How wide is the horse's visual field?
- The shape and location of the eyes means the horse can see almost 360 degrees around him although some places are barred from the horse's vision, including the area directly in front and behind him and anything above eye level. As a general rule, predators (including us humans) have binocular vision, or eyes situated on the front of our head. Prey animals have monocular vision, or eyes located on either side of their head. This monocular vision is essentially a safety mechanism enabling them to watch out for danger while they eat. The horse cannot use binocular and monocular vision at the same time. While he makes the switch between the two, objects may appear to 'jump' into view and distort until he focuses once again.
Think: What might a horse think when an object suddenly appears from his blind spot? How does the horse's visual range affect your horse's vision when you ride and what incidents might it explain?
Is the horse long-sighted or short-sighted?
- Due to the nature by which horses must bring objects into focus, they cannot easily focus on anything closer than 1 metre, making the horse a long-sighted animal. In the wild, horses would not to see things up close as the main function of their eyes was to look out for approaching predators.
How do horses bring objects into focus?
- When looking at an image, light passes through the pupil and lands on the lens. The lens then focuses the image onto the retina. The human lens can be altered by powerful ciliary muscles, through the process of 'accommodation', to help bring objects at varying distances into focus. This also takes place in the horse's eye, just not as accurately. (It was originally thought that the horse's lens could not alter in shape because of a 'ramped retina', which would have given reason for the horse's moving his head to bring objects into focus, but the reason has now been found to be the 'visual streak'.)
Horses have a 'visual streak' or linear-shaped area within the retina which has a high concentration of ganglion cells (up to 6,100 cells/mm² in the visual streak compared to 150 and 200 cells/mm² in the peripheral area). Horses have much better acuity when the image of the object they are looking at falls into this region and they often raise, lower or tilt their head or move closer or farther away, until it does.
How accurate is the horse's vision?
- It is generally thought that horses have a visual acuity of about 20/33 as opposed to the ideal human 20/20 vision. This means that while a horse may see something clearly from twenty feet away a person would see it as clearly from thirty-five feet away.
How well can horses sense motion?
- It is also thought that horses can generally sense motion more acutely than humans. In the wild, motion is a common predator alert. Such motion is detected in their periphery, where there is little visual accuracy, and they must raise or lower their head to bring the respective object into clear focus (into the 'visual streak').
Think: How does the horse's instinct to run from danger tie in with his visual acuity? Could this explain why horses raise their heads in alarm or sometimes just run before bringing anything into focus? Could a trusting relationship help to override instinctive, fearful reactions?
What is eyesight's part in having to train horses the same thing from both sides?
- All good horse people know that when you teach the horse anything, you must teach him 'from both sides'. For instance, you may train him to lead perfectly from the left, but when you then try to lead him from the right, act confused and walk wonky like you hadn't taught him a thing. What a horse sees in his right eye is processed in the left hemisphere of the brain and vice versa. The information gathered through one eye is not processed efficiently by both sides of the brain. This is because, unlike humans, the horse does not have an efficient corpus callosum, which is the connective tissue that transfers information between the two hemispheres of the brain.
Can horses see in the dark? - While horses don't see colour as well as humans, they have more rod cells than we do. Rod cells are light sensitive and the more the eye has, the higher the accuracy in distinguishinging light and dark differences. Horses also have a tapetum lucidum which reflects visible light back onto the retina, allowing for greater light absorption in dark conditions. These factors result in much better night vision in horses than people.
How well do horses adjust to changes in the light?
The amount of light the eye allows in is affected by the pupil which contracts and dilates appropriately and the corpora nigra, or black, cloud-like protrusions located near the pupil. Because of the extent to which horse's eyes can adjust to darkness and pick up images due to their light sensitivity, they can also take a longer period of time to adjust back to a different light, especially a sudden or acute change. It is important to understand that horses' eyes take longer than ours to adjust to light changes and also that something that looks somewhat shaded to us (e.g. inside of a trailer) may look pitch-black to horses (who also happen to be claustrophobic by nature).
Think: How long do you give you horse to adjust his eyesight? How might illuminating the inside of a trailer help when loading?
DID YOU KNOW?
Horses have a third eyelid (nictating membrane) situated in the corner of the eye. It is often unseen because it is usually the same colour as the iris, but in some horses it is white and much more obvious. In fact, some people do not buy horses that have white nictating membrane because they believe the horse to be temperamental. Similarly, some horses' eyes are 'too small for their sockets', making the surrounding white sclera visible and this can also look like the horse is frightened because he is showing 'the whites of his eyes'.
