Why do fish look down when they swim?

Just as you can look down at the pavement as you walk, fish look down when they swim, a new Northwestern University international collaborative study has confirmed.

The study is the first to combine simulations of the zebrafish brain, native environment and spatially varying swimming behavior into a computational model. By analyzing this model, the researchers concluded that this quirk—looking down while swimming forward—is an adaptive behavior that evolved to help fish stabilize themselves, such as when swimming against a current.

As the water moves, fish are constantly trying to self-stabilize to stay in place—rather than being swept away in a moving current. Focusing on other fish, plants or debris can give the fish a false sense of movement. The stable bed below them, however, gives the fish more reliable information about their swimming direction and speed.

“It’s similar to sitting in a train car that isn’t moving. If the train next to yours starts to pull away from the station, it can trick you into thinking you’re moving too,” said Northwestern’s Emma Alexander, the which led the study. “The visual signal from the other train is so strong that it overrides the fact that all your other senses are telling you that you are sitting still. This is exactly the same phenomenon we study in fish. There are many misleading signs of movement above these, but the most abundant and reliable signals are from the bottom of the river.’

The study will be published Nov. 2 in the journal Current Biology.

Alexander is an assistant professor of computer science at Northwestern’s McCormick School of Engineering, where she directs the Bio Inspired Vision Lab.

Back to the source

To conduct the research, Alexandros and her colleagues focused on zebrafish, a well-studied model organism. But although many labs have tanks full of zebrafish, the team wanted to focus on the fish’s natural habitat in India.

“It was recently discovered that fish respond to movement below them more strongly than movement above them. We wanted to dig deeper into this mystery and understand why,” explained Alexander. “Many zebrafish we study are raised in laboratory tanks, but their native habitats shaped the evolution of their brains and behaviors, so we had to go back to the source to investigate the context of where the organism developed.”

Armed with camera equipment, the team visited seven locations across India to collect video data from shallow rivers where zebrafish naturally live. The field team placed a 360-degree camera inside a waterproof diving case and attached it to a remote-controlled robotic arm. They then used the robotic arm to submerge the camera in the water and move it around.

“It allowed us to put our eyes where the fish’s eyes would be, so we see what the fish see,” Alexander said. “From the video data, we were able to model hypothetical scenarios where a simulated fish was moving arbitrarily within a realistic environment.”

‘Wait for me!’

Back in the lab, the team also tracked the zebrafish’s movements inside a ball of LEDs. Because fish have a large field of vision, they don’t need to move their eyes to look around like humans do. So the researchers played motion stimuli to the lights and watched the fish’s responses. When patterns appeared on the bottom of the tank, the fish swam along with the moving patterns—more evidence that the fish were getting their visual cues from looking down.

“If you play a video with moving stripes, the fish will move with the stripes,” Alexander said. “It’s like they’re saying ‘wait for me!’ In the behavioral experiment, we measured their tail beats. The more they wagged their tails, the more they wanted to keep up with the moving stripes.”

The team then extracted data from their videos and combined it with data from how motion signals are coded in the fish’s brain. They fed the data sets to two pre-existing algorithms used to study optical flow (or the movement of the world in our eyes or camera lenses).

Ultimately, they discovered that in both scenarios—in the wild and in the lab—zebrafish look down when swimming forward. The researchers concluded that fish look down to sense the movement of their surroundings and then swim to counteract it – to avoid being swept away.

“We put everything together in a simulation that showed that, in fact, this is an adaptive behavior,” said Alexander, who led the computational part of the study. “The surface of the water is constantly moving and other fish and plants are passing by. Fish are better off skipping that information and focusing on the information below them. Riverbeds have a lot of texture, so fish see strong features that they can track. .”

Building better robots

Not only does this information provide some insight into fish behavior, but it could also inform designs for artificial vision systems and sophisticated bio-inspired robots.

“If you were building a fish-inspired robot and you were just looking at its anatomy, you might think, ‘The eyes point sideways, so I’m going to point my cameras sideways,'” Alexander said. “But it turns out that the eyes point laterally because they’re balancing multiple tasks. We think they point laterally because it’s a compromise—they look up to hunt and down to swim.”

The study is titled “Visual flow in zebrafish natural habitats supports spatial biases in the visual estimation of self-motion.”