The map details the connections between brain cells called neurons in the mouse, which has about 71 million of them.
To create it, researchers injected a virus engineered to be green and fluorescent into a particular region of a living mouse brain. The virus infects the neurons near the injection site, and in three weeks it propagates all throughout the brain.
Here is the pattern of connections originating from one brain area, shown in fluorescent green after the virus has infected the surrounding neurons. Among other functions, this area of the brain is responsible for interpreting certain sensations, primarily the sense of touch.Allen Institute for Brain Science Once the virus has propagated, they take the brain out of the mouse and image it in several different ways, eventually generating about 1 terabyte of data per mouse. By following the path of the virus and analyzing these images down to a very small scale, they can create a picture of how that particular region connects to the rest of the brain.
For example, here is a 3-D visualization of the connections between four visual areas in the mouse cortex (green, yellow, red, orange). "These areas are highly interconnected with each other and with additional areas involved in vision in the thalamus (pink) and mid-brain (purple)," a press release explains.Allen Institute for Brain Science
Once they've mapped the connection from one specific brain area, the the process is repeated in other regions. The researchers analyzed and imaged about 1,700 mouse brains in all; the report in Nature is based on 469 of those.
Here is a visualization of that data that includes connections from multiple areas throughout the brain:
Allen Institute for Brain Science
The connections they found can also be represented as a circular wiring diagram. This diagram maps the connections between 215 regions of the mouse brain. Connections originating from 11 specific regions are highlighted in different colors, and the rest are shown in gray. Allen Institute for Brain Science