The horse's requirement for large amounts of fibrous herbage, and the open grassland that provides this, leaves the horse vulnerableto predation. The horse is subject to little or no threat from aerial predation, and typically, predators include wolves, lions, and snakes such that ground level visual stimuli are particularly salient to the horse.
Furthermore the risk of predation is not limited to daylight hoursand, similar to the larger carnivores and some other ungulates (e.g., cows and sheep); the horse is active during both the day and night. Although feeding activity in the horse has beenfound to occur throughout the day and night, this activity peaks after dawn and before dusk. Therefore this crepuscular lifestyle requires a visual system that is sensitive in low light levels (scotopic conditions), provides early warning of approaching predators, and can assess the immediate ground conditions quickly for a speedy escape from imminent danger.
SOME ABOUT THE HORSE'S EYES
-The equine pupil is horizontally elliptical. Because the pupil islocated in front of the nodal point of the optical system, the field of view will be greatest in the horizontal plane, a situation that is best suited for scanning the horizon.
(-Monocular visual field is about 150ª -The binocular visual field is located down the nose of the horse and is limited to between approximately 65ª and 80ª .-There are two Blind zones: at back of the horse 20º aprox. for each eye, and in front.-Vertical visual fiel is about 178º )
-There are some interesting structures within the eye call tapetum and granula iridica (or corpora nigra):
The large corpora nigra on the central superior border of the iris also improves vision in bright light by augmenting pupillary constriction and acting as an internal visor that decreases the amount of light entering the eye from the superior visual field (where the sun is located), further reducing glare and improving vision in bright light.
With large pupils and the Tapedum lucidum location of the streak further enhances vision in dim light, but at the expense of light scattering and degraded visual acuity in bright light.
(We show you two videos to understand inside the eye where we can watch the eye structure, the second video is about a sheep eye butthe parts are equal than a horse.)
-To see objects at a greater distance, the horse rotates its nose upwards because its binocular overlap is oriented down the nose. When the horses taking off for a jump up they upward the head and sometimes a subtle turn of the head allows a horse to focus in on an object.
photo:www.equuesmagazine.com
-Human eyes require about 25 minutes to adjust from bright sunlight to darkness. Equine eyes need 45 minutes, almost twice as long. So, upon entering a dim building from daylight, your horse will be blinded by darkness long after your eyes have adapted.
-The horse could better visually discriminate between objects if they were located on the ground versus being placed 70 cm above the ground.
-In one study of 215 equine eyes in Europe, 3% were hyperopic, 15% were emmetropic, and 82% were myopic, although most of these were within a diopter of emmetropia. Significant astig-matism was not found in this study.
-Horses have dichromatic colour vision, with two cone types, sensitive to short (428nm peak) and mediu wavelenghts (539 nm peak). This mean thant they have reduced colour vision compared to humans, seeing colours along a continuous range from blue to yellow, and therefore cannot distinguish between many of the colours that humans see as red, orange and green, unles they also differ in brightness.
(Thrichromatic color vision of the Human and a simulation of the Dichromatic color vision of the horse.) Therefore, when we use some colours in the fence or hurdles could be hard to see for the horses under a variety of conditions. (Horse vision and obstacle visibility in horseracing.pdf)
The horse relies more on his eyes than his brain for visual identification. Horses aren't capable of much categorical perception. A different view, or the disparate position, of a familiar object is almost the equivalent of a new object. Our neural networks are firing simply for an object, for explemple a "hose" when we see one. We take a glance, and yep, it's a hose.
Our horses' neural networks are firing "new hose," "faded hose," "muddy hose," "green hose," "black hose," "hose wound in
24-inch loops," "hose wound in 14-inch loops", Every perception is separated. The same noncategorical effect occurs with sounds, touches, smells and tastes.
References:
-Horse vision and obstacle visibility in horseracing.Sarah Catherine Paul, Martin Stevens.
-"Equine Behavior". British Library.
-"Equine Ophtalmology". Brian C. Gilger, DVM, MS, DACVO Professor of Ophthalmology Department of Clinical Sciences North Carolina State University Raleigh, North Carolina
