What does MEG measure?
MEG measures the magnetic signals produced by activity in your brain. These signals are very small (on the order of femtoTesla, or 10-15). By comparison, your heartbeat produces a signal in picoTesla (10-12). The earth's magnetic field is in the order of tens of microTesla (10-6). A technology such as magnetic resonance imaging (MRI) uses magnetic fields up to 3 Tesla (a signal 3,000,000,000,000,000 stronger than the signal produced by your brain).
Is undergoing MEG risky?
MEG is a passive tool for looking at magnetic activity in the brain. Unlike technologies such as functional magnetic resonance imaging (fMRI) or positron emission tomography (PET), MEG does not "put" anything into you - rather, it records the magnetic activity that your brain naturally produces. There are no known risks associated with MEG.
What is the MEG machine like?
Because magnetic signals in the brain are so small, we must do everything possible to screen out electric or magnetic noise that would interfere with recording brain signals. Therefore the actual experiment takes place inside a Magnetically Shielded Room (MSR), which is rather large (about 8' wide by 8' high by 12' long), and contains a bed where you recline during testing.
Your head rests on a dewar containing the superconducting MEG sensors and liquid helium to keep them at the correct temperature. These sensors measure the activity in your brain while you look at words or pictures on a screen above your head, or listen to sounds or words through ear buds. There are no loud noises while the machine is recording, as with MRI, so you may not even notice that anything is happening. The experimenter will try to make you as comfortable as possible in the MSR.
What happens during the experiment?
Before the experiment begins, we do two things. First, we attach five small electromagnetic markers to your head. We do this so that we can figure out where exactly your head is when it is underneath the MEG sensors (this is something we can't guess about, or use a rough estimate of; the brain is very specialized, and two regions an inch apart can have very different functions). This is a painless process, and poses no danger. When these electromagnets are activated, for about 1 minute during each experiment, they are much weaker than refrigerator magnets, and slightly stronger than the magnetic signal of your heartbeat.
Second, we need to trace the shape of your head. We do this for two reasons. The most important is that the algorithms we use to figure out which parts of your brain are active are relatively dumb: they don't care much if they find something inside or outside your head. We, on the other hand, assume that most of the activity in your brain actually occurs in your brain, so we use the shape of your head to constrain the algorithms. When we later try to figure out what parts of your brain are active, it is much easier to understand the results if they are situated inside something that looks like a head, rather than just floating around in space, so we also use head shapes for graphics displays. We record your head shape using special digitizing equipment. This equipment records, in 3-dimensional space, where your head is as we "paint" a laser over it; interestingly, this equipment is also used in virtual reality technology.
Once those two steps are completed, the experiment moves into the MSR. There, you will lie down while the experiment takes place. Some experiments examine language processing, and involve reading or hearing words and then making decisions about them (for example, whether they are real English words or not). Some experiments look at speech processing and involve listening to tones and speech-like sounds; sometimes for this type of experiment you don't have to do anything other than listen quietly, while other times you have to categorize the sounds (for example, decide whether the sound you heard was more like a B or a P). We also usually perform a couple of short baseline tests that are used to identify where the basic visual and auditory parts of your brain are, and how their activity looks on our MEG system. These are very short and simply involve watching patterns or hearing tones.
Why are there such strict requirements for eligibility?
Most of our restrictions on participation relate to the fact that MEG measures magnetic signals in the brain, which means that any other magnetic materials near the MEG machine will interfere with our recording. It is important to note that magnetic materials are not at all dangerous around the sensors, but simply result in lower quality data.
For this reason, we require that participants remove all watches, earrings, and other jewelry. Eyeglasses are also not permitted; participants must therefore have normal vision or use soft contacts. We may also ask you to remove belt buckles, keys, etc. before beginning.
Many types of cosmetics contain traces of iron -- in particular, some hair gels and coloring mousses, as well as mascara and rouge-- so makeup is to be avoided.
Clothing that contains metal (especially bras with metal underwire or clasps and shirts with metal zippers or buttons) cannot be worn in the MSR. The fly on a pair of pants is permitted. We ask that, if possible, you wear clothing without metal to the experiment, but can provide appropriate garments to change into if necessary.
Volunteers with permanent magnetic objects on the body (e.g. braces, permanent retainers, etc.) may not participate. Most fillings and crowns are fine.
Two additional restrictions unrelated to magnetic interference are the requirement that participants be native English speakers, because our experiments usually concern language processing, and the exclusion of left-handed volunteers, because of potential differences in the way right- and left-handed people's brains are organized.