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MRI and fMRI

This lesson is a part of an audio course Brain Science for Non-Scientists by Wilf Nelson

This lesson is going to talk about the other very famous method of studying the brain, MRI, or magnetic resonance imaging. It is an amazing technique that lets us study what is going on in the brain without cutting it open. In fact, since the blood flow of the brain needs the brain to have the pressure of the skull, it is only through fMRI that we can study how people think when they are healthy and awake in the way thousands of researchers do today.

So what is MRI, what is fMRI, and why does it matter? Quick version, MRI is magnetic resonance imaging, if you magnetise the whole brain, which you can do safely, the hydrogen atoms in your body align with the field, don't worry about it past that, we give the atoms energy, radio energy not x rays like cat scans so completely safe again, and those atoms respond to the energy, more particularly as they give up the energy. This means we get a signal, we get a signal from every hydrogen atom in your brain and we can map the brain. fMRI is done by measuring the oxygenated and deoxygenated blood, as blood loses oxygen to give it to the brain its magnetic property changes, we measure that signal and know an area is using oxygen. An MRI measures structure, an fMRI measures blood flow, a big difference, and it means using MRI we can never see neurons firing, only the oxygen they use and if you have been listening to that is actually fMRI.

So what do we use fMRI for? Well first off it can measure the whole brain, it's amazing, but it is slow. Blood responses take place over 15-30 seconds; the changes in oxygen are instantaneously like neurons are so we have to work making assumptions as we go. Let's say you want to know how the brain processes things visually, well we put someone in the scanner, show a blank screen and a really bright visual scene, and we find the back of the brain uses much more oxygen, ok vision might be processed at the back. We actually need to do it again and again, and that is what we do, our visual field is at the back of our brains. We can also measure how memories are formed, they are formed in the hippocampus and also retrieving memories, imagining things based on old memories all this uses our hippocampus, and we learn this through fMRI. Hearing, well those regions are by our ears, movement, the crown of our head if you are curious. fMRI let us map the brains of healthy awake people in ways we just couldn't before.

fMRI can also do something amazing, that is measure how two areas are responding at once and if they are communicating. It is all well and good saying a region goes active, but the brain is about communication, a signal doesn't end until it reaches the motor systems, thinking by its nature is a never-ending cycle of thoughts going round and round. So saying vision is at the back of the head doesn't mean much, it is a start, but we need to go further. What we can do is measure two regions, let's say the visual region and the parietal, or the area on the back of your head where some people have a bald spot. Well if vision is sending signals there, we should see the back of the head use oxygen, use neurons, and process something, then a few seconds later the parietal area will respond in the same way. We know neurons are communicating a lot faster but remember we are measuring blood flow, and blood flow is slow. This is the first step to showing how the brain communicates; this involves measuring functional connectivity, how the brain is connected, and how it uses those connections to function. What we learned was the brain doesn't just send information only one way, even the highest levels of visual processing influence the start, an intricate set of loops, what you expect to see influences what you see before you even realise you see it, that is how connected the brain is.

MRI has taught us all of this, and on my show, Water Cooler Neuroscience I have spoken with experts like Alan Jasanoff about how new chemicals are being made that can bond to neurotransmitters we know are used by the brain, bond safely to the chemicals in the brain and are read by fMRI like blood. It opens up the chemistry of the brain to fMRI scanning just like we have done with blood flow. This is the new frontier of fMRI science, and it is amazing, but it is so new there is no more I can talk about here. As new scientific techniques become available, on my show, we will talk about the new findings and how scientists are using this amazing method.

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Written by

Wilf Nelson