A rat's-eye view of the world, in two parts

Domestic pet rat
One idea that we’re grappling with at the moment is that rats have foregone fusion. As humans, we fuse our two eyes into one world view; this has many advantages for us, allowing us to do very fine, detailed manipulation in front of our faces.
Dr Jason Kerr
With the use of miniature cameras, a multi-disciplinary team finds that rats forsake a unified view of the world for the ability to constantly watch the sky...

Researchers from the Max Planck Institute for Biological Cybernetics in Tübingen have found that rats have a rather surprising way of looking at the world. Using specially-designed miniature cameras and tracking devices, the multi-disciplinary team discovered that freely moving rodents moved their eyes in opposite directions in both the vertical and horizontal planes, depending on the position of their head.

Rats have a wide panoramic view due to the placement of their eyes at the sides of their head, but 3D vision requires a certain amount of overlap between the visual fields. With tiny cameras weighing around one gram, the scientists were able to record the rapid eye movements of rats. They were also able to reconstruct their line of sight. They found that when the head tips forward, the eyes tend to move back. Conversely, when the rat lifts its head the eyes move forward towards the nose, with the appearance of being crossed.

In an interview with ScienceOmega.com Dr Jason Kerr, leader of the Network Imaging Group at the Institute and senior author of the paper, which appears in Nature, explained that these unusual eye movements are such that a binocular view of the region above the rat’s head is maintained at all times…


What led you to study eye movement in rats?
We were interested in cortical circuits and the underlying principles of vision in the rodent. We realised very early on that we would have to work with freely moving rodents in order to take advantage of the full array of behavioural strategies the animal uses to interact with its environment as well as the full suite of brain processes involved, such as the vestibular system and corollary signal. Also, some of the biggest discoveries made on how the brain operates have come from place cells and grid cells, which were first discovered in freely moving animals.

We needed to know how the eyes move because ultimately we want to understand what the animal is looking at and how this is deconstructed or reconstructed in the brain. The existing literature basically said that rats don’t move their eyes. We were a bit nervous about this and said, ‘We’d better just have a look’. What we saw was unbelievable – we thought it was a mistake – so we started studying it.

How far removed is rat vision from human vision?
One of the paper’s reviewers made an interesting point; 40 per cent of mammals are rodents, so it may be that monkeys’ and humans’ conjugate eye movements are the real outliers, which is a fascinating idea. The problem is that eye movement measurements have been almost exclusively taken in monkeys and humans; they’ve had the most attention for obvious reasons.

When we got over the egocentric point of view – rats are not little humans or monkeys – we started looking at the literature on all the other animals in the animal kingdom. Rats seem to move their eyes more like some birds, so we’re pursuing that as an idea.

How does this suit the evolutionary pressures these rodents have been subject to?
Rats are generalists – they can survive and thrive almost anywhere. Like other rodents, they have a range of well-developed tools for perception, such as their whiskers, hearing and olfactory senses. However, vision is really the only sense that can allow them to detect predators from a distance. If you start whisking your whiskers against a predator it’s a bit late.

How important was the technology you used in allowing this study to take place?
It was fundamental. With the advent of cell phones and so on, this technology would seem straightforward, but cell phone cameras only transmit millimetres onto a board, so there are no issues with capacitance and electrical interference. We had to make them super-light, super-fast and able to transmit data over large distances, plus we had to synchronise both cameras. We had to synchronise the tracking cameras on the head to follow the animal around in three dimensions. Then we had to assemble everything in the three-dimensional world; you have to track the animal’s head, and track the eyes relative to their head, in order to project where the eyes are pointing in space.

First author Damian Wallace is a zoologist, so he does all the animal wrangling and experiments. First co-author Juergen Sawinski is the physicist who built the cameras, electronics and the data transfer cluster heat tracking equipment. Another of the authors, David Greenberg, put everything together mathematically. We couldn’t have done it any other way.

Does the rat’s visual system disadvantage it in any way, despite the benefit of being able to spot aerial predators?
One idea that we’re grappling with at the moment is that rats have foregone fusion. As humans, we fuse our two eyes into one world view; this has many advantages for us, allowing us to do very fine, detailed manipulation in front of our faces.

The ‘disadvantage’ for the rodent is that they don’t fuse, but we cannot tell to what extent this does actually disadvantage them. Although we now know how they move their eyes, we don’t know how they see or what they see per se.

Rats’ optics mean that they have a huge panoramic view, with over 200 degrees of acceptance into each eye, with an overlap in the middle. For every animal that is preyed upon, there’s a trade off between panoramic surveillance and binocular overlap. Now we are curious as to what binocularity is to a rat; why do they need it? Why don’t they have eyes that are completely separated with a little bit of overlap rather than this huge overlap in the middle?

You talk about these new questions, but what will your future research be focussing on?
The same question that we originally set out to answer! If anything we’ve got further away from that, although things got very interesting along the way. Fundamentally, we’re trying to understand how these animals stabilise their perception. With two eyes roving around the world like that, how does the rodent brain keep track of the information it’s receiving?

Is it lateralised, like a bird brain, so they can block one eye off and focus on the other? Do they somehow piece it back together in some higher order process like a frontalised field, where they can keep track of both eyes and reconstruct their view into a kind of grid? There may be some other mechanism that we don’t know of. The way we’re attacking the problem is to look at anatomy, because it will give us insights into the way the whole system is ‘wired up’.

At the moment, we are designing experiments to monitor what rats actually look at when they make decisions and how this then relates to neural activity.

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