It has long been recognized that the two eyes of a horse seem to behave and react independently of each other. Where we humans have both our eyes facing the same direction, a horse's eyes have most of the head between them, and therefore they see scenes that are very different from each other. That almost certainly means that a horse cannot sense "distance" as we do, (except for a very small angle directly in front of its head where both eyes can see the same object) because their brains are not able to use the parallax effect of bi-focal vision, of two eyes seeing the same scene from slightly different angles and therefore being able to figure out the distance of some object. But that is a different subject!
The significant fact here is that nearly everyone trains horses to become used to a rider approaching and mounting FROM THE LEFT SIDE! IF a rider should not know this and approach from the right side or attempt to mount from that side, many horses seem to panic and react badly. The generally accepted understanding is that the two halves of the horse's brain (which is much like our own with two independent hemispheres), learn things relatively independently of each other. In all animals and people that have such a brain structure, there is a bundle of nerves called the corpus callosum which connects the two hemispheres.
It figures that in the 55 million years that horses have existed, if the two eyes can rarely see the same object, there has been little point in their brains needing intimate connection between the two hemispheres, as there has been little need for co-processing the optic signals from the two eyes to determine distance information. This suggests that the corpus callosum in a horse may either be much smaller (less developed) than in us or that it is used less often. In either case, that can explain why the horse reacts so differently regarding riders approaching and mounting. A similar situation sometimes occurs when a person walks around a horse without touching it or making noise. A horse seems to sometimes be surprised and even scared when the person comes into view, from behind, by the second eye! That characteristic has always been considered a "quirk" of horses, and I don't know if anyone has ever tried to investigate it scientifically.
So far we have that horses generally have to be trained FROM ONE SIDE, which essentially trains the brain hemisphere on the opposite side, and that the corpus callosum seems to have minimal functionality.
Several years ago, a Russian researcher discovered that certain dolphins sleep in a peculiar way, where one eye closes for around two hours, then the other eye does. In this way, one eye is always open. In the millions of years that dolphins (and porpoises) have existed in the open ocean, so have predatory sharks. It would obviously be a tremendous disadvantage regarding survival to be completely asleep for several hours if there were sharks in the vicinity. That may be the reason for this peculiar sleeping mode of dolphins. Non-injurious EEG readings have been recorded from both hemispheres of a dolphin's brain which confirm this. It is interesting that in the United States, it is illegal to virtually touch a dolphin much less do any research on one, so there is NO ongoing research in this area. The Russian research has not involved any intrusive (invasive) procedures so has probably not injured any dolphins (as far as is known) but western countries seem to forbid any such research now!
The placement of the eyes on a dolphin are similar to that of a horse, where there may be little call for co-processing visual information simultaneously. This MAY have resulted in the corpus callosum in a dolphin being either much smaller (less developed) than in us or that it being used less often. In either case, that can explain why the dolphin can sleep one half of its brain at a time. The alternative is that the dolphin has somehow DEVELOPED THE ABILITY TO SHUT DOWN THE CORPUS CALLOSUM, which is an intriguing idea! If that were the case, then the fact that our human brains are structured much the same, maybe it could be possible that a human might be able to learn how to do that, of having half the brain sleep at any one time! I am not sure how "intelligent" such a person might act, but it is still an interesting concept!
The significance might be great. I have long wondered how a horse can sleep and still stand up! This might provide an explanation of that, where only one half of the horse's brain is actually sleeping while the other half is still alert enough to sense danger or of falling over. Both eyes might easily be closed even if one half of the brain happens to be active like that. This possibility seems even more possible after a friend informed me recently that horses sometimes lay down to sleep. I was told that it seemed that the horse sleeps much more soundly during that type of sleep. THAT could be a situation where both halves of the brain are sleeping!
The friend also mentioned something else that I found very interesting! She said that horses sometimes SNORE! (Which seemed amusing!) BUT she said that she had only ever seen her horses snore when they were lying down! This could make sense with the premise suggested above! A standing horse MIGHT be alert enough to maintain air passageways to avoid snoring, because one half of the brain was alert to such things. But a horse that was lying down might be prone to snoring due to the entire brain being asleep and the air passageways therefore partially closing due to gravity.
Regarding this last issue, I guess I make a call to all horse owners! Have you ever heard your horse(s) snore when standing up? If so, then part of my premise must be wrong! But if we could collect enough data (around 1500 horses, for statistical purposes) regarding whether a horse was standing or lying down when snoring, we might be able to achieve some scientific value in that research!
On the other, main, issue, I suppose it will be up to some Veterinarian to do an EEG brain scan for a number of hours on a sleeping horse, to get actual evidence regarding whether both hemispheres sleep together or whether one stays partially active.
It appears that such research is starting to get underway! Serious scientific research on the brains of horses is (finally) getting under way!
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C Johnson, Theoretical Physicist, Physics Degree from Univ of Chicago