As we are reminded anytime we try to swat a fly, the problem of following erratically moving objects with our vision is a tricky one. This is partially due to having to move both our head and our eyes while staying aware of our surroundings. If we now add self-movement, aka running or chasing, to the equation, following a moving object while the environment around you moves might seem impossible. Solving this problem is crucial for animals, from hawks to ferrets, that pursue prey while also navigating through their environment. To better understand how head and eye movement align to keep a clear picture of prey, an international team of researchers – Damian Wallace, Jason Kerr and colleagues from the Max Planck Institutes for Neurobiology of Behaviour, Germany, and the Max Planck Florida Institute for Neuroscience, USA – tracked how ferret's eyes and heads moved as they chased down a ball, representing prey, in an arena.
The researchers attached cameras to the animals’ heads to record the movement of the ferret's eyes, while also recording the whole animal from above as they chased the ball being pulled by a string around the arena. The team was then able to track the movement of the eyes and reconstruct what each eye saw to recreate the ferret's point of view. In tracking the eye, a key question was where the center of the eye, known as the area centralis, was pointed. This region of the eye, present in most mammals, has a structure that provides detailed, focused vision. In ferrets, this region in each eye overlaps to form the binocular region of their vision, where they see in 3D. Initially, the researchers had expected that the ferrets would keep the prey, or ball in this case, centered in their vision. However, the ferrets pointed the center of their vision in the direction that they intended to travel using synchronised eye and head movements.
With this surprising result, the researchers wondered why the ferret looked at the direction in which it intended to go. Having previously reconstructed what the ferrets saw, they were able to test what the ferrets’ eyes would see if the head and eyes did not move together. They found that the ferrets focused their optic flow – the way that edges of images appear to move away from you as you move through the world – on the direction where they were headed to avoid the ill effects of motion blur when following prey. This is the same strategy that athletes use when tracking an object, such as a ball, while running, keeping their gaze stable to maintain clear vision despite rapid movement.
However, the team realized that the ferrets were presented with a problem when the eyes move quickly back and forth – known as a saccade – to shift their gaze and gather information. These eye movements cause their vision to blur, which loses information about the environment. It turns out that the ferrets move their heads during and after eye movements to compensate for blurring in the image. This not only keeps the picture of the surroundings sharp by stabilizing the image, but also reduces motion blur.
The researchers then wondered what other animals might use these strategies. To find out, they used the same camera tracking setup to look at head and eye movements of tree shrews, mice and rats, and found very similar patterns. This suggests that this visual strategy is fundamental to keeping a sharp eye on both prey and surroundings in mammals while on the move.
