In his words:

We used a laser scanning microscope to take images of neurons deep in the brain. These neurons were genetically modified to express a fluorescent protein that glows only when you excite it with a laser. By scanning the laser across the whole 3D volume of the brain tissue, we could generate 3D images of almost all of the neurons present in our tissue sample. This technique lets us look at how different genes are expressed in different kinds of neurons in the brain. Understanding how many different kinds of neurons there are in the brain is important, because each type of neuron might have a unique role in processing information.