Do Horses & Humans See Eye to Eye?

Photo: Adam Hill

Learning about how your horse sees can improve performance and communication

BY Elle Gibbs

As humans, we have developed the habit of assuming our horse’s sensory processes are just like our own. Although this may seem like an innocent fault, these assumptions can cause miscommunications and harmful outcomes in the show ring. So, how do our horses see? How is their vision different from our own, and how is it similar? And how can we, as riders, take this information to better understand our equine partners and improve our performance?  

Background to Equine Sight

Before jumping into the nitty gritty of eye anatomy, we must first have a brief understanding as to why horses have adapted their unique vision abilities. Before the days of heated barns and individual paddocks, most horses roamed open grasslands. As herbivores, horses spend 50-60% of their time with their heads lowered grazing; however, this left the animals open for attacks from predators like wolves (Pilliner et al., 2013).

With a high demand for herbage, horses needed to be equipped with extensive vision that allowed them to focus both on their food and assess their surroundings for danger. In order to increase survival, the horse has undergone years of visual evolutionary adaptations and natural selection pressures to evolve into the modern animal we know today.  

Anatomical Differences  

The horse has one of the largest mammalian eyes, bigger than that of either the whale or elephant. Unlike humans, who have their eyes set on the front of their head and their line of sight directed straightforward, horses have their eyes set high up on their head and positioned to the sides of the face.

The result is a horse with extensive monocular vision and limited binocular capabilities, diverging from humans who possess only the latter. Monocular vision is when each eye perceives images separately, where binocular vision is where both eyes work together in parallel to see (Pilliner et al., 2013). Different from the spherical shape of the human eyeball, horses have an eye that is flattened front to back and more level on the bottom. This results in the animals being able to focus both near and far images at the same time. The horse also has a structure not found in humans known as the tapetum lucidum, this allows horses to perceive images in the dark well, a result from wild horses’ nocturnal habits (Pilliner et al., 2013).  

Visual Acuity Disparities  

Binocular vision results from a pair of eyes receiving the same visual stimuli, also known as stereopsis, which in turn increases visual acuity (discrimination of detail) and allows a better judgment of depth. In turn, monocular vision in horses caused by the side placement of the eyes allows the animal with panoramic sight abilities at the expense of great detail discrimination. More influential on the ability to focus is a concept known as accommodation: the ability of the eye to focus images at different distances accurately (Murphy et al., 2009).

Humans have been observed to have great accommodation powers allowing them to discriminate minor details quite well. Unfortunately, the horse has undeveloped ciliary bodies and therefore have impaired accommodation abilities. However, this does not mean that the ciliary bodies are entirely useless in horses as they still are the mechanism that allows the animal to bring images into focus.

Due to this limitation, horses may need to move their head up or down in order to place the image at the correct distance for best detail processing (Pilliner et al., 2013). 
Horses do possess a form of binocular vision that aids in their depth discrimination. But much of their binocular abilities are limited due to their frontal blind spot. Much of this blind spot is due to the horse’s facial structure which essentially cuts off any sight beneath the muzzle (Pilliner et al., 2013).

It has also been observed that horses, like many other mammals, will have difficulty in discerning objects less than a meter away (Murphy et al., 2009). Horses do enjoy a wider range of sight than humans having approximately 215 degrees of sight capabilities from either eye (“Vision in the Equine,” n/a). On the other hand, humans only have about an 180-degree view of a field, or a 150-degree capacity from either eye (“How Normal Vision Works,” 2020).  

Light sensitivity   

Both horses and human retinas contain two photosensitive cells: rods and cones. In either species, cones allow us to perceive detail, color, and respond to light quickly, where rods allow us to see in low-light levels (scotopic vision) (Murphy et al., 2009). Humans have shown better visual acuity than horses in photopic (bright) conditions. But, the horse has developed a tapetum lucidum: a structure that provides a “second chance” for photoreceptor cells to react to low-light stimuli.

So, although humans may have better daytime vision, horses could be more adapted for sight at night, this hypothesis still needs further testing to be completely sure (Shinozaki et al., 2013). Even though the tapetum lucidum provides the horse better sight in darker conditions, it comes at the costs of yet again decreasing horses’ visual acuity. The horse also has a pupil that is less quick to adapt to new levels of light exposure; for this reason, they have gained a structure known as the corpora nigra (Murphy et al., 2009). The corpora nigra provides a shade to the eye to prevent potential sun damage due to their delayed optic responses to light levels.

The presence of the corpora nigra also means that the horse will be more easily “blinded” when exposed to bright light or have lengthened adjustment times when exposed to darker conditions (Warren, 2021). This concept is important when it comes to horse shows with different portions of competition at various light levels. The presence of cone photoreceptors that allow wavelength discrimination and daytime behaviors provide evidence that horses have color vision. Humans are trichromatic, possessing three cone types, where horses are dichromatic, only have two cones, and are more similar to color-blind people.

