Neuroscientists have found a way to watch neurons fire in an independently moving animal for the first time. The study was done in fish, but it may hold clues to how the human brain works, Science Now reports.

Junichi Nakai of Saitama University’s Brain Science Institute in Japan and colleagues selected a glowing marker known as green fluorescent protein (GFP) and linked it to a compound that would light up in the presence of large amounts of calcium.

The researchers then inserted the DNA that codes for this marker into the zebrafish genome, tying it to a specific protein only found in neurons.

This means that only actively firing neurons would fluoresce, and scientists could track neural activity without applying dye. Because the signal was stronger and clearer, researchers didn’t have to immobilize the larvae.

Using their newly developed imaging system, Nakai and colleagues associated the sight of moving paramecium and prey capture behavior with the activation of a group of neurons in the optic tectum, the visual center of the zebrafish brain.

The neurons pulsed in tandem with the movements of the paramecium — a sudden dart of the one-celled organism caused a bright flash of neural activity in the zebrafish tectum (see videos).

The tectum went silent if the paramecium stilled. Only moving prey interested the larvae, the team reported in Current Biology. These particular neurons, Nakai proposes, are part of a specific visual-motor pathway that links the sight of moving prey with swimming behavior.

All animals, from zebrafish to humans, contain an optic tectum, which coordinates eye movement and the organism’s response to objects in their visual field.

The neurons in the larvae continuously make new GFP, which allows ongoing detection of neural activity. “It means we can take the same measurements today, tomorrow, and the day after tomorrow,” Nakai says. “This technique makes long-term measurement possible.” He hopes the approach will allow scientists to associate a variety of specific behavior patterns with specific neural circuits.

That, in turn, could improve the development of psychiatric drugs, as scientists will more easily be able to tell if a particular drug has the desired effects on the brain.