An electroencephalogram (EEG) produces a map of the electrical activity on the surface of the brain. Fortunately, the surface is often what we’re interested in, as the cortex—responsible for our complex, high-level functions—is a thin sheet of cells on the brain’s outer layer. Broadly, different areas contribute to different abilities, so one particular area might be associated with grammar, another with motion detection. Neurons send signals to one another using electrical impulses, so we can get a good measure of the activity of the neurons (how busy they are doing the work of processing) by measuring the electromagnetic field nearby. Electrodes outside the skull on the surface of the skin are close enough to take readings of these electromagnetic fields.
Small metal disks are evenly placed on the head, held on by a conducting gel. The range can vary from two to a hundred or so electrodes, all taking readings simultaneously. The output can be a simple graph of signals recorded at each electrode or visualised as a map of the brain with activity called out.
The EEG technique is well understood and has been in use for many decades. Patterns of electrical activity corresponding to different states are now well-known: sleep, epilepsy, or how the visual cortex responds when the eyes are in use. It is from EEG that we get the concepts of alpha, beta, and gamma waves, related to three kinds of characteristic oscillations in the signal.
Great time resolution. A reading of electrical activity can be taken every few milliseconds, so the brain’s response to stimuli can be precisely plotted.
Relatively cheap. Home kits are readily available. OpenEEG ( http://openeeg.sourceforge.net ), EEG for the rest of us, is a project to develop low-cost EEG devices, both hardware and software.
Poor spatial resolution. You can take only as many readings in space as electrodes you attach (up to 100, although 40 is common). Even if you are recording from many locations, the electrical signals from the scalp don’t give precise information on where they originate in the brain. You are getting only information from the surface of the skull and cannot perfectly infer what and where the brain activity was that generated the signals. In effect this means that it’s useful for looking at overall activity or activity in regions no more precise than an inch or so across.
—Myles Jones & Matt Webb
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