How
Horses See
by Evelyn B. Hanggi, M.S., Ph.D.
Beginner,
trainer, owner, lover, weekend rider, American, Canadian, European,
Asian, Australian – these are the folks who come to the nonprofit
Equine Research Foundation (ERF) in California . Determined to improve
their relationships with horses, they seek knowledge about why horses
behave as they do, how they think and learn, and how they perceive
their world.
Along
with ambition, participants bring questions, conceptions and, often,
misconceptions. “Many people may have years of horse experience,”
says ERF program director Jerry Ingersoll, “yet they still do
not fully understand what makes a horse tick. They groom, saddle,
and ride as they’ve been taught but don’t often question
the soundness of those teachings.”
And
so, riders continue to steer their horses around puddles because,
according to student volunteer and horse veteran Rebecca Grand on
her first day at the ERF, “horses can’t tell how deep they
are, at least that’s what my instructor said.” And people
persist in pulling and pointing and pushing horses over jumps because
of an idea that these animals lack perception of depth; in other words,
that they have a tough time judging distance. When asked if any instructor
ever spent time talking about equine vision, ERF participant Nancy
Siddens replied “The only thing I remember being told was that
since horses have eyes on the sides of their heads, they do not have
binocular vision . . . I am assuming depth perception is not strong.”
Good
with depth, trouble with depth; colourblind, not colourblind; sharp
vision, blurry vision – which is it? Answers differ, depending
on who is doing the talking. More often than not, the speaker is someone
people trust and, therefore, do not question — the well-known
clinician, the feared riding instructor, the respected old ranch hand.
Unfortunately, some of these authorities base their comments on hearsay,
on folklore, on myth.
Hearsay
is a worry. Generally untested, most likely undocumented, it is the
cause of much misunderstanding. How, then, does one distinguish fact
from fiction? Simple really, just ask the experts. Today, scientific
study into equine vision is at an all time high, with discoveries
offering evidence that horses see a world not so far removed from
ours. Here is what science tells us:
Field
of View: Having eyes in the back of their heads, almost.
Remember
in school when your teacher, with her back to the class, always knew
what everyone was up to? Seemed she had eyes in the back of her head.
Or maybe she had eyes like a horse.
Horses
have lateral eye placement, meaning their eyes are located on the
sides of their heads. This gives them a much larger field of view
compared to humans; in fact, when holding their heads level, they
possess a nearly spherical field of vision. An advantage in detecting
predators, many prey animals have this sort of eye placement. But
horse eyes are also placed somewhat frontally, affording them binocular
overlap ranging from 55 to 65 degrees.
With
vision that extends almost a full horizontal circle, horses have no
problem seeing what goes on around them. Blind areas exist only within
a narrow region to the rear as well as a small area perpendicular
to the forehead and directly below the nose. Even so, a tiny shift
of the head suffices in bringing these areas into view.
Visual
Acuity: On a clear day, can they see forever?
The
ramp retina theory is passé. Long popular, even in veterinary
textbooks, this concept sought to explain how the equine eye focuses.
It was thought that objects at various distances were brought into
focus by the horse raising or lowering its head so that images could
fall onto different areas of the retina. Upon physical examination,
however, scientists found little indication that the retina was ramped;
on the contrary, they discovered that what small slope did exist slanted
opposite to that which might have helped focusing.
More
exactly, the horse’s retina contains a narrow horizontal streak
across the center of the eye densely packed with receptor cells called
cones. Providing an elongated band of acute vision over much of the
lateral range, this visual streak probably serves the same purpose
as the fovea in the human eye. Acuity drops off rapidly outside the
fovea in humans as it does outside the streak in horses. Heads bouncing
up and down, like a bobblehead dog in the rear window of a car, serves
no advantage to horses in terms of keenness of vision.
Does
the horse see fairly clearly at distance? Well, yes. In fact, if you
are even moderately nearsighted, chances are your horse sees better
than you! Whereas normal human vision is 20/20, horses are rated at
20/30. In comparison, cats come in at 20/75, rats at 20/300.
