You can look into the brain of a zebrafish

Zebrafish larvae can be so transparent that you can see their brains. Researchers make individual active neurons light up. This allows them to study the brain function of the fish while they freely swim around and display natural behavior: exploring their environment, interacting with each other, and foraging for food. 

Studying the human brain is an extremely difficult task: With its 100 billion neurons and over 100 trillion synaptic connections, its size and complexity can be daunting for anyone trying to unravel how it works. To make matters worse, measuring human brain activity is tricky: Non-invasive imaging techniques such as fMRI and EEG are a bit coarser than what one would hope for. On the other hand, any method capable of detecting the activity of individual neurons would require surgery, which cannot be done in humans for obvious ethical reasons; moreover, such techniques do not allow to observe the entire brain at once. For this reason, some neuroscientists interested in how neurons work together on a large scale turn to smaller, more easily observable brain models.

Active neurons light up

Zebrafish are one such model. The tiny brain of young fish contain 100,000 cells – just a fraction of the size of ours. And yet, they are vertebrates just like us. As such, we share many important characteristics: Zebrafish use the same neurotransmitters such as dopamine, serotonin, and acetylcholine, and in many ways, the structure and function of their brain networks can be similar to ours. Another advantage: They are completely transparent when only a few days old. This makes it possible to look inside their brains! Using fluorescent calcium indicators, one can even make the active neurons light up. This is possible because every time a neuron fires, the concentration of calcium ions changes. Using this method, researchers can simultaneously observe all the individual neurons in the entire fish brain!

Fish with individual personality traits

But of course, to understand how the brain works, it is best to study it while it works, that is, while the fish behave naturally. An invention by Jennifer Li and Drew Robson, co-leaders of the Systems Neuroscience & Neuroengineering research group, makes this possible: tracking microscopes that move with the fish. With these, the team can watch what is going on in the little brains of the fish while they swim around freely. This opens the door to a myriad of questions to explore: What neural circuits are active when the fish explore a new tank? Are there neurons that light up when the fish remembers a familiar environment? How do the fish forage for food, and what happens in their brains when they find it? How are memories stored? Which brain structures are involved in social behavior?
Despite their small brains, zebrafish are capable of complex and varied behaviors. What makes them even more like us is that they are individuals with their own personalities! Some like to explore more, while others prefer to stick to what they know. They can be risk-takers or risk-averse, bold or shy, or everything in between!