On the other side of the spectrum are the weird rectangular pupils of goats, sheep, horses and some toads. "This is the right arrangement to maximize stereo and blur as cues to distance simultaneously," Banks said. In other words, the ideal shape is narrow horizontally and wide vertically - precisely the arrangement of a cat's eye. And to maximize blur for horizontal lines, the pupil must open wide from top to bottom.
To maximize blur, the pupil must open wide. To judge horizontal distances, though, cats and other slit-pupil predators likely use blur, Banks said. Thus, Banks said, the vertical pupil provides the best view for stereopsis.
The other method, blur, takes advantage of the fuzziness of objects behind and in front of the spot on which you're focusing.Īs it turns out, the side-to-side displacement used in stereopsis is easier to gauge using vertical lines and contours than it is with horizontal ones. That "jump" you see is the distance used in stereopsis.) (Hold out your finger, focus on it and close each eye in succession. In one method, stereopsis, the brain compares the distance between the two images returned by each eye to gauge depth.
There are two ways to gauge depth without moving. Ambush hunters need to be very good at gauging depth so they can effectively leap out at their prey. The reason for this correlation most likely has to do with the mechanics of the eye, Banks told Live Science. Ambush predators, like many cats and snakes, were most likely to sport vertical-slit pupils, particularly when those animals were active at night.
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