Instead of perception of a full color spectrum, horses have the ability to best determine yellow and green, with little ability to see blue and least of all red. It should also be noted that being dichromatic allows horses to see the texture of an object regardless of high-contrast colors, which we as trichromats cannot do (Murphy et al., 2009). Therefore, it’s plausible that certain surfaces are more visible to horses than us, not on a basis of color but texture. This could explain how horses may be able to discriminate against natural-colored jumps on earthy ground, like on cross-country courses, better than humans.   

Application to the Horse World 

Photo: Adam Hill

So, how can sight differences between horses and humans affect performance? We know that horses have underdeveloped ciliary bodies that prevent strong visual acuity, sometimes forcing the animal to move his/her head around to gain better image processing. Unfortunately, this act of moving the head can be mistaken as a behavioral issue that can lead to unfair punishments and further restriction of sight.

Riders should take time to discern if certain head movements stem from poor manners or just lack of visual clarity before acting; this can ensure that any potential eye problems can be addressed and reprimand is not delivered unfairly. Riding “on the bit” or flexing the head and neck towards the chest has grown in popularity in the equestrian world. This riding style does utilize the horse’s back well but it comes at the cost of limiting frontal vision.

Riders who work their horses in this manner need to be extra cautious of their surroundings to ensure that the horse is comfortable working visually impaired and that they are putting themselves in positions and environments that could not exacerbate chances of harm. This may mean opting to ride in a smaller, less-crowded arena or allowing the horse to have some freedom to raise its head so that they may have a better view of the ring.

Some researchers have stated that the horse should be allowed their head when jumping a course, this is especially important right before the jump as horses can no longer see the fence when 1.5m away (Pilliner et al., 2013).  We have covered the idea of light-sensitivity in horses and how they have a delayed adjustment period to new light levels compared to humans. Many shows, like the famous Capital Challenge Horse Show, ask competitors to transition from an outdoor arena into a dim indoor show ring or vice versa.

When tasked with transitions such as this, it is important to try and aid your horse in adjusting to the light levels before attempting to jump a course or work in these new conditions. This can be resolved by allowing the animal to stand in low-light environments, perhaps standing near the indoor (this may not always be possible or the safest option considering the density of people and other horses) or finding other ways to replicate the new environment before showing.

Fence color and decoration can also play a role in determining a horse’s sight capability and even safety while on course. Fences are designed for aesthetic appearances and are often created without regard to the animal’s vision. For example, jumps with only a single color, especially white, make it more difficult for horses to distinguish. They have also been shown to have difficulty discerning jumps where green is selected with yellow or blue.

There’s also an interesting implication in trailering. Most horse transportation have black mats for their stalls; however, it has been studied that by changing the color to green it decreases loading hesitation and makes the animals more comfortable while being shipped. Another feature of the equine dichromatic eye is that they can sometimes see textures and movement where humans may only perceive color. This may be linked to the widely held belief that red objects, especially flowers, are the spookiest jump feature to horses.

This is not the first time that red has been considered a “provoking” color to animals, in fact bullfighters also rely on this concept for their performances. But, scientists have figured out it is not the color but rather the movement of the cape that triggers the bull, just like how it is most likely not the red flowers but rather small movements undetectable by humans but visual to horses that causes their alarmed reaction. Riders should be aware of this idea while jumping so they can understand whether refusals or spookiness is simply the result of being naughty compared to when their horse could be perceiving the fence in a different way.

The relationship between horses and humans is one of the greatest friendships of all time; therefore, we owe it to both our teammates and friends to gain a better understanding of how they see the world. This knowledge will ensure greater com-fort, safety, and improve performance of our adored partners for years to come.   

Elle Gibbs is a first year at Cornell University where she is majoring in Biological Science as well as pursuing a minor in Economics and second minor in Animal Science. Her dream is to attend Cornell University’s College of Veterinary Medicine and pursue a career as an Equine Veterinarian.

Sources: M;, Shinozaki A;Takagi S;Hosaka YZ;Uehara. “The Fibrous Tapetum of the Horse Eye.” Journal of Anatomy, U.S. National Library of Medicine, pubmed.ncbi.nlm.nih.gov/24102505/# • Murphy, Jack, et al. “What Horses and Humans See: A Comparative Review.” International Journal of Zoology, Hindawi, 22 Apr. 2009, www.hindawi.com/journals/ijz/2009/721798/ • Pilliner, Sarah, and Zoe Davies. Equine Science. Wiley, 2013 • “Vision in the Equine.” Equine Science, www.extension.iastate.edu/equine/vision-equine#:~:text=Each%20eye%20 sees%20across%20an,between%2065%20and%2080%20 degrees • “How Normal Vision Works.” Chadwick Optical, Inc., 23 Nov. 2020, chadwickoptical.com/how-normal-vision-works/#:~:text=As%20you%20can%20see%20 from,overlap%20 is%20 called%20 binocular%20 vision • Warren, Rob. “Three Equine Specialties (and a Dedicated Family) Come Together to Heal Horse.” School of Veterinary Medicine, 7 Apr. 2021, www.vetmed.ucdavis.edu/news/three-equine-specialties-and-dedicated-family-come-together-heal-horse