Science In Real Life

Last time, on a very special Science In Real Life …

“But doctor! What will we do?”
“Stand back! I’m going to magnetically resonate his nuclei!”

And now, the conclusion … wherein we discuss two methods of imaging the brain that I didn’t have room for in the previous essay. Welcome to SiRL, where we have more surprise twins than a fertility clinic. Now, some of you will remember all the way back to the very first thing that was ever posted to this blog. The very first Science Fact of the Day: The electrical activity in your brain, i.e. your thoughts, constantly produces measurable magnetic fields outside your head. This phenomenon is, dare I say it, the brainchild behind imaging methods known as MEG and EEG, Magneto- and ElectroEncephaloGraph, respectively.

Let’s go back to basics. What is the brain? Putting aside for now the much broader question of how the brain does all those wonderful things it does, the relevant description for our purposes is a big messy jumble of very tiny wires that each carry pulses of electrical current. Like a telephone switchboard with thoughts, ideas, and concepts whizzing every which way. So now we’ve got memories flying everywhere, passing each other. The electric field is like AAAAAHH – which is to say it’s complicated. This is no exaggeration. You have neurons firing signals in all directions. You might then think that measuring the brain’s activity is hopeless, since one neuron’s signal is far too miniscule to measure. Keep that defeatist attitude out of the lab, buddy. We’re trying to do science here.

It so happens that, given the tens of billions of neurons in your brain, and hundreds of trillions of connections between them, that groups of signals heading in more or less the same direction spontaneously form like mobs of children heading for the ice cream truck. This analogy becomes especially apt when one considers that different regions of the brain correspond to different processing subroutines at our disposal. For example, if the electrical impulses currently comprising your consciousness all decide that it’s a really good idea to pack the kids into the car, stop the mail delivery, and head for scenic Brodmann area 4, it’s a good bet you’re about to move some part of your body, since that’s the motor cortex. So what the EEG does is detect these (relatively) massive throngs of collected neural impulses.

How? It’s all about fields. The business end of the EEG device is a collection of metal electrodes attached by wires to an amplifier and a computer. One of the electrodes is placed apart from the others and used as a base reference value. The sensor watches for a difference between any of the other electrodes and the reference, like you might watch for a flash of green and brown while playing Duck Hunt. The amplifier boosts the signal and you’re in business. The advantage to using an EEG is its amazing temporal accuracy, but what you gain there you lose in spatial accuracy. Since a massive convocation of neurons is required for a signal, the EEG cannot pinpoint exactly where it is happening.

Another disadvantage is a directional dependence. Imagine we divide all the neurons into two groups: those that aim signals along the surface of your skull (tangential), and those that aim signals outwards or inwards (radial). Like a biased reporter, EEG is much better at detecting the latter than the former. Enter MEG. By measuring the magnetic fields resulting from firing neurons, we can focus on the tangential signals. In addition, due to the properties of skull and skin, MEG measurements are less distorted and more spatially localized. So what we have are Jack Sprat and his wife – by using them simultaneously we can lick the plate clean and measure both kinds of signals with increased precision.

Now, the really cool part is that in order to design an instrument sensitive enough to detect the brain’s magnetic fields, we have to use a SQUID. Put the tentacles away – SQUID stands for Superconducting QUantum Interference Device. Instead of plain vanilla electrodes, we now have paired loops of superconductors (superconductivity is an inherently quantum mechanical process). When one of your idle musings sends out a magnetic field, it disrupts the balance between the two loops, producing an electrical voltage. The reason this is measurable is due to the dramatic difference between superconducting and not superconducting. The tiniest push across that critical threshold is enough to reorganize the entire electromagnetic structure of the sensor loop, like a librarian who finally sees one book too many out of place.

Of course some of the imaging methods I discussed in the previous essay can also be used to measure the brain, certain types of MRI in particular. Often an EEG and specialized MRI scan are performed simultaneously to better suck the patient’s brain dry of tasty, tasty information. Yum.