NIGHT VI­SION: SUR­PRIS­INGLY SLOW ADAP­TA­TION

EQUUS - - Conversati­ons -

We’ve all heard that horses can see in the dark, so they have no trou­ble jump­ing at dusk or load­ing onto the trailer at dawn. But take a look at your horse’s pupil some­time. See how much longer and larger it is than a hu­man pupil? Large, hor­i­zon­tal pupils take in more light across a wider range of vis­ual an­gle. Hav­ing en­tered the eye, that lu­mi­nos­ity fil­ters to­ward the retina, a patch of cells that changes par­ti­cles of light into neu­ral im­pulses. Those im­pulses are then routed to ar­eas of the brain that in­ter­pret their mean­ing.

Horses also have irides­cent col­la­gen fibers in an up­per area of the eye­ball called the tape­tum. Th­ese fibers re­flect light from the ground into the eye­ball, al­low­ing the horse to see as he moves in dark­ness. Hu­man eyes have a sim­i­lar mech­a­nism, the choroid coat, but it is not as large or strong as the tape­tum and can­not re­flect light to an equiv­a­lent de­gree. In both species, th­ese fibers be­come vis­i­ble when re­flected in a cam­era flash or head­light. In horses, the eye-shine varies among green, yel­low or blue de­pend­ing on a horse’s color and age.

With large pupils and a re­flec­tive tape­tum, horses have pretty good night vi­sion---enough to wan­der from hay bin to wa­ter trough in the dark and no­tice move­ments in the shrubs. A horse’s night vi­sion is more acute than a per­son’s, but it’s still not sharp enough to dis­cern de­tails, hop crosspoles or load into trail­ers com­fort­ably.

Then there’s the real rub: It takes a horse’s eyes much longer than a per­son’s to adapt to dark con­di­tions. Hu­man eyes re­quire about 25 min­utes to ad­just from bright sun­light to dark­ness. Equine eyes need 45 min­utes, al­most twice as long. So, upon en­ter­ing a dim build­ing from day­light, your horse will be blinded by dark­ness long af­ter your eyes have adapted. Af­ter

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