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The Ramp Retina - What is it and does it exist
Ref: AM Harman, S Moore, R Hoskins and P Keller

It is commonly observed that when a horse looks at something in the distance it raises its head and lifts its nose and when observing much closer objects the horse may arch the neck and rotate the head on one side. In the past it has been explained that the horse needs to make up for an inability to focus on objects when viewing at different distances (accommodation) by using a ‘ramp retina’. This concept was used to denote that there is a variable distance from the lens to the retina (the deepest part of the eye) in the eye of the horse. It was assumed that this retinal feature accounted for the alterations in head position exhibited by horses. The horse was presumed to be changing the angle of the head in order to find a visual axis along the ‘ramp retina’ with a distance from the lens to retina appropriate for focussing on the image.
Researchers from Western Australia have conducted a study to examine the eye of the horse to confirm or deny the presence of a ‘ramp retina’. Other aims of the study were to discover the anatomical features that might account for the head moving behaviour, to confirm whether horses have limited accommodation as well as to estimate the visual field of the horse.

Results from this study show that the horse does not have a ‘ramp retina’. It was found that the horse has a retina that is the same distance from the lens in all areas except in the peripheral regions where the retina is closer to the lens. This is opposite to previous theories of the ‘ramp retina’. The study also found that the horse can see a very narrow but panoramic view clearly, and this area of vision is perceived by a retinal region called the visual streak. The horse’s peripheral vision is poor; a ‘ramp retina’ would be of little benefit to the horse, as it would give the horse no advantage to use any part of the retina other than the visual streak for direct observation.

Results from the visual field tests show that the binocular overlap (areas where the horse is viewing objects with both eyes at once) is down the nose and not straight-ahead. In fact the horse has a blind spot straight in front of the forehead. Therefore the horse has to raise its nose considerably to see directly in front. When the horse lowers its head, the binocular field is directed towards the ground for grazing and the monocular fields are in position to scan the lateral horizon. When the head is raised with the nose pointing forward, the horse uses the binocular field (both eyes) to scan the horizon and the monocular (single eye) lateral vision becomes limited. The monocular fields are used during calm moments of grazing when the head is in the position that the nose approaches the vertical and it is this head orientation that many riders attempt to obtain from their mounts when their horse is ‘on the bit’ and which results in relaxation and submission from the horse. When the horse is distracted by a sound or sees something in its peripheral vision, it will turn its head or whole body towards the object and raise its head. The researchers believe the head is raised so the horse can utilise its binocular vision to see something in the distance and not to alter the focussing power of the eye as suggested by the concept of the ‘ramp retina’. When the horse is looking at something close, for example a treat held in the hand, the researchers believe that the horse arches its neck and rotates its head in order to utilise the binocular vision down the nose.

Another important finding is that the researchers believe the blind spot in front of the horse is actually the width of the horse and when the horse is ‘on the bit’ it cannot see anything directly in front of it and must rely on the rider. Therefore a fair degree of submission is requested by the rider and given by the horse to achieve this position. If a showjumping horse is to see and judge the distance of a fence that it approaches, it must be allowed the freedom to raise its head and direct its binocular field forward.

Click here for photos that try to represent the difference between equine and human vision.
Photo Information
The view seen by the left eye of a human and
a horse facing the same direction, looking over
the city of Perth (Australia). (A) is the front view
of the city and (B) is the rear view behind the horse
and the human. (C) is the view seen by the human,
a small high acuity central region surrounded by a
lower acuity area and (D) is the view seen by the
horse, running from the city almost to the rear as a
horizontal strip with much lower acuity above and below.
Photo courtesy of the Equine Veterinary Journal.

Click here for the transcript of a report featured on ABC television in Australia.

Ref: AM Harman, S Moore, R Hoskins and P Keller Horse Vision and an Explanation for the Visual Behaviour Originally explained by the ‘Ramp Retina’ (1999) Equine Veterinary Journal Vol. 31, No. 5, Pg. 384.