Brain imaging of zebrafish sheds new light on how animals find their way in the dark. Specifically, researchers from Harvard University identified brain activity in animals that helps them find food and other vital resour ces without the guidance of environmental cues, such as lights and sounds.
When animals are placed in unfamiliar environments, their foraging behavior generally appears random and spontaneous. While these behaviors have been observed in many animals, the corresponding brain activity has remained elusive, as previous studies have had difficulty knowing where to look for neural signals in large vertebrate brains.
In the latest study, however, researchers used whole-brain imaging in larval zebrafish to unravel how their brain activity translates into spontaneous behaviors. It turns out that the animals' behavior is not actually all that random. Instead, it is characterized by alternating left and right turn "states" in the brain, causing animals to perform repeated left and right turning maneuvers.
"We noted that a turn made by the zebrafish was likely to follow in the same direction as the preceding turn, creating alternating 'chains' of turns biased to one side and generating conspicuous, slaloming swim trajectories," said Timothy Dunn, first author of the study and a postdoctoral researcher at Harvard University. "Freely swimming fish spontaneously chained together turns in the same direction for approximately five to 10 seconds on average, and sometimes for much longer periods. This significantly deviates from a random walk, where movements follow no discernible pattern or trend."
After analyzing the relationship between spontaneous brain activity and spontaneous behavior in the larval zebrafish, researchers created whole-brain activity maps of neuronal structures that correlated with the patterns in the animals' movements. This revealed a nucleus in the hindbrain of a zebrafish, which is thought to be a vital behavioral algorithm for optimizing foraging when little information about the surrounding environment is available to the animal.
Researchers believe such behavioral strategies, and therefore neural systems, must exist in other animals that explore unknown territory.
"Overall, our whole-brain analysis, neural activity experiments, and anatomical characterization of zebrafish revealed a circuit contributing to the patterning of a spontaneous, self-generated behavior," explained Yu Mu, a postdoctoral researcher at Janelia Research Campus. "As our study makes very specific predictions about this circuit, future experiments will be required to validate its critical components. It will also be interesting to see if different environmental contexts and the motivational state of zebrafish influence their spontaneous swim patterns."
Their findings were recently published in the journal eLife.
