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by Tammy
Miller Michau, and Michele Stengard
Diplomates of the American College of
Veterinary Ophthalmologists
For millennia, the horse has depended on its visual
abilities for it’s survival.
In the current world, survival has become less of an issue but the
visual function of the horse is still critically adapted to a “flight” response
from threats or predators.
Therefore, understanding horses normal vision is critical to understand
normal behavior and the effects of disease on vision. Changes in vision secondary to disease can result in
abnormal behavior and poor performance.
WHAT DOES THE HORSE “SEE”?
The horse’s vision is adapted to function in both
bright light and dim light. The
act of seeing is a complex process that depends upon: 1) light from the outside
world falling onto the eye, 2) the eye transmitting and focusing the images of
these objects on the retina where they are detected, 3) the transmission of
this information to the brain, and 4) the brain processing this information so
as to make it useful.
VISUAL
PERSPECTIVE AND FIELD OF VIEW
Visual perspective varies greatly depending on
whether the horse’s head is up or down (i.e. grazing) and how tall it is (i.e.
miniature horse or a draft breed).
The position of the eyes in the skull of a horse allows for a wide,
panoramic view. Their visual field is enormous (up to 350°) and provides nearly
a complete sphere of vision with few small “blind spots”.
DEPTH PERCEPTION
Stereopsis (binocular depth perception) is the
fusing of 2 images from slightly different vantage points into one image. If the image from both eyes did not
fuse, double vision would result.
A horses depth perception is generally good. From 2 meters away horses
can detect a 9-cm difference in depth, which approximates the ability of a cat.
The work to detect variation among horses has not been done. This comes in very handy when jumping over objects!
MOTION DETECTION
Horses maintain the image on the retina during
motion in much the same way as do people, and like people, they more easily
detect moving objects than stationary ones. The ability to detect motion is greater in the horse’s
peripheral visual field. Visual
acuity is also low in this area and may explain, in combination with the
horse’s prey mentality, why horses shy so easily from objects located in their
peripheral visual field.
VISUAL ACUITY
Visual acuity is the ability to see the details of
an object separately and without blurring. In humans, the most familiar method of measuring visual
acuity is to ask an observer to read an eye chart. The horse cannot read an eye chart, however, visual acuity
has been estimated at 20:30 to 20:60.
This means the horse’s visual acuity is among the best of the domestic
mammals and better than that of many people.
REFRACTIVE STATE
The refractive state is based on an image being
accurately focused on the retina.
If not focused on the retina, the image can be focused in front of the
retina (i.e. myopia) resulting in near-sightedness or behind the retina (i.e.
hyperopia) resulting in far-sightedness.
Although some individuals are slightly myopic or hyperopic, the average resting refraction is near
normal in the horse. Various
diseases can result in refractive errors and possibly result in behavioral
changes or poor performance.
COLOR VISION
Because many predators also evolved coat colors
that closely matched the background in terms of color, and in some cases
texture, color is relatively poor way for the horse to “break the camouflage”
of a predator. This may mean that color detection became relatively unimportant
and therefore not very useful to the horse.
Humans typically have trichromatic color vision. The 4 basic hues seen are blue, green,
yellow, and red. Horses have only
dichromatic color vision. This
results in horses seeing only 2 unique hues, yellow and blue. It is probable
that this also results in colors appearing as washed-out pastels or sepia. Horses most likely have difficulty in
differentiating orange and blue much like some color-blind men cannot
differentiate between red and green.
In effect, the horse is believed to have a form of color blindness and
sees only yellow, blue, green and gray.
Why does your horse always spook at the red fence then? It is probably
not the color but other factors such as its brightness, etc.
VISION ASSESSMENT IN HORSES
Currently, veterinarians are unable to recognize
anything but the most serious visual changes in horses because of the crudeness
of the visual testing tools the clinician has available to them. Known diseases such as cataract
and corneal scarring probably affect vision in the horse in much the same way
as the diseases do in humans.
Behavioral or orthopedic issues can frequently masquerade as visual
problems. Any time you suspect a
visual abnormality in your horse you should have him evaluated by your primary
care veterinarian. A referral to a
veterinary ophthalmologist can then be obtained for further examination and
testing, if an ocular problem is suspected.
For more information you may contact Surgi-Care
Center for Horses at 813-643-7177 or by email lkuebelbeck@surgi-carecenter.com.
This article was originally published in Horse & Pony magazine and is reprinted with their permission.