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Rats have “double view” of world

May 28, 2013
Courtesy of Max-Planck-Gesellschaft
and World Science staff

If you’ve ev­er ex­pe­ri­enced how hard it is to sneak up on a fuzzy but un­wanted lit­tle ro­dent, here may be your ex­plana­t­ion.

Rats see the world very dif­fer­ently from us, re­search­ers say, be­cause they’re able to keep the world above them in per­ma­nent view—but ap­par­ently can’t merge the views from both eyes in­to one im­age.

With hawks and the like trail­ing ro­dents at ev­ery op­por­tun­ity, na­ture seems to have just de­cid­ed the first abil­ity is im­por­tant enough to sac­ri­fice the sec­ond, said the sci­en­tists. 

De­scribing them­selves as to­tally sur­prised by the discovery, they found that the ro­dents move their eyes in op­po­site di­rec­tions, not to­geth­er like us, when run­ning around. Their pre­cise eye move­ment, which de­pends on the change in the head po­si­tion, pre­vents a “fu­sion” of both vis­u­al fields, the re­search­ers added.

With peo­ple, “both our eyes move to­geth­er and al­ways fol­low the same ob­jec­t,” while coun­ter­act­ing head move­ments, said neuro­bi­ol­o­gist Ja­son Kerr of the Max Planck In­sti­tute for Bi­o­log­i­cal Cy­ber­net­ics in Tü­bin­gen, Ger­ma­ny, one of the re­search­ers.

“In rats, on the oth­er hand, the eyes gen­er­ally move in op­po­site di­rec­tions.” 

Kerr and col­leagues not­ed that de­spite the dif­fer­ent ways of look­ing, the im­ages from the rats’ eyes are fed in­to pret­ty much the same brain path­ways as hu­mans.

Like many mam­mals, rats have their eyes on the sides of their heads. This gives them a very wide vis­u­al field, good for de­tect­ing preda­tors. But three-di­men­sion­al vi­sion re­quires over­lap of the two vis­u­al fields. So the rats’ vis­u­al sys­tem must meet two con­flict­ing de­mands: max­i­mum sur­veil­lance, and de­tailed bin­oc­u­lar vi­sion.

The sci­en­tists fit­ted ti­ny cam­er­as to the an­i­mals’ heads, which could rec­ord the lightning-fast eye move­ments. They al­so used anoth­er new meth­od to meas­ure the head’s po­si­tion and di­rec­tion of the head, so they could re­con­struct the rats’ ex­act line of view mo­ment by mo­ment.

“When the head points down­ward, the eyes move back, away from the tip of the nose,” Kerr said. “When the rat lifts its head, the eyes look for­ward: cross-eyed, so to speak. If the an­i­mal puts its head on one side, the eye on the low­er side moves up and the oth­er eye moves down.”

This system en­sures that the ar­ea above the an­i­mal is al­ways in both eyes’ view sim­ul­ta­ne­ously, they added.

In peo­ple, the eyes’ gaze di­rec­tion must be pre­cisely aligned; even a one-degree de­via­t­ion can cause dou­ble vi­sion. In rats, the op­pos­ing eye move­ments mean that the line of vi­sion varies by as much as 40 de­grees in the hor­i­zon­tal plane and up to 60 de­grees in the ver­ti­cal plane, Kerr and col­leagues said.

The find­ings were pub­lished on­line May 26 in the jour­nal Na­ture.


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If you’ve ever experienced how hard it is to sneak up on a fuzzy but unwanted little rodent, here may be your explanation. Rats see the world very differently from us, researchers say, because they’re able to keep the world above them in permanent view—but apparently can’t merge the views from both eyes into one image. With hawks and the like trailing rodents at every opportunity, nature seems to have just decided the first ability is important enough to sacrifice the second, said the scientists. They described themselves as totally surprised by the discovery. The scientists found that the rodents move their eyes in opposite directions, not together like us, when running around. Their precise eye movement, which depends on the change in the head position, prevents a “fusion” of both visual fields, the researchers added. With people, “both our eyes move together and always follow the same object,” while counteracting head movements, said neurobiologist Jason Kerr of the Max Planck Institute for Biological Cybernetics in Tübingen, Germany, one of the researchers. “In rats, on the other hand, the eyes generally move in opposite directions.” Kerr and colleagues noted that despite the different ways of looking, the images from the rats’ eyes are fed into pretty much the same brain pathways as humans. Like many mammals, rats have their eyes on the sides of their heads. This gives them a very wide visual field, good for detecting predators. But three-dimensional vision requires overlap of the two visual fields. So the rats’ visual system must meet two conflicting demands: maximum surveillance, and detailed binocular vision. The scientists fitted tiny cameras to the animals’ heads, which could record the lightning-fast eye movements. They also used another new method to measure the head’s position and direction of the head, so they could reconstruct the rats’ exact line of view moment by moment. “When the head points downward, the eyes move back, away from the tip of the nose,” Kerr said. “When the rat lifts its head, the eyes look forward: cross-eyed, so to speak. If the animal puts its head on one side, the eye on the lower side moves up and the other eye moves down.” Still, regardless of head movements, the eyes always move in such a way to ensure that the area above the animal is always in view simultaneously by both eyes, they added. In people, the eyes’ gaze direction must be precisely aligned; even a one-degree deviation can cause double vision. In rats, the opposing eye movements mean that the line of vision varies by as much as 40 degrees in the horizontal plane and up to 60 degrees in the vertical plane, Kerr and colleagues said. The findings were published online May 26 in the journal Nature.