How does fMRI show us what’s going on in the brain?

FMRI, by Washington irving on Wikimedia Commons.

An fMRI, showing your brain on something. (Washington Irving/Wikimedia Commons)

[Ed. note: Today we bring you a double first: the first response to a reader request (by a Twitter user called @slentinator) and the first guest post, by Cat Ferguson, who will soon join the science writing program at UC Santa Cruz. Take it away, Cat!]

Functional magnetic resonance imaging – a way to use a standard-issue MRI machine to study the workings of the brain – seems simple when you look at it. Paint splashes of ‘activity’ stand out against a dull grey background, showing where blood is flowing in the brain and, therefore, what part of the brain is ‘on’ when a person is thinking or doing something.

But as with most things in life, the truth behind the pictures is way more complicated. Scientists tend to fall into three camps when it comes to fMRI. Some look at it as the key to answering our deepest philosophical questions. On the opposite end of the spectrum are those who call it the modern answer to phrenology (the now long-discredited study of skull shape to determine personality and intelligence). Most fall in the middle, believing that fMRI is a useful but imperfect technology to help understand the human brain.

We’ve heard that using fMRI scientists can tell that Republicans use their amygdala more than Democrats and spot a pedophile. But since non-scientists – even college students studying neuroscience! – are more likely to believe a bad explanation of psychology research if it’s accompanied by a completely irrelevant fMRI scan, it’s important to understand what fMRI pictures actually show.

Let’s start small…really small. Your body is full of hydrogen atoms, each one a single proton with an electron orbiting it. These protons don’t sit still, but rather spin on an axis, like the Earth does every day. An fMRI uses an incredibly powerful magnetic field, about 600 times the strength of your refrigerator magnets, to line some of those protons up.

The machine then fires radio waves into the brain. When those waves hit the lined up protons, the protons start to vibrate. When the waves are turned off, the protons shoot back radio waves of their own. And that’s what the scanner picks up.

Now let’s get a little bigger, and talk about proteins – specifically hemoglobin, the protein that carries oxygen. Like the rest of your body, the brain needs oxygen, and parts of the brain that are burning more fuel than usual need lots of oxygen.

Protons in hemoglobin with oxygen behave differently in magnetic fields than those in hemoglobin without. That means fMRI can be used to tell the ratio in the brain of hemoglobin with oxygen to hemoglobin without. Scientists call this ratio the BOLD, or blood oxygenation-level dependent, signal, and use it to calculate where the most oxygen was just dropped off for cells to use. That tells them, theoretically, where the brain was most active a few seconds before.

So how do we get from radio signals bouncing around in the brain to pictures that show us the ebb and flow of blood (and oxygen)? Statistics. By crunching the numbers, scientists can measure how different the signals from different parts of the brain are. Computers then use colors to highlight those differences, showing which parts of the brain have the least oxygen, and voila.

Now, that math is incredibly important. Last year, a team of scientists won an Ignobel Prize for a study in which they interviewed a dead salmon in an fMRI machine. When they used the same statistical math as some 25 to 40 percent of the fMRI papers published in 2009, they found significant activity in the brain and spinal cord of the fish. Either its spirit was communicating from beyond the grave, or those papers had serious issues.

The dead fish thinks! (Courtesy of The Scicurious Brain)

There is no question that fMRI has redefined neuroscience, whether it’s changing our ideas about free will or helping define the biology of spirituality. However, there is danger in its seeming straightforwardness. Study results often fit neatly with ‘common sense,’ like when a part of the brain associated with impulse control seems to predict recidivism for criminals.

But before we hurry to put every parole applicant in a scanner, we need to remember that the truth is far thornier than the headline-grabbing reduction: fMRIs show the brain in shades of grey, not black and white.

Cat Ferguson has a dog, a Twitter, and an email address.

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