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Authors: Temple Grandin
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why.
Another huge difference between animals and people is that most animals have panoramic vision. The eyes of prey animals like horses, sheep, and cows are set so far apart that they can literally see behind their heads. Thatâs why some hansom cab horses wear blinkers; theycan see everything going on behind them, and they get distracted. Most racehorses donât wear blinkers for the same reason: their trainers want them to know exactly where the horses behind them are, and how fast theyâre moving.
Prey animals donât have perfect 360-degree vision, although they come close. Thereâs one small blind spot directly behind a cow or horse that you have to be careful not to sneak up to. The animal canât tell what you are, and he might get scared and lash out and kick you. Prey animals also have a small blind spot directly in front of their heads because their eyes are set so far to the sides.
Even though their eyes are so far apart, prey animals do have depth perception, though it seems to be different from ours. We use binocular vision, which means each eye is seeing the same thing from a slightly different angle. When our brains combine the angles, we get our sense of depth.
Prey animalsâ eyes are so far apart that a lot of researchers have assumed their left eye was seeing something completely different from the right eye, so they couldnât have binocular vision. But theyâve tested this in sheep, and sheep do have at least some binocular vision. We know this because sheep can see the cliff in visual cliff experiments. In the original visual cliff studies the experimenters put a baby on top of a table covered in a sheet of glass thick enough to crawl on. Directly underneath the glass there was a checkered surface that, midway across the table, suddenly dropped off way below the glass surface. It was a visual cliff, not a real one, so the baby couldnât actually fall over the edge if he crawled out over the drop-off. Very young babies will refuse to crawl over the cliff even if their mothers stand on the opposite side of the table and call them. They can see the cliff, and they instinctively know itâs dangerous. It turns out that sheep wonât walk over the cliff, either, which means they have to be seeing the difference in depth. (On the other hand, sheep donât appear to have depth perception while theyâre moving, only when they stand still.)
Youâve probably seen bulls in bullfights lower their heads before they charge the matador. Border collies do the exact same thing when theyâre herding sheep. They lower their heads below their shoulders and stare at the sheep. They do this because their retinasare different from ours. The human retina has a fovea, which is a round spot in the back of the eye where you get your best vision. Domestic animals and fast animals who live on the open plains like antelopes and gazelles have a visual streak instead of a fovea. The visual streak is a straight line across the back of the retina. When you see an animal lower its head to look at something, itâs probably getting the image lined up on its visual streak. Most experts think the streak helps animals scan the horizon.
Researchers have also found that of the meat-eating animals that have been tested so far, the two fastest animalsâthe cheetah and the greyhoundâalso have the most highly developed visual streaks. Their visual streaks are dense with photoreceptors, giving them extra-acute vision. To test visual acuity you can use a bar code design. The more acute your vision, the tinier a bar code you can look at, from a greater distance, and still see the stripes as separate rather than as a gray square. Animals with super-acute vision can also see separate grains of sand on the beach.
S EEING C OLOR AND C ONTRAST
A third area where animals and people diverge is in the ability to see color and contrast. At least ten of the eighteen distracters are
Another huge difference between animals and people is that most animals have panoramic vision. The eyes of prey animals like horses, sheep, and cows are set so far apart that they can literally see behind their heads. Thatâs why some hansom cab horses wear blinkers; theycan see everything going on behind them, and they get distracted. Most racehorses donât wear blinkers for the same reason: their trainers want them to know exactly where the horses behind them are, and how fast theyâre moving.
Prey animals donât have perfect 360-degree vision, although they come close. Thereâs one small blind spot directly behind a cow or horse that you have to be careful not to sneak up to. The animal canât tell what you are, and he might get scared and lash out and kick you. Prey animals also have a small blind spot directly in front of their heads because their eyes are set so far to the sides.
Even though their eyes are so far apart, prey animals do have depth perception, though it seems to be different from ours. We use binocular vision, which means each eye is seeing the same thing from a slightly different angle. When our brains combine the angles, we get our sense of depth.
Prey animalsâ eyes are so far apart that a lot of researchers have assumed their left eye was seeing something completely different from the right eye, so they couldnât have binocular vision. But theyâve tested this in sheep, and sheep do have at least some binocular vision. We know this because sheep can see the cliff in visual cliff experiments. In the original visual cliff studies the experimenters put a baby on top of a table covered in a sheet of glass thick enough to crawl on. Directly underneath the glass there was a checkered surface that, midway across the table, suddenly dropped off way below the glass surface. It was a visual cliff, not a real one, so the baby couldnât actually fall over the edge if he crawled out over the drop-off. Very young babies will refuse to crawl over the cliff even if their mothers stand on the opposite side of the table and call them. They can see the cliff, and they instinctively know itâs dangerous. It turns out that sheep wonât walk over the cliff, either, which means they have to be seeing the difference in depth. (On the other hand, sheep donât appear to have depth perception while theyâre moving, only when they stand still.)
Youâve probably seen bulls in bullfights lower their heads before they charge the matador. Border collies do the exact same thing when theyâre herding sheep. They lower their heads below their shoulders and stare at the sheep. They do this because their retinasare different from ours. The human retina has a fovea, which is a round spot in the back of the eye where you get your best vision. Domestic animals and fast animals who live on the open plains like antelopes and gazelles have a visual streak instead of a fovea. The visual streak is a straight line across the back of the retina. When you see an animal lower its head to look at something, itâs probably getting the image lined up on its visual streak. Most experts think the streak helps animals scan the horizon.
Researchers have also found that of the meat-eating animals that have been tested so far, the two fastest animalsâthe cheetah and the greyhoundâalso have the most highly developed visual streaks. Their visual streaks are dense with photoreceptors, giving them extra-acute vision. To test visual acuity you can use a bar code design. The more acute your vision, the tinier a bar code you can look at, from a greater distance, and still see the stripes as separate rather than as a gray square. Animals with super-acute vision can also see separate grains of sand on the beach.
S EEING C OLOR AND C ONTRAST
A third area where animals and people diverge is in the ability to see color and contrast. At least ten of the eighteen distracters are
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