Horses and humans visually conceive the same surrounding environment very differently because our eyes are different and because our evolutionary preconditions and experiences are different, but we do share the same world.
When watching horses interact out on a free range it is easy to realize horses seems to benefit from good eyesight, just as we humans do. No one has a complete explanation of the differences between equine and human visual perception, but the more we know about it the better we can understand the sublime details in equine behavior most people never even see.
This subject has undergone several changes since the days of Xenophon (431-355 BC) and his written works on horsemanship of his time. By his experience horses had an excellent ability to see over vast distances. Legends of the Arabian horse cultures tell the same. Then during the early years of natural science, another less flattering view upon the blurry equine vision was proposed, which has lived on until quite recently. We need to change that, because now we know better. However it is wise to point out that even if it is with the most humble intentions of accuracy, errors are inevitable as time will always prove time and again, which by the way reminds me of a poem by Alexander Pope:
“A little learning is a dang’rous thing;
Drink deep, or taste not the Pierian spring:
There shallow draughts intoxicate the brain,
And drinking largely sobers us again.”
Eyes and vision, especially equine vision is complicated business, so even if this text has no ambitions to be a work of science whatsoever, it is necessary to “drink deep” otherwise it only leads to more misconceptions. Since we (yet) cannot connect an equine optic nerve to our own brain we cannot know exactly how horses see. At best we can find out about the technical specifications of the equine eye, but that alone does not tell us exactly how horses experience their own eyesight. Let us begin from the basics; vision is just one of many senses that help us interpret the reality around us. Our eyes receive light and transform it into signals compatible to our brain, as an input to create one important piece in our complete perception of the immediate surrounding reality.
What is light?
Light is electromagnetic radiation. In some ways it behaves like a wave and in some ways it behaves like a particle, or a package of energy. The best present explanation is that it is a point-like particle called a photon, an elementary particle which has no mass and no electric charge. In vacuum it travels at a speed more or less equal to seven laps around the earth in one second. The shorter the wavelength the more energy each photon represent. The visible spectrum for us humans is between 380nm and 780nm. (Equivalent to about 200 oscillations across the width of a single human hair.) Short wavelengths are perceived as blue and long wavelengths are perceived as red. Even longer waves, beyond the visible spectra, is low energy radiation; infrared radiation, micro waves and radio waves. On the shorter side, shorter than the visible spectra we find high energy radiation; ultraviolet radiation, x-rays and gamma rays. It is basically the same physical phenomena, only different wavelengths.
How does it work?
When these photons of light, originating from nuclear processes in the core of the sun (or from another source like a light bulb), comes rushing in a complete mix of a variety of wavelengths and hits an object they bounce off, but not all of them. Some wavelengths of light are absorbed or changed, depending on the objects’ properties. If e.g. only short wavelengths are reflected and we happen to look in the direction of the object, some of the reflected light enters our eyes, hits some of the photoreceptor cells in the retina of our eye and a nerve impulse is sent to our brain which in our consciousness creates a virtual picture of a blue object. So we do not really perceive anything as it really is; we only imagine virtual models out of bouncing photons. However it serves its purpose very well for humans as well as horses; we can see.
In the next part of this series on Equine Visual Perception we will begin to scrutinize the internal features of the equine eye as compared to our human eye.