In this episode on Equine Visual Perception we will begin to scrutinize the internal features of the equine eye as compared to our human eye. First we need to get a general understanding of how an eye works. The science behind the eye, the different cells, their connections and functions, is a universe of its own and the more you dig into it the more you realize how little you know. So, right or wrong in order to reach a graspable explanation, we need to make simplifications.
There are basically five different types of cells in the eye retina that receives light and transforms it into signals compatible to our brain:
Light receptor cells; rods for black and white and cones for color.
Horizontal cells; primarily unite signals from cones. There are three types.
Amacrine cells; some converge signals from rods, but there are 22 different kinds of amacrine cells and they do a variety of different jobs, many yet unknown.
Bipolar cells; a kind of amplifiers connected between light receptors and ganglion cells. Some connects light receptors via horizontal cells.
Ganglion cells; the transmitters that sends the visual information to the brain. They come in different sizes and connections.
Rods and cones
Rods are very sensitive and thus good for faint light conditions (scotopic conditions). While a rod under perfect conditions only needs to be hit by one single photon to react, a cone needs to be hit by a thousand photons. This makes a big difference in very poor light conditions, but not in full daylight. However once a cone reacts it responds more quickly than a rod does.
Rods detect differences in light intensity, but not in wavelengths (colors). Their sensitivity peak in short (blue) wavelengths, that’s why colors seems diluted in faint light conditions. Cones however dissolve wavelengths very precisely, meaning they give us the sense of color, but cones need fairly good light conditions to do their work. We will return to that later.
Rods are often connected in bundles to each bipolar cell, which means great sensitivity, but poor resolution. Cones in the fovea, in the center of the eye, are each connected to one bipolar cell, providing excellent resolution in good light conditions.
Horses have the biggest eyes of all land mammals, 10 times bigger by volume than a human eye, which is important to consider as it probably enhances every aspect of horses’ visual perception, including acuity. Keep this in mind as you read on!
Differences regarding rods and cones
A human eye has 120 million rods and 6 million cones, which means a 20:1 ratio. In the absolute center of the macula (the yellow spot) called the fovea, there are tightly packed cones (160 000 cones per square millimeter) but no rods. The further away from the center of the retina the less cones. In the peripheral there are “only” 5000 per square millimeter.
A horse eye has the same 20:1 ratio between rods and cones. Horses have a horizontal streak or band instead of the human yellow spot. The streak have about 20 000 cones per square millimeter and 120 000 rods per square millimeter, but at the peripheral there are the same cone density as in the human eye. The area of highest density corresponds to the binocular field, but more on the binocular field later.
The horse has a stretched out horizontal pupil, as compared to the circular human pupil. It gives the horse a wide lateral field of vision, which is great for a prey animal on open range. When exposed to high light levels from one part of the visual field it can be problematic for the pupil to adjust the amount of incoming light without making other parts of the visual field too dark. This problem is solved by the third eye lid, corpora nigra, in the inside corner of the eye that closes diagonally over the eye. It can limit the amount of light that comes in from above as it shades the lower half of the retina. So in bright sunlight the horse can still keep a good sensitivity for faint light coming from the dark shadows. It is however not as effective if the light is reflected and evenly spread in all directions, like in a stable. Therefore it can be a good idea to paint the ceiling white, but the walls and floor in a darker color, to correspond better to natural outdoor conditions.
It is often said; the horse has difficulties to adapt to fast changes in light intensity, as when an artificial light is turned off or on. I am not sure that is correct, or at least it is a bit off subject. Because horses have such a large field of view they cannot easily get away from a sudden bright light by turning away the head as we humans can. Besides a horse is naturally suspicious to enter a dark place like a room, stable or transport, she may also react when the light changes quick, but all this is natural and logic reactions for a prey animal, it does not necessarily have anything to do with the eye being less good at adapting to changes in light intensity.
Horses have an inter-ocular transfer between the eyes, just as humans do. It means that e.g. a change in light intensity that affects one eye is also recognized by the other eye.
Horse’s scotopic vision is further enhanced by a reflective layer, tapetum lucidum, acting like a mirror behind the light receptor cells, a feature the horse shares with many other species. It gives the rods and cones a second chance to capture the photons, at the expense of loss in acuity since it scatters the light. The tapetum lucidum primarily reflects light coming from a downward angle. Thanks to this, horses can confidently move around in what we humans perceive as total darkness. The difference between day and night is much less dramatic for a horse than for a human.
Another feature with equine eyes is that detected light photons seem to be added from nearby light receptors to increase the visual contrast, although at the expense of some loss in acuity. (In CCD photography this is called the Blooming Effect, which must be considered by astronomers involved in measuring light variations from distant stars with high precision thru photometry e.g. as a method for detecting exoplanets. The definition of blooming is when the charge in a pixel exceeds the saturation level and the charge starts to fill adjacent pixels.)
The next part of this series on Equine Visual Perception will deal with color vision and binocular vision.