How do predators use their vision to both navigate through the terrain whilst tracking prey running for its life? Pursuing prey through a complex environment is a major challenge for the visual system as not only do the prey constantly change direction, sometimes in the opposite direction to the pursuer, but running after something evokes self-induced motion-blur which degrades vision. In a study, published in Current Biology, researchers reconstructed the visual fields of freely moving ferrets as they chased a fleeing target. They discovered that the eye saccades, like those that normally track objects when sitting still, aligned the motion of the environment, or optic flow, to the retinas of the eyes while running, instead of tracking the fleeing target. This enabled the reduction of motion blur in the high-resolution retinal specialization, allowing mammals to have clear vision of the environment that they are coursing through.
Saccades do not fixate the target being chased
When sitting still, very rapid coordinated eye movements, saccades, are used to track objects of interest such as a moving object on a screen. But what happens when an animal is running at top speed chasing after prey that is making fast turns to avoid capture? Using head-mounted cameras, a team of researchers from the Max Planck Institute for Neurobiology of Behavior (MPINB) and the Max Planck Florida Institute for Neuroscience measured head and eye rotations in freely moving ferrets to reconstruct the visual fields of both eyes. When running in straight lines after the target, with the target directly in front of the animal's head, the target's position was directly on top of the retinal specialization that has the sharpest vision, which was unsurprising. As the ball deviated its course and the animals turned to chase it, saccades were elicited and the target's position was still held directly on top of the retinal specialization. Surprisingly, digitally removing either eye saccades or head rotations did not significantly change the target's position on the retina. It was still clustered around the area of the sharpest vision. This suggested that saccades do not serve to align the area of sharpest vision and the target during pursuit as assumed. So, what are the saccades being used for?
Saccades align the world motion to the retina
To solve this, the researchers looked at the pattern of motion of the environment, or optic flow, on the retina and showed that the saccades directed the retinal specialization to point in the animal's intended direction of movement, which is also the area of least motion blur. This was independent of the target, because the saccades also occurred when the target was absent. "It is very hard to track something that is erratically running for its life, especially when you are also running, but it is easier to predict your own motion and then counteract that motion to enable a clear view of where you are going, the visual systems of mammals seem to have exploited this" explains Damian Wallace, scientist at MPINB.
A generalized mechanism
The same synchronized head and eye movements underlying this were also found in freely moving tree shrews, rats and mice, suggesting a generalized mechanism in mammals. The precisely timed mechanism allows the animal to react very flexible to erratic changes in direction, for example when the prey makes a sharp turn trying to evade capture.
