Functional magnetic resonance imaging (fMRI) has allowed scientists to watch areas of the brain “light up” in response to various stimuli. And this has told them about where in the brain certain responses reside—like where fear resides. But fMRI has limitations. The lighting up of certain brain regions leads to increased blood flow to active areas, and this is what you see in an fMRI image. But scientists only knew this as a correlation, not causation—meaning they weren’t sure if increased blood flow happened because neurons were firing.  

But a study published today in Nature has finally brought vindication. Using a technique called optogenetics, they were able to turn on genetically engineered brain cells in rats using a blue light delivered directly to those cells via an optic fiber. And then they put the rats in an fMRI and saw that the blood flow indeed matched precisely those excited cells. This shows that neural excitation is what produces the fMRI images of active brain areas.

Being able to control individual neurons with light has not only justified fMRI, but has opened up an entirely new way to study the brain. Optogenetics works at micro scale, and fMRI covers wide regions of the brain—together this means that scientists have a way to intervene and experiment with entire brain circuits, to finally see how a certain type of brain cell affects the wider global activity of the entire brain.

—Christie Nicholson

[The above text is an exact transcript of this podcast.]