Vision in horses: More than meets the eye

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Horses have dichromic vision, meaning they have a fundamentally different view of the world to people.
Horses have dichromic vision, meaning they have a fundamentally different view of the world to people.

Our understanding of how horses view the world is not yet 20/20, but researchers are beginning to get down to the fine print.

How do horses see the world?

It’s a question horse owners may occasionally consider as they go about their daily lives, enjoying the benefits of their finely tuned binocular vision and the rich tapestry of reds, blues, greens and yellows we see in our world.

Horses, however, have followed a very different evolutionary path. The world they see is different to our own, but in what way?

Scientists have been researching that question for at least 65 years.

A 1942 study observed that horses, like other ungulates, were active during the day, at dusk, dawn, and during the night. Their eyes, it was found, were designed to provide high sensitivity for vision in dim light and better vision still under higher light levels.

Before looking at horses, we need to understand something about the eye itself.

Eyes are believed to have started evolving some 540 million years ago. They have evolved in different ways to meet specialist needs throughout the animal kingdom.

In insects, for example, we find compound eyes. Horses and humans have what could best be called a camera-type eye.

Light passes through the lens and focuses the image on the retina at the back of the eye – much like a camera lens throwing an image on to a piece of film.

The eye’s curved retina is linked to the optic nerve, which transmits information about the visual environment to the brain.

How we see the world, left, and how a horse would see the same images. The horse's view of the world has also been blurred to reflect their lower levels of sharpness (acuity), as identified in 1992 research. The horse's view has been likened to that of a colour-blind person with problems in the red-green spectrum, but with variations in gray regions. The researchers say the illustrations give only a sense of the colour world of the horse. There are many differences between the horse and human visual systems, and the way in which the eye "sees" colour is but one of them. These differences cannot be captured in this simulation.
How we see the world, left, and how a horse would see the same images. The horse’s view of the world has also been blurred to reflect their lower levels of sharpness (acuity), as identified in 1992 research. The horse’s view has been likened to that of a colour-blind person with problems in the red-green spectrum, but with variations in gray regions. The researchers say the illustrations give only a sense of the colour world of the horse. There are many differences between the horse and human visual systems, and the way in which the eye “sees” colour is but one of them. These differences cannot be captured in this simulation.

Not all cones are created equal. In humans there are three types of cone cells – each one is most sensitive to a different wavelength (colour) of light. Inside each cone is a photopigment, and it is this pigment that gives a particular cone its characteristic sensitivity.Retinas are lined with two types of light-sensing cells, called rods and cones. Rods look after vision in low light, and cones handle colour. This is why colours are barely, if at all, distinguishable to us in low light, as the cones are not sensitive enough in these conditions.

Horses – in common with pigs, goats, cows, sheep and deer – have only two different cone types on their retina, providing them with what scientists call dichromatic vision.

Primates, which include humans, are the only placental mammals to have three cone types at the back of their eyes, called trichromatic vision.

So, the way that humans and other primates see is fairly unique in the world of mammals. However, 1993 research revealed that birds go one better, having four cone types (tetrachromacy), as well as ultra violet (UV) sensitivity.

Dichromatic mammals such as horses have one type of cone most sensitive in the middle-to-long wavelength of the light spectrum and a second cone with maximum sensitivity to short wavelengths – either UV, as found in many rodents, or a more traditional short-wavelength-sensitive cone.

One of the most detailed studies into equine vision was carried out in 2000 by researchers based in Wisconsin, in the United States. Their findings were published the following year in the Journal of Vision.

The team used a flickerphotometric electroretinogram (ERG) in their study – a device not unlike an electrocardiogram (EKG) which measures the electrical activity of the heart to provide an assessment of its performance.

Six ponies, all with healthy eyes, were used in the trial.

The animals were first anaesthetised, then electrodes were placed gently on the eye and just beneath the skin as part of the monitoring process.

Different-coloured light was then shone into their eyes and the change in electrical response of the retina was measured. The stronger the response, the more sensitive the cones were to that colour.

The team found evidence, as previous researchers had, for two cone types in the horse that provide the basis for their dichromatic colour vision.

But by analysing the strength of the response to the different colours, the researchers, lad by Dr Joseph Carroll, of the Department of Ophthalmology at the Medical College of Wisconsin, were able to paint perhaps the best picture yet of how horses might see the world.

