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How can lost fish get back on track?

When zebrafish swim toward their intended targets, strong currents frequently push them off course. However, the tiny fish swim back to their original location, determined to finish their journey. In order to clarify how do animals know where they are in their environments and how this knowledge determines their choices, a team of researchers led by the Howard Hughes Medical Institute (HHMI) has investigated the brains of zebrafish when they are involved in such behaviors. The analysis revealed that the hindbrain – an evolutionary conserved or “ancient” region in the back of the brain – helps them compute their location and use to information to decide where they need to go next.

The scientists placed several zebrafish in a virtual reality environment which simulates water currents. When the current shifts unexpectedly, the fish are initially pushed off course, but they are able to correct for those movements and go back to their initial position. By using a whole-brain imaging technique, the researchers managed to search the animals’ entire brains to see which neural circuits are activated during their course-correcting behavior. 

Although they expected to observe a significant amount of forebrain activation – where the hippocampus, a brain structure containing a “cognitive map” of the environment, is located – the experts were surprised to notice activation in several regions of the medulla, where information about the fish’s location was being transmitted via a hindbrain structure called “the inferior olive” to the motor areas of the cerebellum which enable the animals to move. When this pathway was blocked, the fish were unable to navigate back to their initial location.

These findings suggest that regions of the brain stem “remember” a fish’s original location and generate an “error signal” based on its current and past locations. “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. 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,” explained study lead author En Yang, a postdoctoral fellow at HHMI.

However, it is still unclear whether the same neural networks are involved in similar behaviors in other animals. Moreover, further research is also needed to clarify whether this hindbrain network could be the basis of other navigational skills, such as when fish swim to a specific place for shelter.

The study is published in the journal Cell

By Andrei Ionescu, Staff Writer

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