In my first post about the equine eye (found here), I explained that I think it is important to understand how every part of the horse functions in order to be able to train and work with them efficiently and effectively. So why take so much time talking about the horse’s eye? The eye is an incredibly complex structure and the way that it functions in horses and humans is vastly different. If we can’t understand how a horse views the world differently from us, how can we understand how to effectively communicate with him? Therefore, by taking the time to learn about the equine eye, we can become better trainers, riders and handlers.
There has been much debate over the way horses see color, or if they see color at all. Through many tests and studies however, researchers have made many discoveries about the way that color and light works from a horse’s perspective. Before delving into the color debate, let’s go over two important parts of the eye:
- Cones are the part of the eye that distinguish color.
- Rods are the part of the eye that perceive light intake.
The first item in the color debate is whether or not horses actually see colors and to what extent. The answer to this is that horses do indeed see color, but not quite as vibrantly as humans do. Horses have dichromatic vision, which means that they have two types of cones in their eyes. Humans have trichromatic vision which means that they have three types of cones. The cones in human eyes perceive blue, green and red. With these three types of cones, we can see thousands of colors in sharp contrast. The cones in a horse’s eye perceive blues and greens, although the greens that horses see has more of a yellow hue to it that it does to our vision. When a horse views red, it appears as a kind of earthy tone with yellow and blue hues. Certain colors can not be distinguished from one another, similar to when we put a color picture into grayscale and one color has the same gray tone as a different one. Researchers believe that equine color vision is similar to a person’s vision who has a red-green deficiency where there is very little or no problem with blues and yellows but not all reds and greens are distinguishable. The colors a horse sees are washed out in comparison to what we see when looking at the same image and tend to resemble a sepia tone. Here is an example:
The cones of a horse’s eye are also arranged differently than the cones in a human eye. Our cones are packed centrally on our retina. A horse’s cones are spread out along a “visual streak” on the retina that mimics the line where earth and sky meet making it easier for them to scan the horizon. As a prey animal who needs to be continually aware, this is useful!
Next we come to the function of rods in the eye. Many horse people, especially avid trail riders, are familiar with the advice to let your horse be the guide if you find yourself stuck out on a ride after nightfall or as it begins to get dark. This is great advice to follow because horses have much better night vision than us because humans have fewer rods than horses do. The human cone to rod ratio is 20:1 while the equine ratio is 9:1. The larger amount of rods, in conjunction with the tapedum lucidum, a reflective coating on the back of the retina, allows for optimum light intake and makes a horse’s scotopic vision (vision in dim light) much better than ours. The tapedum lucidum bounces light back to be absorbed by the rods that would otherwise be lost.
So, while we can see more color, trust your horse to safely carry you home if you find yourself in the dark!