might see a predator when viewing the same animal. Why? Because horses have very different ocular function than do humans.

At the 2012 American Association of Equine Practitioners' Focus on Ophthalmology conference, held Sept. 6-8 in Raleigh, N.C., Andrew Matthews, BVM&S, PhD, Dipl. ECEIM, ACVO (Hon.), FRCVS, an equine practitioner and ophthalmologist from Ayrshire, Scotland, gave an overview on what we know about equine vision.

"Although there is much information as to how the eye functions as an optical instrument, the function of the higher visual pathways and optic cortex--in particular the way the assimilate and integrate nonocular cues to produce the sensory interaction with the animal's environment we call 'vision'--is wholly unknown," Matthews said.

Evolutionary Roots

Horses, Matthews explained, evolved as an open-country prey species with an arrhythmically photic lifestyle (meaning they are active during both the day and night). Thus, he relayed, their eyes are designed to give them every possible advantage in staying safe from predators in those environments:

• The horse's eye position in the skull allows for a large, nearly 360° field of vision--approximately 146° of monocular vision on either side and about 65° to 80° of binocular vision ahead of them; the horse's two blind spots are directly behind the tail and directly in front of their interocular space;
• Horses pupils are oblique in shape and positioned horizontally within the eye, which optimizes the animals' ability to scan horizons;
• The structure of equine retinas appears to be designed to detect motion (or predators, for example) in low lighting conditions;
• Additionally, the horse's large globes and pupilary structure allow him to capture the maximum amount of light possible in low-lighting conditions; and
• A structure within the eye called the granulae iridica appears to serve as a "natural visor" in bright light; the structure also probably helps improve the horse's limited depth perception, Matthews said.

What we Know and What we Don't

Researchers have made great strides in understanding how the horse's eye functions.

For example, researchers have studied the accuracy at which horses see, and believe it to be fairly good. "Visual acuity in horse has been variably estimated to lie between 20/30 and 20/60," Matthews said, noting that horses' visual acuity is believed to improve if objects are elevated up to 70 centimeters (roughly 2.3 feet) off the ground

Additionally, color perception studies have shown that horses have dichromatic vision with a reduced cone density, meaning they're able to see washed-out versions of colors including green, yellow, blue, and gray. It does not appear that horses can see reds, Matthews noted.

Several aspects of equine vision, however, remain a mystery to even the foremost equine ophthalmologists and researchers. For example, "there is at present no means of objectively assessing vision or visual disability in the horse that is not overtly blind," Matthews said.

While some tests are available to test vision, they all carry with them drawbacks for evaluating equine visual quality. For example, he explained that the dazzle response test tells whether or not the horse has a functional retina and, therefore, the potential for sight; however it can't provide additional information about how well or poorly a horse can see. Likewise, blindfold tests can often pinpoint completely blind horses; however it doesn't take into the horses' other sensory cues that could help or hinder the results of the test, he noted.

And of course, he said, the fact that "horses with severe and extensive eye disease often show no apparent difficulty in 'seeing' remains one of life's mysteries."

Take-Home Message

Although researchers have learned much about how the horse's eye functions, exactly how horses see remains a mystery. A basic understanding of equine vision can help horse owners better understand why their animals act the way they do.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.

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