The ability to perceive color is just one of the features that differ between horses and humans, without claiming that one is better than the other. The field of view including the binocular vision is another aspect that we will discuss in this third episode on equine visual perception.
Humans have three kinds of cones (trichromatic vision) with sensitivity curves that peak at different wavelengths close to the colors; blue, green and red. When combined the maximum sensitivity peaks at 555nm, which is the color green. Some people have only two kinds of cones (dichromatic vision), a kind of defect color perception.
Horses color vision is difficult and some studies show different results, but the most recent and reliable express that horses have only two kinds of cones (dichromatic vision), most sensitive at blue wavelengths 429-456nm and green 539-556nm. Horses do detect light in wavelengths we call red, but they perceive it as shades of grey. This scheme gives a sense of the differences in human (left) and horse (right) color perception:
In some situations the human trichromatic vision, which puts much attention to various colors in the environment, can actually disguise visual features a horse will never miss. After all every land mammal have dichromatic vision, with very few exceptions, for some evolutionary reason. Dichromatic vision is especially effective in detecting movement and to see changes in texture based on light intensity and not being distracted by a wide range of colors. Only humans can be fooled by a fancy camouflage-hunting-suit, seldom the hunted prey. This is a very important difference to ponder about; color is important to humans and movement is important to horses!
Field of view and Binocular vision
The horse, also known as Equus ferus caballus, evolved in open rangelands and that is where they are untouchable, not without thanks to their wide field of view. The herbivores hugely outnumber the carnivores in any open grassland habitat, if free from human interference. A predator’s only chance to feed frequently is to catch the weak and injured, which of course only pushes evolution towards even stronger and healthier herbivores. Horses’ visual field covers almost a full circle, which makes it virtually impossible to sneak up on a band of horses out on open range. Each horse eye covers 200 degrees and they overlap by about 60-80 degrees in a triangular shape up front, so the combined field of view is something like 320-340 degrees, leaving a small blind spot behind the horse, which is easily covered by a slight movement.
We humans have a combined visual field of about 180 degrees, a half circle. A large part of that field (120 degrees) where our eyes overlap is called the binocular field. It gives us a good sense of distance, or 3D to put it straight.
Horses’ ability to estimate distance is affected by their smaller binocular field and also by the fact that it has a triangular shape, with the base horizontal to the ground. They need to lift their head and point their eyes and head more exactly to see a distant object in full 3D. However the distance between their eyes is wider than ours which is to their advantage. The sum of it all is that a smaller binocular field, but a larger inter-ocular distance (the distance between the eyes) gives the horse a narrower, but deeper binocular vision compared to us humans. When moving fast and looking forward a wide binocular field is no advantage, but a deeper binocular field means high precision. So given the horse has loose reins so that she can point her head for optimum binocular vision, she out conquers her human riders ability to estimate distance. This is an important conclusion.
Another way to judge distance while moving fast is to sense the speed at which the size of an approaching object is increasing. It is probable that horses generally are better at this than the average human. Besides as everyone knows it is possible to see depth in a flat image too and it is easy to realize by looking with just one eye that there are several different ways to sense distance and depth. Together they sum up the total experience.
As you all probably know it is often recommended that you train a horse from both sides because horses seems to have no connection between what happens on the left and on the right side. It is also common practice to mount from the left side every time, or the horse may get spooked. The reason for all this is said to be because horses have no, or insufficient, connection (corpus callosum) between their brain halves and they have no inter ocular transfer. Unfortunately these assumptions are absolutely false!
A well renowned researcher and horse person Dr Evelyn B Hangii has done a lot of interesting work on this subject. According to physiological evidence horses have in fact more crossed optic nerve fibers than humans and horses also have a substantial corpus callosum perfectly capable to transfer sufficient data between the two brain halves.
The above mentioned behaviors that horses seem to react different depending on which side, or which eye they face things, need another explanation. Horses are prey and humans are predators and we use a different frame of reference in everything we do. Evolution has made horses very careful; they keep track of every change in their environment in good memory, even small things like the ones mentioned above. For us a trailer is a trailer, but for a horse one trailer is very different from another trailer, not because they cannot make associations or categorizations and not because there is something wrong with their eyes or brains, but because they are prey, and therefore careful. The only way a horse can relax is in company with trusted friends, horse or human, and in places they trust. Their concern about their surrounding environment is tiresome for them, so they constantly seek safety in trusted friends and trusted places. It takes time and patience, but it comes to every horse with her own experience. Horses get their experience from social connections with other horses and humans in a varied environment they can control themselves, the more the better.
You are undoubtedly also familiar with the old scientific truth that humans, and horses alike, are either left-brain dominant (methodical/analytical) or right-brain dominant (artistic/creative). It sounds logical and well established and is used by several famous horse trainers, but this does not prevent it from being completely false!
In a very recent work of science (2013 Nielsen et al) more than one thousand human individuals were tested thoroughly by magnetic resonance imaging. One clear conclusion is that an individual brain is not “left-brained” or “right-brained” as a global property. You can of course put a label on individuals and categorize different personalities to make a pedagogical point out of that, but it has nothing to do with left and right brain halves being more or less dominant and there is no reason why horses would be any different.
The following section of this series on Equine Visual Perception that will appear here in a few days is dedicated to acuity, the ability to focus to create a sharp image on the retina; a subject full of myths and assumptions.