After a quarter-million scientific papers, you'd better hope your methodology was solid. Most of the studies you've probably heard of that try to tie a specific region of the brain to an action or feeling probably relied on a functional MRI technique that tracks the flow of oxygenated blood--so when you see a region "light up" on an fMRI image, that's not the fMRI picking up the actual neurons firing. Rather, it watches for small changes in blood oxygen levels in the region. This method, called blood oxygenation level-dependence (BOLD), presumes that active neurons use more energy and thus require more oxygen. Now, in a study in Nature, researchers at Stanford Medical Center have provided direct evidence that the inference is correct. Lead researcher Karl Deisseroth employed a technique called optogenetics to prove the point. He and his colleagues engineered brain cells that respond to a flash of blue light; when they did this trick on cells in the motor cortex of rats, the flash of light acted as a trigger to active the neurons there. The idea was that they would examine these rats with fMRI at the same time they stimulated those motor neurons with the blue light. If the fMRI lit up in the same places where the researchers knew they were stimulating neurons, they could be confident that fMRI was really picking up brain activation.
Sure enough, when the neurons were turned on with a pulse of blue light, the researchers detected a strong BOLD signal emanating from the motor cortex neurons’ neighborhood. The BOLD signals were exactly what was expected. “It was very compelling and reassuring,” Deisseroth says. “Everyone can breathe a sigh of relief” [Science News].
Still, the brain's complexity never ceases to amaze: While the optogenetic stimulation produced neuron activity that the fMRI scans registered as a BOLD signal, there was other activity besides that showing up as BOLD activity. But, Deisseroth says, those seem to be secondary signals caused by the initial neuron activity.
“We’re certainly not saying that other processes don’t contribute to these signals,” he says. “We’re saying that driving these excitatory neurons kicks it off” [Science News].
Besides reassuring neuroscientists, the Stanford work could also open doors for them, like allowing them to see when brain activity is one region is connected to activity on the other side of 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 [Scientific American].
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