The horse has one of the biggest eyes of any land animal.
Some information about horse vision:
 There is a blind spot directly behind the horse's hindquarters.
 The horse can't see directly below its head.
 The horse has to lower its head to see faraway objects.
 The horse has to raise its head to see close objects.
 If an object is closer than a few feet, the horse can't see it with its binocular vision.
 It takes time for a horse to adjust its eyesight going from a dark area to a light area or vice versa.
 The horse needs to use binocular vision for depth perception and distances.
 The horse vacillates between use of monocular and binocular vision.
Horses, with their eyes being located on opposites sides of their head, are mostly "monocular".
Their "binocular" vision is limited to an area in front of them, in a line down their nose.
Monocular vision is used to view separate things with each eye at the same
time, to the side and rear of the horse. Binocular
vision is frontal vision, using both eyes to look at an object.
Horses have a blind spot behind them and directly in front of them. As they move
forward, they are unable to see within a few feet. It is felt that they "memorize"
as they go forward, to know where to place their feet. The horse cannot see
approximately two to four feet in front of its face due to the placement
of the eyes on the head. Therefore, the horse cannot see the food it eats.
To judge distances, a horse uses its binocular vision. The horse must have use
of his head to be able to use his vision to his best advantage.
From a site about horse vision: "So if a horse needs to look down
his nose to see where he is going, what happens when he is 'on the bit'
as in showing or dressage? A horse who is flexed
at the poll will have his head vertical (at right angles)
to the ground and cannot see straight in front of him,
only down his nose towards the ground.
Recent research found this blind spot in
front of the horse is about the width of his body and a horse 'on the bit'
must rely on the rider for direction as he is almost working blind!"
A similar situation exists with the "star gazers"; they are unable to see the ground
to memorize where they should be putting their feet.
Don Blazer says: " The horse's eye does not focus well on objects that are closer than four
feet, and when a horse has his head high in the air, he cannot see the
ground in front of him. Because of a special system within his eye, the
corpora nigra which absorbs light rays from above, the horse cannot see
clearly that which is above the level of his eyes."
From a study of horse vision: "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."
From a study in Veterinary Science: "The horse has a frontally placed blind field,
such that when the nose is drawn in and the face approaches the vertical,
the animal is unable to see directly in front. This situation occurs when
the horse is being ridden 'on the bit' with neck arched and the nose just in
front of the vertical. If a show jumping horse is to see and judge the distance
of a fence that it approaches, it must have the ability to raise its head and
direct its binocular field forward."
An article from the University of Sydney:
Interocular transfer (IOT) occurs when an animal that has learned a visual
discrimination task with only one eye is able to perform accurately using
the other eye. It is thought that this is possible because visual information
from the right and left eye comes together onto the same area in the brain.
The corpus callosum is a structure in the brain responsible for communication
between the two hemispheres and has shown to be the major pathway for IOT in
several species including cats, primates and some rodents. It has been
hypothesised that horses are incapable of or have difficulty in transferring
information between the two hemispheres of the brain or that the lateral
placement of the eyes is the reason for the behaviour of horses when presented
with an object from both sides. This reasoning however is incorrect.
The horse has a substantial corpus callosum connecting the two sides of the
brain and lateral eye placement does not preclude the transfer of information.
Even with this anatomical information, it is still difficult to believe that
the horse does not have a problem with transferring information from one eye
to another because the behaviour is seen so often. A researcher in California
has conducted a study which asks the question; will a discrimination learned
with only one eye be retained when tested with only the other eye? The study
comprised of a series of tests conducted with one eye blindfolded and the
general premise was that horses were trained to particular visual stimuli,
one of which was associated with a food reward. The other eye was then
blindfolded and the behaviour of the horses to the stimuli was observed.
The results show that the horse is capable of interocular transfer with the
horse showing high levels of transfer of visual tasks from the trained test eye.
This data supports the anatomical and physiological evidence and contradicts a
popular belief based on anecdotes and misconceptions.
The question still remains: why do horses behave in such an unpredictable way
when faced with the same object but from the opposite side? The author is
continuing investigation into how horses perceive their environment. Some
of the possible reasons put forward by the author are: the horse may not have
been attentive the first time it passed an object and therefore never learned
about it; or perhaps the horse startled, not because it did not recognise an object,
but because that object suddenly came into focus when the horse’s attention
was elsewhere; or perhaps the horse did not perceive that the object was one a
nd the same because the item had a different appearance from the opposite direction.
This would create an element of surprise because the horse saw something contrary
to what it expected in a certain location.
Ref: EB Hanggi (1999) Interocular Transfer of Learning in Horses
(Equus Caballus) Journal of Equine Veterinary Science Vol. 19, No. 8, pg 518.
We see two separate issues going on with the Icelandic Horse: the star-gazing,
and the current trend of putting the horses behind the vertical. Both issues
are a problem for the horse and it's vision, and it's confidence in it's rider. It
could be said that it's an issue of horsemanship or ignorance of the subject
of the horse's vision and how it works.