Colour
Vision: Looking at the world through rose-colored glasses?
Cones
are multifunctional. They control fineness of detail but they also
control colour when more than one type of cone exists. Humans with
normal colour vision are trichromats: we see four main hues –
red, green, blue, and yellow, as well as many subtle variations.
The
topic of equine colour vision has raged controversial for years. Backyard
owners swear their horses prefer buckets and blankets based on colour;
top trainers sell books proclaiming horses are colourblind. Yet, when
asked how they know, most people cannot support their statements,
falling back on the adage “Well, so and so said so.” Enter
again the researcher. Equine vision expert Dr. Brian Timney, of the
University of Western Ontario , explains that horses are like humans
with colour deficiencies. For example, humans with red-green colour
vision defects have only two hues (blue and yellow) rather than four.
Such dichromats do not see intermediate hues, seeing instead white
or gray or a faded form of the two colours.
What
colours, then, do horses see? “Horses have little difficulty
in discriminating red or blue from gray. With respect to green and
yellow, the results are mixed,” says Timney. His findings are
similar to those from earlier studies but other researchers’
results differ. Exact answers have proven elusive in colour vision
research because of the difficulty in designing experiments with adequate
controls (such as for brightness). Nonetheless, we can reasonably
say that horses do see some colours and may react positively or negatively
to them.
Scotopic
Vision: It was a dark and stormy night.
And
your horse probably saw more than you did. But scientific research
into equine night vision remains sparse. What there is of it is based
on anatomical and physiological factors, not behavioral. What we do
know is that the equine retina has many more rod receptors than cones,
about 9:1. These receptors are responsible for vision in dim light.
The eye also contains a tapetum lucidum (the reason eyes of nocturnal
animals shine in the dark), which reflects light and enhances the
light-gathering properties of the rods. All this indicates good night
vision but there may be a hitch. The tapetum lucidum, while increasing
sensitivity to dim light, may also, because of light scattering, reduce
visual discrimination. Nonetheless, horses are active during the night,
grazing, moving about, avoiding obstacles. This gives us a behavioral
clue that their scotopic vision is decent.
Depth
Perception: Are puddles the terrestrial equivalent of celestial black
holes?
Think
about it. How can an animal gallop full speed over uneven ground,
screech to a halt mere inches from a fence, easily clear high hurdles,
step over rocks and logs, or nudge a friend gently, with inadequate
depth perception? Put that way, it sounds rather silly, does it not?
Yet, no small number of horse people (even equine professionals) persist
in their beliefs that horses do not possess this visual ability.
This
notion stems from the idea that because horses have laterally placed
eyes, they have no binocular vision, hence, no depth perception. This
is wrong. Firstly, horses do have substantial overlap in their fields
of view and secondly, some depth perception exists even when only
one eye is used.
Let
us look once more at the scientific evidence. In one study, horses
were tested for recognition of depth in pictures using the famous
visual illusion, the Ponzo Effect. “The sense of depth is conveyed
by a flat image when lines that appear as separate at the edge of
the picture converge near the center,” explains Timney. Horses
had to choose between a photograph showing strong depth cues –
converging railroad tracks – and a photograph that lacked obvious
cues. Overwhelmingly, the horses went for the railroad tracks. “Not
only could they recognize depth in pictures,” Timney says, “but
they also must be susceptible to the same kinds of visual illusions
that we perceive.”
Timney
next investigated the role of binocular vision in the perception of
depth with the use of random-dot stereograms – pairs of pictures
that look like flat patterns of random dots when viewed normally.
However, when these pictures are tinted and viewed through red and
green filters (remember those weird 3D glasses at the movies?), a
shape emerges to individuals with stereoscopic capability. Amazingly,
when horses wore red and green goggles, they behaved as if they, too,
saw depth in such pictures. “Apparently, horses have many of
the same depth-detecting skills that we have,” Timney says. “They
have true stereoscopic vision, despite having lateral eyes.”
