A myth exploded – Until very recently, all vets and vision scientists knew the horse had a “ramp retina”. It was unique to the horse. This meant that the distance from the lens in the eye to the back of the eye varied so that the top part of the retina was further away from the lens than the bottom part. It had been “discovered” at the beginning of the 20th century and it was thought that the ramp retina accounted for the changes in head position we see in horses. For example, when looking at far away objects a horse lifts the head up high and when looking at near objects, such as a hand holding a carrot or a small object rolling on the floor they might tilt the head sideways. The horse was thought to be finding a distance from the lens to the retina suitable for focusing on things that were different distances away. I noticed that even though many other species of mammals have poor focussing ability, they do not have a ramp retina, so I decided to have a look for myself and found that the horse does not have a ramp retina at all. Their eyes are, in fact, not all that different to those of all other species.
What horses see – “Visual acuity” means the ability to see fine detail. We have found that a horse has a visual acuity about half a good as ours so they can only see the detail we can see if it is twice as close to them as to us. The whole eye does not have good acuity, just a small part does. In humans, there is a really small round part right at the back of the eye, which has good acuity. It is not like this for a horse. A horse has a long thin region running right across the middle of the eye from front to back, a “streak” of good acuity. This means that the horse has a sort of narrow panoramic view of the world with each eye, quite unlike us. We cannot really imagine what it is like to see like a horse.
If you look straight ahead out of both eyes at once, you are looking in a binocular way (binocular means “both eyes”) and the part of the world you see this way is called the binocular field. Binocular vision is useful as it helps with perception of depth. We have a huge binocular field, which means that we see most of the world with both eyes at once. Only a small part of the world is seen by us with one eye only, right at the far sides, these tiny areas of the visual field are called the monocular (one-eyed) regions. However, horses have much bigger monocular fields than we do and a much smaller binocular field. This is due to the fact that their eyes are on the sides of their heads. Horses can see almost back to their rumps. Their monocular fields are used to look out to the side and watch for predators. The horse’s binocular field is pointing down the nose and not straight ahead like us. Down the nose is rather useful for the horse as it then has a good view of the ground it approaches with its nose when it wants to eat (which, lets face it, is most of the time!). Not only is there no binocular overlap directly in front of the animal’s head, but a blind area exists in front of the forehead. Remember you have been told not to approach a grazing horse from directly in front? That’s why.
The horse, then, has to raise its nose up in the air to see directly in front or, more specifically, to use its binocular vision. As you will all know, horses respond to something unusual by turning towards it, lifting the head high to use the binocular field located down the nose and freezing. When the horse adopts this head orientation and freezing, it is highly distracted and very likely to attempt to turn around and run off in the next few seconds. This is therefore a head orientation disliked and avoided by riders. It is important also to remember that the horse is blind directly in front of its head such as when the nose is lowered during what some people believe is riding “on the bit”. A horse that is trained to perform dressage movements is balanced to cope with the added weight of the rider in such a way that the back and neck are, to some extent, arched and the nose should approach the vertical. However, the horse cannot see directly in front if the head is overbent and behind the vertical and so must rely on the rider. Therefore, a fair degree of submission is requested by the rider and given by the horse to achieve this balance. Perhaps this is why there is such a lot of overbending seen in dressage competition these days, a very overbent horse cannot see where it is going and therefore probably feels much safer!
Alison Harman, BSc PhD