A primitive part of the zebrafish brain helps them find their way home

An evolutionarily-ancient region of their brains work as a cognitive map.
Zebrafish swimming in an aquarium.
Zebrafish swimming in an aquarium. Deposit Photos

In 2003, a lost fictional clownfish named Nemo swim his way to box office success. If Nemo hadn’t ended up in a dentist’s fish tank, it’s possible an older region in the back of his brain could have kicked in to help him find his way back to his home to a reef, according to a new study.

A team of scientists at Howard Hughes Medical Institute’s (HHMI) Janelia Research Campus are now better understanding how animals know where they are in relation to their environment—and how to find their way back on the path they were taking. A study published December 22 in the journal Cell details how a region called the hindbrain helps animals determine location and use that information to plan where to go next.

[Related: Fish brains provide insight on how humans store memories.]

The hindbrain is an older region located in the back of the brain that has been evolutionarily conserved, or virtually unchanged throughout the process of evolution. The authors looked at tiny translucent zebrafish. They have historically been used in research, especially in genetics, for many reasons, including their quick growth rates, translucent bodies that can help scientists peer inside, and similar genetic structure to humans. The zebrafish genome was fully sequenced in 2013.

The fish were placed in an environment that simulates currents, and were then pushed off course when the currents shifted unexpectedly. However, they were able to course-correct and get back where they started. While the zebrafish were swimming, the researchers used a whole-brain imaging technique to measure what was going on inside the fish’s brain. The scientists could search the entire brain to note which circuits were activated when the zebrafish course corrected and separate the individual activities.

The team expected to see the forebrain, where the hippocampus that houses an internal map of an animal’s environment is stored. Instead, they saw several regions of the medulla activate. This is where information about the animal’s location was being transmitted through a newly identified circuit. A part of the hindbrain called the inferior olive used motor circuits to move the information to the cerebellum that made the fish move. The fish were unable to move back to its original spot when these pathways were blocked.

[Related: These jellyfish don’t have brains, but still somehow seem to sleep.]

“We found that the fish is trying to calculate the difference between its current location and its preferred location and uses this difference to generate an error signal,” says En Yang, the first author of the new study, and a post doctoral researcher at Janelia’s Ahrens Lab, in a statement. “The brain sends that error signal to its motor control centers so the fish can correct after being moved by flow unintentionally, even many seconds later.”  

Previous studies have shown that the inferior olive and the cerebellum performed actions related to reaching and locomotion, but not this type of navigation. According to the team, this hindbrain network could also lay the basis for other navigational skills, including when a fish swims to a specific spot to take shelter.

“This is a very unknown circuit for this form of navigation that we think might underlie higher order hippocampal circuits for exploration and landmark-based navigation,” said Misha Ahrens, Janelia Senior Group Leader, in a statement.

Further study is needed to determine whether these same networks are involved in similar behavior in other animals.