Whole-brain recordings of the larval zebrafish taken while it was in the virtual reality environment. Credit: Misha Ahrens
Key Points:
- An “ancient” region in the back of the brain helps zebrafish compute their location and figure out where they need to go next.
- The research reveals a new function for the inferior olive and the cerebellum.
- It is unclear whether the same brain networks are involved in similar behavior in other animals, like mammals.
How do animals know where they are in their environment, and how does this determine their subsequent choices? Scientists at HHMI’s Janelia Research Campus discovered that the hindbrain—an evolutionarily conserved or “ancient” region in the back of the brain—helps animals compute their location and use that information to figure out where they need to go next.
For the study, published in Cell, researchers put tiny translucent zebrafish in a virtual reality environment that simulated water currents. When the current shifted unexpectedly, the fish were initially pushed off course; however, they were able to correct for that movement and get back to where they started.
While the zebrafish were course correcting, the scientists were using a whole-brain imaging technique developed at Janelia to measure what was happening in the fish’s brain. They expected to see activation in the forebrain, where the hippocampus is located. To their surprise, they instead saw activation in several regions of the medulla, where information about the animal’s location was being transmitted from a newly identified circuit via a hindbrain structure, called the inferior olive, to the motor circuits in the cerebellum that enable the fish to move. When these pathways were blocked, the fish was unable to navigate back to its original location.
These findings suggest that areas of the brainstem remember a zebrafish’s original location and generate an error signal based on its current and past locations. This information is relayed to the cerebellum, allowing the fish to swim back to its starting point. This research reveals a new function for the inferior olive and the cerebellum, which were known to be involved in actions like reaching and locomotion, but not this type of navigation.
It is still unclear whether these same networks are involved in similar behavior in other animals. But the researchers hope labs studying mammals will now start looking at the hindbrain for homologous circuits for navigation.
“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.
Information provided by HHMI.