ASHBURN, Va. — “Finding Nemo” isn’t just a kid classic, you might be able to call it a documentary! New research reveals lost fish can find their way home thanks to the “ancient” wiring in their brains.
The neuronal circuit evolved up to half a billion years ago and could have been passed to humans — before people diverged from other animals. It enables our slippery ancestors to get back on course, even after being swept away by fast flowing currents.
The discovery sheds fresh light on the workings of the ancient brain and may apply to other vertebrates as well. There are potential implications for treating Alzheimer’s, since patients frequently lose their way in familiar and unfamiliar surroundings.
“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,” says Janelia Senior Group Leader Misha Ahrens in a media release.
Surprising activity in the back of the brain
The study found the key chemical pathways cross different regions at the back of the brain, helping the animals regain their bearings. In experiments, the tiny translucent fish traversed a 2D virtual reality environment in the presence of a simulated flow. As they swam toward a target, strong water unexpectedly pushed them off course. The fish still swam back to where they started, determined to finish the journey. Scans show that the hindbrain — a conserved area at the back of the brain — computes their location. They use the information to figure out where to go next.
First author Dr. En Yang and colleagues used a “whole imaging” technique developed at their lab at the Howard Hughes Medical Institute to measure what is happening. It allowed scientists to search the entire brain of fish to see which circuits are activated during course-correcting and disentangle the individual components involved.
They expected to see cells triggered in the forebrain where the hippocampus — which contains a cognitive map of an animal’s environment — is located. To their surprise, they saw activation in several regions of the medulla. Information was being transmitted from a newly identified circuit.
It passed through a hindbrain structure called the inferior olive to the motor circuits in the cerebellum that enable the fish to move. When scientists blocked these pathways, fish were unable to navigate back to their original location.
The brain sends out an error signal when you’re lost
The findings suggest areas of the brainstem remember a zebrafish’s original location and generate an error signal based on its current and past locations. This is relayed to the cerebellum, allowing the fish to swim back to its starting point. It reveals a new function for the inferior olive and the cerebellum. Scientists say these brain regions were known to play a role in actions like reaching and locomotion, but not this type of navigation.
“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 Dr. Yang. “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.”
It is still unclear whether these same networks are involved in similar behavior in other animals. However, the researchers hope labs studying mammals will now start looking at the hindbrain for homologous circuits for navigation.
This hindbrain network could also be the basis of other navigational skills, such as when a fish swims to a specific place for shelter, Dr. Ahrens adds. Zebrafish are colorful creatures, named after their stripes. They are popular additions to aquariums due to their social, peaceful nature.
It shouldn’t surprise people that fish have such a keen sense of direction. A previous experiment found that goldfish were actually capable of driving a robotic car!
South West News Service writer Mark Waghorn contributed to this report.