Rats' eyes move in a manner unlike most other mammals including humans, according to a new study of the animals released in the May 26 issue of Nature. Rats also keep a careful watch on the skies above them at all times, according to the group that reported the unexpected findings.
Researchers from the Max Plack Institute for Biological Cybernetics in Germany used high-speed cameras to record the eye movements of rats. What they found is that the highly-intelligent rodents could move both eyes independently from one another, both horizontally and vertically.
Eye movement also appeared to be coordinated with the position of the rat's head to provide the largest field of view possible at all times, especially above them. This allows rats the ability to keep the sky above them constantly in view, which researchers propose is an adaptation to keep them safe from predators in flight.
"When the head points downward, the eyes move back, away from the tip of the nose. When the rat lifts its head, the eyes look forward... If the animal puts its head on one side, the eye on the lower side moves up and the other eye moves down," Jason Kerr from the Institute said.
Tiny cameras, weighing about the same as a dollar bill, were fitted to the heads of the animals. These devices were able to record the super-fast movement of the animal's eyes. Another device measured the position and alignment of the head of the rats, and combining information from the tools allowed the researchers to accurately determine what each rat was seeing at all times.
By recreating the field of view seen by each eyes, researchers were able to conclude that rats do not see in the same manner as humans. While our brains (and those of most mammals) process images from both eyes into one coherent image, the eyes of rats deliver two wholly-independent pictures to their nervous system.
"Humans move their eyes in a very stereotypical way for both counteracting head movements and searching around. Both our eyes move together and always follow the same object. In rats, on the other hand, the eyes generally move in opposite directions," Kerr said.
To put this in perspective, human eyes must be aligned within less than one degree of each other, or double-vision could result. The angle between the eyes of rats, the study found, could be as much as 40 degrees horizontally or 60 degrees vertically. This gives rats the ability to make their way around the world while constantly keeping both eyes toward the sky.
This was the only part of the rat's visual field that was kept under constant surveillance by the animals, leading to the conclusion that this adaptation was due to birds.