WATCH: 80,000 neurons fire in a zebrafish brain

The video is based on a new imaging technique called light-sheet imaging, which has allowed researchers to get an unprecedented glimpse into the neural mechanisms of a living zebrafish.

You’d be hard-pressed to find anything on Earth as complex as the brain - even if that brain happens to belong to a simple zebrafish - and these almost inscrutable organs pose a real challenge to those trying to study them. For decades researchers have had to resort to external devices that work by picking up signals in the brain, and often from just a few hundred neurons out of possibly billions, but what if you could see almost all of these neurons firing in real-time inside an active brain? That’s what researchers from the Howard Hughes Medical Institute in the US have been working on.

“There must be fundamental principles about how large populations of neurons represent information and guide behaviour,” one of the team, neuroscientist Jeremy Freeman, told Joshua Batson at Wired. “In this system where we record from the whole brain, we might start to understand what those rules are.”

The team genetically engineered zebrafish to have a chemical indicator in each of the neurons inside their brains. These chemical indicators are so efficient, they become fluorescent within a tenth of a second after a neuron fires. The new light-sheet imaging technology directs lasers to sweep across the wholly transparent bodies of the fish, which makes these indicators glow. A video camera hanging overhead is then able to capture extremely clear footage of the activity going on inside their brains.

The scientists started out by seeing what the fish’s neurons did as it was sitting quietly in its tank, just thinkin’ ‘bout fish stuff. This is what the beginning of the above video shows. But then things start to get interesting, as Batson describes at Wired:

"The scientists then created the illusion that the fish was drifting backwards by sliding bars in front of its eyes. Its intent to swim to catch up was measured with electrodes on its muscles. When the bars start sliding, a few neurons sitting just behind the eyes light up followed by a huge cascade of activity, including massive pulses initiating swimming.”

Publishing in the journal Nature Methods, the researchers report that this is the first time an entire zebrafish brain has been imaged while the creature is moving and thinking. They said that of the 80 percent of the neurons that showed up in the footage, they could identify which ones were involved in specific activities, but not how many individual times they had to fire to get the job done.

Source: Wired