The Pirate's Point of View

Hold up your finger a few inches away from your face. Focus on it, then close one eye. Now switch to your other eye. Did your finger’s position jump? Switch back and forth quickly and see your finger move positions. Open both eyes again and focus on your finger; it will return to a single position in between the two images created by each individual eye. This simple test demonstrates the visual work our brain does so that we can see our world. 

That said, our brain sees two images at once, one from each eye. When we focus our vision, our brain combines the light from each eye to create one image. One process that likely uses this technique is our highly accurate estimate of how far objects are, called depth perception. To test the importance of using both eyes for depth perception, try shooting a basketball with one of your eyes closed. If you have ever worn an eyepatch (like those worn by pirates), you may understand how hard it can be without the images from both eyes.

To understand more about depth perception, we decided to look through the eyes of a mouse. Obviously, mice are a little different from humans. Their eyes are on the side of their head instead of at the front, and they also don’t focus like humans do. Although there are these differences, studying the way they see tells a lot about how we see as well. The visual parts of their brain are highly similar to ours, just scaled down to be much smaller, or mouse-sized. Mice also use depth perception – while navigating the forests, fields, and city train stations in which they live, they must be aware of where things are in the world around them.

The purpose of this study was to figure out how mice use their eyesight when they need to judge distances. To do this, we placed mice in an arena with a platform for them to climb. At the top of this platform, we put up a barrier so they couldn't see what was in front of them while we put down a landing platform on the other side. When we removed the barrier, the mice had to jump across the gap. If they made a successful jump, they were rewarded with a sip of water and a tiny tortilla chip. Since the mice couldn't see the landing platform before the barrier was taken away, they had to use their ability to judge distances. It took the mice a couple of weeks to fully learn this task and jump gaps as long as 22 centimeters. That's like us jumping about 12 feet forward. It turns out that mice are really good at jumping!

There were different landing platforms and different jump distances so that mice couldn’t memorize its size. Overall, mice performed well across all platform sizes and jump distances. To double-check that mice use their vision for this task, we had them perform the same behavior in total darkness. Like we expected, the mice refused to jump when there was no light, suggesting they do need their vision. This may seem obvious to researchers, but it is important to always consider the possibility that other animals don’t see like we do.

Next, we wanted to determine whether the mice still needed both eyes to estimate depth as we do and perform the jumping task. To accomplish this, we kept one of their eyes closed, then had the mice re-perform the task. We found that mice performed the task just as well as when they had both eyes open, changing their jump distance depending on gap size. These findings suggest that mice don’t need images from both eyes to accurately perceive depth and jump various distances – one eye is sufficient. Somehow, a pirate wearing an eyepatch is able to estimate the distance to a treasure-containing island, and this task shows that mice may be able to do this, too, with the treasure being a tortilla chip on a landing platform.

We then wanted to test whether their strategy for depth perception may have changed. We recorded the movement and position of the mouse leading up to them jumping under both two- and one-eye conditions. We identified several categories of movements that the mouse performed leading up to the jump. For example, one category was when the mice nodded their head, and another was a circular head movement.

We found that the mice moved a lot more when they had one eye closed. This resulted in mice taking a longer time deciding how to jump, and moving their heads much more prior to the jump. If we move our head around, we can estimate how far an object is, since closer objects will move more, and farther objects will move less. 

Finally, we tested for the necessity of a brain region important for vision while the mice performed the jumping task. Our technique allows for light beams to temporarily change brain cell activity, turning it off like a light switch. We found jumping performance to be significantly worse on trials when the visual brain area was turned off, suggesting it is highly important for successful depth perception.

The purpose of this study was to determine how mice perform depth perception using their vision, with the ultimate goal of using mouse vision as a model for how we, as humans, may perceive the world around us. We found that mice successfully learned how to jump changing distances, and that they used their vision during the task. We found that closing one eye didn’t impact task success, but that mice took longer to complete each trial, and moved more prior to completing the jump, meaning they changed their strategy for depth perception. Finally, we found that temporary shutdown of the visual brain area disrupted task performance. 

Overall, these findings support the conclusion that mice use vision for processes such as depth perception like we do, but that they may be able to adapt their strategy so that only one eye is necessary. In humans, perhaps this work can be used to help those that may only have one working eye. We also found a brain area involved in vision to be important for this behavior. Future work should dive deeper into why it is important, and how other brain regions with which it connects are involved. Vision, after all, doesn’t just occur at the eyes – without the brain, it wouldn’t make any sense. 

Somehow, like a boat full of eyepatch-wielding pirates is able to traverse the seas and capture treasure, mice are able to jump varying distances accurately with one eye covered to capture a snack. If you ever have to wear an eyepatch, see how your behavior might change as you live your own respective life. These jumping mice may support the notion that, although different and a bit slower, single-eye vision is sufficient for certain aspects of our survival. 

Written By: Emmalyn Leonard

Academic Editor: Biologist

Non-Academic Editor: Lawyer

Original Paper

• Title: Distance estimation from monocular cues in an ethological visuomotor task

• Journal: eLife

• Date Published: 20 September 2022

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