Why
do horses avoid puddles? Perhaps they simply do not want to get their
feet wet.
Interocular
Transfer: Seeing eye to eye.
One
of the purposes of the Equine Research Foundation is to test the validity
of myths in the horse world. One heard time and again involved the
idea that horses cannot recognize with one eye objects they had previously
seen only with the other eye (such as when returning from a trail
ride). Hence, they spook. Reasoning along the lines of the two sides
of the brain not being connected, thus, no transfer of information,
was thrashed about by layperson and veterinarian alike.
This
would make the horse rather bizarre because all placental mammals
have a corpus callosum, the structure in the brain connecting both
hemispheres. But the presence of this structure alone is not enough
to prove transfer, just as the presence of cones does not necessarily
confirm colour vision. To find out for sure, the ERF ran a series
of interocular transfer tests using several different figures. Horses
were trained to select one of a pair of black shapes (say, a circle
instead of a rectangle) with one eye blindfolded. Once they reliably
chose the circle – a learning task that could take hundreds of
repetitions – the blindfold was switched to the other eye. The
horses chose the circle immediately, clearly demonstrating that they
recognized the object previously only seen with the other eye.
Mental
Rotation: A mailbox is a mailbox is a mailbox. Or is it?
Dispelling
one myth does not the story end, however. Yes, the interocular transfer
study demonstrated that eye-to-eye communication was not a problem.
Nevertheless, horses still spook at things they apparently have seen
before. So, same question, different explanation.
Enter
theory #2: horses spook under such circumstances because they do not
recognize that an object is the same when seen from a different angle.
A mailbox, for example, exhibits certain features when approached
from the front. But viewed from the side, it may look entirely different
to a horse. This not so far-fetched possibility was what our summer
participants helped investigate during part of their learning vacation
at the Equine Research Foundation. After raiding toy stores in search
of ideal test items, we trained horses to always choose one of two
objects (several pairs were tested with all the mandatory controls),
presented with the front facing to the left. Next, we rotated the
objects — front to right, front facing horse but upside down,
front straight up with bottom to horse, any and all orientations.
And we wondered. Would the horses still select the same object? Could
they turn it over (or sideways) in their minds?
Although
not yet quite finished, this study has shown some remarkable results.
Notably, horses are not perfect at recognizing rotated objects. Some
rotations are simple; others prove perplexing. But overall, it appears
that horses are pretty good at identifying rotated objects, once learning
has occurred. And therein, I think, lies the key to this conundrum.
Horses need to be given time to consider their surroundings, to look
at details, to understand, to learn enough so things can later be
remembered and compared. Granted that, they do all right.
Re:
View
The
horse’s visual world is a little different from ours – a
little less colourful, a little bit blurry, but oh so much broader
and useful around the clock. The horse’s eyes look quite different
from ours but function quite similarly – with good depth perception,
eye-to-eye transfer, and rotation ability. We are alike but also different
because of what is important to us as a species. As owners, as trainers,
as handlers, as caretakers, we must consider this if we desire to
excel with horses.
Contact
information: The Equine Research Foundation offers learning and riding
vacations and internships. Participants are actively involved in the
research projects and, through hands-on experience, learn bonding
and training methods based on equine cognition, natural horsemanship,
and positive reinforcement. Participants also enjoy riding on the
beaches, through the redwoods and vineyard, and in the arena. For
information, call (831) 662-9577, visit www.equineresearch.org, or
write to Equine Research Foundation, P.O. Box 1900 , Aptos , California
95001 USA .
Evelyn
B. Hanggi, M.S., Ph.D., is an equine cognitive behaviorist and the
president and cofounder of the nonprofit Equine Research Foundation
in California . Horses have always been part of her life and, since
1992, after almost a decade of studying seals and sea lions, she has
been dedicated to the study and teaching of equine cognition, behavior,
training, care and welfare.
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