There is a tremendous amount of information available about horses performance, abilities and behaviors. We also understand a lot about breeding, lineage, diseases and temperament. But little information is available on the senses of horses. Just what does a horse see? What can he hear or taste? Do certain smells affect him?
Recent studies have revealed some information about sight and hearing. In comparison to other mammals, the horse has one of the biggest eyes of any land animal. Horses have a large retina and they are capable of magnifying images twice as much as people. Despite this ability, microscopic examination of the retina has led researchers to believe that horses do not have the best vision. There is, however, one narrow band of dense and organized cells that give the horse at least a narrow horizontal band of good vision. Beyond that band, things can be a little blurry.
Vision is separated into ability to focus, depth perception, field of vision, night vision, color vision and the ability to tell two objects apart when placed closely together (acuity). Another aspect is the ability for the brain to understand and remember what only one eye sees.
Ability to Focus
It was previously thought that horses were unable to focus on objects with a level head. The one narrow band was the only part of their visual field that was in focus. In order to focus on large, near items, the horse would move his head up and down so that the object would enter his narrow focused visual field, thus allowing the horse to "see" the object. More recent research has changed this opinion. It is now thought that horses do have some ability to focus objects outside of the horizontal streak and moving the head up and down does not help with vision.
The ability to perceive distance is very important. Horses seem to have an excellent depth perception and can detect height as well as distance. Horses are also able to tell two objects apart that are a long distance away but their abilities are not as good as humans. In far away objects, we may be able to tell that there are two items standing side by side, but a horse may see it as one large object. In comparison to other species, horses and cats are quite similar in their depth perception abilities.
Field of View
Horses have large eyes on the side of their head. This leads to a wide field of view and increased peripheral vision when compared to people. The field of view for horses can reach up to 200 degrees. Even though horses can see what is going on beside them and behind them, their binocular vision right in front of them is somewhat limited. There is a blind spot just below the nose and just above the head. When holding their head level, horses also have a blind spot directly to their rear.
Horses have limited night vision but it takes a while for them to become accustomed to lower light levels. Their night vision is nowhere near as good as cats and dogs. The rod to cone ratio in the horse is 9:1 which allows for better night vision than species with a higher ratio of cones. More research is needed in this area to determine exactly how much vision horses have at low light levels.
Based on an evaluation of the cells in their retinas, as well as behavior tests, it is felt by researchers that horses have the ability to distinguish red or blue from gray easily. Their ability to differentiate yellow from green is not as good. Beyond this, testing for color vision is difficult. Also, there is the problem of knowing whether or not animals detect color the same way people do.
Visual acuity is the ability to detect the presence of two separate things. It is also considered clarity of vision. For example, black and white stripes are commonly used. When there are wide black and white stripes, each stripe can be identified. As the stripes become thinner and thinner, eventually they get so thin that it appears as a gray object instead of separate black and white lines. The point at which the lines can no longer be differentiated is the visual acuity. Compared with other animals, horses do not have good visual acuity.
Transfer of Visual Information
One other concern regarding the vision of horses is the ability of one eye to transfer information to the other eye. If you cover one of your eyes and look at an object with just one eye, you will still recognize that object when the previously covered eye looks at the object. Some horse owners believe that objects that are only seen by one eye are not understood by the other eye. For example, if a horse is walking along a trail and sees something with his right eye that spooks him, when he walks back down the trail later and sees the object with his left eye, he is again spooked and acts as though he has never seen the object before. This would indicate that the brain of the horse does not allow for the passage of information from one eye to the other. Studies have shown that this is not true. Horses do have the ability to transfer information from one eye to the other.