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Why Fish Look Down When They Swim

Just as you might look down at the sidewalk as you walk, fish look downward when they swim, a new study by a Northwestern University-led international collaboration has confirmed.

The study is the first to combine simulations of zebrafish’s brain, native environment and spatially-varying swimming behavior into one 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 the fish self-stabilize, as when swimming against a current.

As water moves, fish are constantly trying to self-stabilize in order to stay in place — rather than getting swept away in a moving stream. Focusing on other fish, plants or debris might give the fish a false sensation that it’s moving. The stable riverbed below them, however, gives fish more reliable information about their swimming direction and speed.

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

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