The information out there pertaining to how horses see the world around them has been ripe with confusion and misinformation. As new research flows in we can adjust our perspectives and increase our knowledge. Knowledge of how a horse views its world can greatly impact our approach to training because it allows us to understand why a horse does what it does and it allows us to anticipate certain behaviours and adjust accordingly.
Click here for an article concerning a horse's vision as it pertains to orientation and recognition of objects. Essentially, while a horse will likely immediately recognize an object from a new viewpoint, he might find some rotations to be more difficult to recognize than others. Hanggi's research found that horses performed best when objects were upright instead of upside down. Allowing a horse to stand relaxedly and observe its environment might be of benefit because it can allow a horse to observe its surroundings and familiarize itself with certain objects that might otherwise have caused him to spook.
Another article by Evelyn B. Hanggi and her research team that proves our horses have better nightvision than previously thought. I can recall being specifically instructed by my Pony Club instructors years ago to not ride at night because my horse would be unable to see well. There have been countless times since then however where I have had to ride my horses at night (returning home after working cattle, etc etc) and my horses have seemingly navigated with ease. Research seems to prove however that they could actually see better than I could in the dark!
As most horsemen know, the horse has two blind spots - an approximate 3 degree area directly behind the horse and another area just in front of and under the horse's nose. Each of the horse's eyes has a horizontal visual field of approximately 190 degrees and a vertical visual field of about 180 degrees. Interocular transfer of learning between both eyes is possible via the horse's corpus callosum. There seems to be conflicting opinion and research results however in regards to how effective and how developed a horse's corpus callosum is. Rule of thumb: do things on both sides of the horse at all times. For one, a horse could still become accustomed to things being done on only one side (ie, being saddled and mounted only on his left side) and later be alarmed when the human tries to do something on the opposite side. The rider needs to consider that a variety of factors may influence a horse seeming spookier touring the arena on one rein versus the other rein, from the way a horse's vision works to potential behavioural factors.
It has been believed for the longest time that horses have a ramped retina whereby they must raise or lower their head to focus objects at certain distances. Instead, it has been determined that the horse does have the limited ability to adjust the thickness of the lens but that he will raise or lower or swing his head for better visual acuity. For example, he will raise his head to focus better on distant objects because binocular overlap is oriented down the nose - here. The central retina of the horse also is believed to have a higher concentration of photoreceptors and thus the horse will try to focus objects specifically on this part of the retina. Objects viewed outside this area might even have the ability to take a horse by surprise.
The typical horse has good visual acuity though not as good as a human's - approximately 20/30 versus a human's 20/20. Though the horse's eye is much larger, the the receptors (ie, cones and rods) in a horse's eye are not as numerous and are less densely packed. Page 5 of Hanggi's article The Thinking Horse discusses this.
Approximately 146 degrees on either side of the horse is monocular vision and approximately 65 degrees of his vision directly in front of him is binocular vision. This binocular vision is what allows your horse depth perception - however, only within that specific field of course. A horse's depth perception is likely not quite as accurate as that of a human's.
Horses have only two types of cones and therefore their vision is likely similar to that of a red-green colour-deficient human. This means they can likely discriminate red or blue from grey but have greater difficulty differentiating between yellows and greens. This applies to training and riding the horse because certain objects might not be immediately distinguishable to the horse. If an object that seemingly blends in with the background suddenly moves, it is understandable if the horse then spooks. There seems to be some differences in opinion and findings however here is a 2007 study evaluating colour discrimination in horses.
It is crucial a rider understand that a horse perceives his world much different than how a human perceives his world. This is due to a difference in input from visual input to olfactory (smell) and auditory (hearing). As a prey animal they also perceive their world differently from a behavioural standpoint and respond differently than we might immediately understand. The key to training and riding a horse is understanding that horse as a horse and as an individual. In such a manner a rider can better adjust their approach to be effective and so as to work harmoniously with the horse.
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