If you have ever used virtual reality, you may know how disorienting it is when the image you are seeing doesn’t quite match up with your movement – and zebrafish may experience this, too. Studying how these fish react to virtual environments can give us a closer look at how they perceive the world and make decisions.
“It tells us something quite exciting about zebrafish, which is that they appear to have an internal model of the world that they use to predict consequences of their own behaviour,” says Rainer Friedrich at the Friedrich Miescher Institute for Biomedical Research (FMI) in Switzerland.
He and his colleagues used a specialised VR rig to look at the neurons of adult zebrafish to study brain activity in immobilised adult fish. The set-up let the fish mimic horizontal swimming movements and interactions with other fish in a virtual environment.
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To do this, the researchers glued steel bars to the heads of 26 zebrafish and fixed these to a solid mount so the fish could only swim on the spot. The fish were placed in a tank with a 3D virtual environment projected onto the walls.
The researchers could predict which direction the fish were trying to swim based on their tail movements, allowing the projection to be updated accordingly. They also modified the VR projection for a 10-second period, during which the direction that the fish swam in was opposite to the expected direction. For example, when the fish tried to swim left, they saw themselves moving to the right instead.
About 70 per cent of the fish made more intense tail movements when this happened, but there was a 2-second delay in their response on average. Using a fluorescence imaging technique called two-photon microscopy, the researchers imaged the zebrafish’s brains to try to determine what part of them triggered this activity.
The zebrafish were engineered to express a fluorescent protein in the neurons of a specific region called the telencephalon, part of the forebrain that corresponds to areas in the human brain that deal with decision-making, learning and memory. However, it isn’t yet clear whether the telencephalon plays the same role in zebrafish, says Kuo-Hua Huang at FMI.
The researchers saw a significant response to the change in VR in 12 per cent of the telencephalic neurons. Out of the 10 million neurons that adult zebrafish possess, this is still a substantial amount, says Huang. The brain must deal with thousands of different functions, “so I wouldn’t expect the whole brain to respond to this specific case”, he says.
In those neurons, the activity spiked before the fish started to flap their tails more vigorously. Friedrich and his team say this shows the brain activity is related to planning ahead, rather than a response to physical activity.
The work opens up the possibility of examining the role of the dorsal pallium, a little-known part of the telencephalon thought to be analogous to parts of the human brain, says Isaac Bianco at University College London.
Nature Methods
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