Have you ever wondered about what goes through a fish's head? Recent advances in molecular biology and genetics are now making it possible to look at the activity of single neurons in free-swimming fish. Akira Muto from the National Institute of Genetics in Shizuoka, Japan, and his Japanese colleagues have taken advantage of the transparency of zebrafish larvae and recorded neuronal activity in the brain while the fish were swimming and searching for their prey. Their research was recently published in Current Biology and is an important step in understanding how animals perceive their environment, one of the fundamental questions in neurobiology.

First, Muto and colleagues took a strain of zebrafish that was able to emit fluorescent light inside cells when these cells had a rapid increase in calcium concentration, such as when a neuron in the brain is active. They then introduced a mutation to make this strain of fish emit an even brighter light that could be easily detected under a fluorescence microscope. Once they succeeded, they screened and selected zebrafish that emitted light only in the optic tectum, the part of the brain thought to be involved in perception.

Muto and colleagues then wanted to determine how these fish perceive a moving object. The researchers immobilized the zebrafish and placed a screen to one side of them. A fluorescence microscope was used to then visualize changes in the brightness of the neurons that occurred as a dot of light was turned on and off on the screen. The team saw a cluster of neurons ‘light up’ in the neuropil area, where neurons from the retina connect with neurons from the optic tectum. When the dot on the screen was moved up and down, neurons in the optic tectum became active in the same direction, showing that the activation of these neurons was related to the perception of the visual stimulus.

Next, the researchers wanted to see how the fish would respond to a natural object, whilst still immobilized. This time they used a free-moving paramecium, a natural prey of zebrafish larvae. The neurons in the tectal area moved as predicted, tracking the movement of the paramecium, but these neurons were not active when the paramecium was motionless. This showed that a moving object can activate tectal neurons but a stationary object cannot.

Finally, the researchers wanted to study neuronal activity during prey capture in free-swimming larvae. In this experiment, the zebrafish larvae and its prey, the paramecium, were swimming freely under the microscope. Muto and collegues found that the anterior tectum of the zebrafish was more active just before the paramecium was captured than when the zebrafish was locating and swimming towards the prey. These findings indicate that the anterior tectum is responsible for linking visual and motor pathways during the prey capture behaviours.

This is the first study to visualize neuronal activity in free-moving animals. This method is a powerful tool in the future understanding of how animals perceive their world and which areas of the brain are involved in these behavioural responses: one step closer to reading a fish's mind!

Real-time visualization of neuronal activity during perception
Curr. Biol.