The zebrafish is heading towards its target, but a strong current deflects it from its course. Undeterred, the little fish returns to its starting point and completes its journey with determination. How do animals know where they are in the environment, and how does this determine their next choices? Researchers at Howard Hughes Medical Institute’s Janelia Research Campus have discovered that the hindbrain, an evolutionarily conserved or “ancient” region at the back of the brain, helps animals calculate where they have been and use that information to decide where to go next.

A recent study published in the journal Cellreveals new functions for parts of the “old brain,” findings that may apply to other vertebrates.

Whole brain imaging reveals new networks

To find out how animals understand their position in the environment, researchers led by Anne Young, a postdoctoral researcher in Ahrens’ lab, placed tiny translucent zebrafish barely half an inch long in a virtual reality environment that simulates water currents. When the current changes unexpectedly, the fish goes off course first; however, they can correct this movement and return to where they started.

As the zebrafish swim in a virtual reality environment, the researchers are using a whole-brain imaging technique developed at Janelia to measure what’s going on in the fish’s brain. This technique allows scientists to examine the entire brain to see which circuits are activated during course correction behaviors and to resolve the individual components involved.

The researchers hoped to see activation in the forebrain, which is home to the hippocampus, which contains a “cognitive map” of the animal’s environment. Surprisingly, they saw activation in several regions of the medulla oblongata; here, information about the animal’s position was transmitted from the newly identified circuit, via a hindbrain structure called the lower olive, to the motor circuits in the cerebellum that allowed the fish to move. When these paths were closed, the fish could not return to its original position.

This video shows a virtual reality environment for larval zebrafish. A fish moves through a two-dimensional environment in the presence of simulated water flow

These findings suggest that brainstem regions remember the zebrafish’s initial position and generate an error signal based on its current and past position. This information is transmitted to the cerebellum, allowing the fish to return to its starting point. This study reveals a new function for the lower olive and cerebellum, which are known to be involved in actions such as reaching and moving but not in such navigation.

“We found that the fish tried to calculate the difference between the current and desired position and used that difference to generate an error signal,” says Young, lead author of the new study. “The brain sends this error signal to the motor control centers so that the fish can correct itself even seconds later, after being accidentally moved by the current.”

A new multisite diagram of the hindbrain

It is still unclear whether the same networks are involved in similar behavior in other animals. But the researchers hope that labs that study mammals will now start looking for homologous circuits for navigating the hindbrain. This hindbrain network may also underlie other navigational skills, such as when a fish swims to a particular shelter, the researchers say.

“This is a very unknown circuit for this form of navigation, which we think may form the basis of higher order hippocampal circuits for landmark-based exploration and navigation,” says Janelia senior team leader Misha Ahrens.