The Senses: Vision Copy
There are significant differences between horses and humans in respect to vision. Humans (predators) have eyes designed to focus in front of us, on our prey. Horses (prey) developed a wider field of vision to watch out for potential predators approaching from any direction.
While horses have an impressive 350-degree field of vision, including their less focused peripheral vision, there are some blind spots, namely in front of their nose and directly behind them. However, within that wide span the actual acuity of the vision varies. Along the horizon the horse has a span of about 190° with good focus, while the vertical field of vision is about 175°.
Furthermore, the two halves of the horse’s brain are not connected in the same manner as a human. Therefore, each eye registers information independently. As a result, a horse may become familiar with an object viewed from one side but act as if it is being seen for the “first time” from the opposite side.
The term for this left-eye/right-eye independence is monocular vision. This can help a horse scan a much greater range of its surroundings for predators while grazing or resting. The horse is primarily monocular except for a small, but somewhat adjustable area in front where the two fields-of-vision overlap, creating an area of binocular vision which is necessary for depth perception. In order to adjust the size of this area and better judge distances of obstacles or objects of curiosity or fear, the horse must be able to raise and lower its head. Restricting their ability to do so can greatly increase their anxiety and decrease their ability to navigate uneven terrain or clear jumps successfully. You may observe horses utilizing their binocular range of vision when they intently study something unfamiliar, turning their heads fully toward the object of interest.
Furthermore, the lens of the horse’s eye does not change shape for the purpose of focusing, whether for monocular or binocular vision, in the same fashion as the human eye. The horse depends on the mobility of his head and neck to create a visual focus within the sharpest area of his vision. The horse will generally raise its head in order to focus, while a lower head position, as when grazing, allows the full range of peripheral “soft focus” vision. He may also lower and tilt the head to focus on an object that is closer to him. Again, it is easy to understand why horses will commonly act restless if the movement of their head is too restricted.
The question of how the horse sees color and which colors horses see is still a matter of study, with disagreement between several camps. What is certain is that the horse does not possess the same anatomical “cone cells” that register color in other mammals, namely humans. If horses do see color it is by another as yet undetermined physiological process. There does seem to be observable reaction to shades and many horse people provide anecdotal evidence of color preferences or dislikes.
For additional information on equine vision, you may refer to an informative article in Equus magazine, issue 268, or read Understanding the Equine Eye by Michael Ball.