The visual meaning of "looking down" when fish swim

A new study by an international collaboration led by Northwestern University confirms that just as you might look down at the sidewalk as you walk, fish look down when
swimming.

This study is the first to combine swimming behavior that simulates changes in the brain, native environment, and space of zebrafish into a computational model
.
By analyzing this model, the researchers concluded that this quirk of looking down while swimming forward is an adaptive behavior that evolved to help the fish stabilize itself, just as it did
when swimming against the current.

When the water flows, the fish constantly tries to stabilize itself to stay in place, rather than being washed away
by the flowing current.
Focusing on other fish, plants, or debris may give the fish the false feeling
that it is moving.
However, the stable riverbed beneath them gives the fish more reliable information
about the direction and speed at which they swim.

"It's like sitting in
a stationary train car.
If the train next to you starts leaving the station, you will mistakenly think that you are also moving
.
The visual cue from another train is so strong that it transcends all your other senses to tell you the fact that
you are sitting.
It's exactly the same
phenomenon we study in fish.
There are many misleading movement signals on them, but the most abundant and reliable signals come from the river bottom," Emma Alexander said
.
She is an assistant professor of computer science at Northwestern University's McCormick School of Engineering, where she manages the Bio-Inspired Vision Laboratory
.

The study was published in the journal
Current Biology.

To conduct the study, Alexander and her collaborators focused on zebrafish, a widely studied model organism
.
But while many labs have tanks filled with zebrafish, the team wanted to focus on the fish's native environment
in India.

"It has recently been found that fish respond more strongly
to movement below than to movement above.
We wanted to dig deeper into the mystery and understand why
.
Many of the zebrafish we studied grew up in laboratory tanks, but their native habitat shapes the evolution of their brains and behaviors, so we need to go back to the source and investigate the context
in which this organism developed.
”

Armed with camera equipment, the team visited seven locations across India to collect video data of
shallow rivers where zebrafish naturally live.
The field team loaded a 360-degree camera into a waterproof diving tank and attached it to
a remote-controlled robotic arm.
They then threw the camera into the water with a robotic arm and moved it
.

"It allows us to put our eyes where the fish's eyes should be, so what we see is what
the fish see," Alexander said.
From the video data, we are able to simulate imaginary scenarios that simulate fish moving
freely in a real-world environment.
”

Back in the lab, the team also tracked the zebrafish's movements
inside an LED ball.
Because fish have a large field of vision, they don't need to move their eyes around like humans do
.
So the researchers played motor stimuli through lights to observe the fish's response
.
When a pattern appears at the bottom of the tank, the fish swims with the moving pattern – more evidence suggests that the fish get visual cues by 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 behavioral experiments, we calculated their tail beating
.
The more they wag their tails, the more they want to keep up with the moving stripes
.
”

The team then extracted the data from the video and combined
it with data on how the movement signals were encoded into the fish's brains.
They fed the dataset into two existing algorithms for studying optical flow (or the movement of the world over our eyes or camera lenses).

Eventually, they found that in both the wild and in the lab, zebrafish looked down as they swam forward
.
The researchers concluded that the fish looked down to understand the movement of their surroundings and then swam to neutralize it — to avoid being washed away
.

"We combined everything into one simulation, and the results 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 so are other fish and plants
.
Fish are better off ignoring this information and focusing on the information
below.
The riverbed has a lot of texture, so fish can see strong features
that they can track.
”

Build better bots

Not only does this information give us some insight into how fish behave, it can also inform the design of artificial vision systems and sophisticated biomimetic robots
.

"If you're building a fish-inspired robot and you just look at its anatomy, you might think 'its eyes point to the side, so I'm going to point the camera to the side,' but it turns out that the eyes are looking to the side because they're balancing several tasks
," Alexander said.
We think they look sideways because it's a compromise – they look up to hunt and down to swim
.
”

Optic Flow in the Natural Habitats of Zebrafish Supports Spatial Biases in Visual Self-Motion Estimation