Over the past ten years researchers have developed a variety of techniques to get more and more precise information on the working of a brain, lately becoming able to "look" inside a working brain to follow its activities.
fMRI has been the first tool used and it provided the first glances on a working brain. However, its spatial resolution and the time delay could only afford a very coarse view of what was going on.
The discovery of specific markers that can bind to individual neurones and fluoresce when the neurone is active have increased the spatial resolution but requires observation tools, like sensors or optical fibres, to be placed in the vicinity of the neurone to pick up the fluorescence.
A first dynamic capturing of a working brain is probably the one of the zebrafish back in 2014 where almost 80% of the baby fish neurones were tracked as the fish vision system was solicited. That was possible because the baby fish is transparent.
However, the correlation between neurones activity and the stimuli was coarse.
Now for the first time (at least that I am aware of) researchers at Princeton University have managed to track individual neurones activity in a nematode (a worm) by looking at intracellular calcium transients (the moving of calcium from the inside to the outside of a neurone is an indication os a change of membrane electrical potential, hence of a signal from that neurone to others).
The researchers have been able to track the movement of fluorescent calcium molecules using 3 cameras to obtain a 3D spacial resolution living the nematode unconstrained in its movement.
One needs to be aware that the nematode brain contains just 302 neurones, nothing if you compare it to our brain (and also to a fly brain) but still it is an amazing feat.
Researchers hope this tracking can improve our understanding of how control is achieved and how behaviour emerges.