Last updated May 2, 2018 at 9:54 am
How to maintain spatial perception when your eyes moves independently of each other.
How do you keep a steady gaze when your eyes are moving relative to the world? Animals have a variety of ways of doing this, with the most internet-famous being birds, especially chickens, who have their own inbuilt image stabilisation system where they try to keep their head in the same spot regardless of where their body is.
Doing this with two eyes that face forwards is difficult enough, but what about when it comes to the mantis shrimp? They can not only move their eyes side-to side and up and down, they’re also able to rotate their eye by rolling it around the axis of their eye-stalk.
To make it even more confusing, the left and right eyes can move independently of each other, so that one can be oriented horizontally and the other twisted to be at right angles.
Previous work has shown that this eye rolling is important in allowing the mantis shrimp to see polarised light.
“Like other animals, mantis shrimp do make stabilising side-to-side movements that help keep their vision steady as they move through the world, but we found that even while stabilising in the horizontal direction, they can’t resist rolling their eyes” says Ilse Daly from the University of Bristol, who has just published a new study on mantis vision in Proceedings of the Royal Society B.
“This is completely counter-intuitive; the whole point in stabilizing gaze is to keep the appearance of the world around them steady, but by rolling their eyes ‘up’ suddenly becomes ‘sideways’ and things get complicated” she says.
Daly found that no matter what position they’d rolled their eyes to, or how quickly they were rolling, the mantis shrimp were able to reliably track sideways motion.
Following this surprising finding, she next wanted to test to see how the mantis shrimp would respond if the world started to roll around them. Would their eyes roll to track the motion?
“In humans, such a stimulus would induce severe vertigo, as visitors to certain theme parks may have experienced with rides which challenge people to walk through a tunnel along a solid, fixed gangway while the walls of the tunnel rotate around them – which is nearly impossible to do without falling over.” says Daly.
“We expected that, in response to the world around them apparently twisting, mantis shrimp should roll their eyes to follow their surroundings. They did not.
“The mantis shrimp visual system seems entirely immune from any negative effects of rolling their eyes. Indeed it appears as though rolling has absolutely no effect on their perception of space at all: up is still up, even when their eyes have twisted sideways.”
This means that the mantis shrimp might have a neural arrangement of their motion detection circuits that is unique to them among the whole animal kingdom, that allows them to detect motion accurately despite how their eyes are rolled, and understanding it might have applications to how we program robots to make sense of their visual worlds.
“Many of the issues that computer vision systems have are shared with biological vision systems.” says Daly, “it seems only natural to look towards animals for the solutions; after all, they’ve had millions of years of evolution to get it right!
“Mimicking the motion detection circuitry of the mantis shrimp eye could be valuable for any robotic vision system that needs to maintain visual stability whilst moving with complex or unpredictable trajectories.”
The next set of experiments Daly will be doing will try and understand exactly how this unique motion detection system works and how it lets mantis shrimp see the world clearly through their constantly twisting eyes.
“I have the feeling that despite all our scientific understanding, we can never really know what the world looks like to a mantis shrimp, and I think that feeling of complete unfamiliarity is what is so wonderful about working with them.”