Behaviourial experiments by other researchers had already proved the ability of horses to discriminate colours. However, what colours were they actually seeing?

“People often wonder what the visual world is like for an animal whose eyes and nervous system are different from our own,” Dr Carroll says.

“Because of its special relationship with humans as a companion and a form of transportation, as well as a beast of burden and source of recreation, there are probably few animals that have more often been the subject of curiosity about alternate sensory worlds than the horse.”

Using data gathered in the study, the research team believe they derived a sense of what the daytime colour experience of the horse might be like.

People, with their trichromic vision, see four basic unique colours: red, green, blue, and yellow, as well as a range of intermediate hues.

The research showed that horses, with their dichromatic vision, cannot distinguish red.

“It’s not that they don’t ‘see’ red,” says Dr Carroll. “They just can’t discriminate in the red/green region of the spectrum. They are also slightly less sensitive to red light.”

Human dichromats who have inherited red-green colour vision defects – commonly referred to as colour blindness – indicate that instead of having four basic colors, they have only two hues, the ones most similar to blue and yellow.

“One of the most dramatic differences believed to differentiate the visual world of the dichromat from the trichromat is that, for dichromats, there are no intermediate hues,” Dr Carroll says.

Two colour wheels show the differences in colour perception between people, with their trichromic vision (left) and the dichromatic colour vision of horses. Dichromic vision sees a big reduction in the number of different colors seen.
Two colour wheels show the differences in colour perception between people, with their trichromic vision (left) and the dichromatic colour vision of horses. Dichromic vision sees a big reduction in the number of different colors seen.

“For a dichromat, when colours from the two ends of the spectrum are mixed, rather than getting an intermediate hue, the result is either achromatic (white or gray) or a desaturated version of one of the two basic hues – that is, a pastel blue or yellow.”

Using the information from the study, Dr Carroll’s team was able to produce a colour wheel showing how a horse’s eye perceives colour, compared with people.

Dr Joseph Carroll
Dr Joseph Carroll

Colour perception is, however, only part of the equation.

A 1992 study showed that horse vision is not as sharp as human sight. If good human vision is 20/20, a horse rates as 20/60. This means that details a person with 20/20 vision can see at 60m are only visible to a horse at 20m. The findings were obtained by measuring brain activity when horses saw different sets of lines on a television screen.

The series of lines in the accompanying illustration demonstrate the difference between 20/20 and 20/60.

Dr Carroll’s team combined both sets of findings and doctored two typical daytime scenes to give some insight into what a horse might be seeing.

It is not, however, an exact science.

There are many differences between the horse and human visual systems, Dr Carroll points out. These include the positioning and optics of the eye, as well as differences in the retina and the brain. All these factors would contribute to differences between horse sight and our own.

Many questions remain.

Dr Carroll sees scope for further measurement of equine colour discrimination, as well as other visual skills, such as motion, depth and spatial abilities.

Why then, have primates ended up with three cone-types and other mammals only two?

“This is pretty well known,” Dr Carroll says. “Old World primates, including humans, evolved trichromacy as a result of a gene duplication some 40 million to 60 million years ago in a primate ancestor.”

Why should red be the colour that horses have trouble distinguishing?

It probably had a lot to do with the environment in which the species evolved. “It bears thinking about how much long-wavelength light [the red end of the spectrum] that an animal is exposed to.”

Dr Carroll says identifying the cone pigments in any given species is an important first step toward understanding its colour-vision ability, but he cautions that it is an incomplete picture.

Research into horse vision has also been conducted in New Zealand. Tania Blackmore, when a graduate student in the Department of Psychology at the University of Waikato, designed a research study for her Masters thesis.

Her test was behavioural-based, involving four horses who were incentivised to distinguish between a colour and grey. They had to achieve an 85% success or better for a pass.

Her study found that horses could definitely see the difference between blue and grey, between yellow and grey and between green and grey. They could tell red from grey to some extent but they found this much more difficult, as would be expected when considered in tandem with Dr Carroll’s findings.

 

FOOTNOTE: Research led by Dr Joseph Carroll, entitled “Photopigment basis for dichromatic color vision in the horse” was published in the Journal of Vision in 2001. The other researchers involved in the study were Christopher J. Murphy, Maureen Neitz, James N. Ver Hoeve, and Jay Neitz.

First published in May, 2008

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