EEG

As discussed in Section 2, neurons transmit electrical signals around the brain. The electrical activity of large numbers of neurons synchronise into large-scale oscillations called brain waves, which can be measured directly by placing electrodes on the scalp. These electrodes detect voltage fluctuations due to the brain waves, which are then recorded as an electroencephalogram (EEG). The result is a picture of how the brain waves change over time.

This technique has shown that different frequencies of brain waves are associated with different mental states, from highly active learning to deep sleep (Figure 11).

This is a composite of two images. Figure 11 (a) shows a dummy’s head with a cap covering the skull, but not over the ears and with a chin strap fastening. The cap is covered with multiple points (greater than 50) to which electrodes can be connected. There are three electrodes inserted in the cap. Figure 11 (b) shows typical electrical signals that can be recorded from EEG equipment. They are drawn as wavy lines and are depicted in order of frequency and amplitude. Gamma waves, whose role is not clear but are observed during problem solving, show small rapid changes in amplitude. Beta waves, observed during active concentration, are lower frequency and have a slightly larger amplitude. Alpha waves, have lower frequency still and are observed during mental and physical relaxation with eyes closed. Theta waves are observed during a drowsy, meditative state, and finally delta waves, are very low frequency and high amplitude, and are observed during deep sleep.
Figure 11 (a) EEG headcap, used to apply electrodes to the surface of the head; (b) different types of brain wave activity measured using EEG

While the EEG technique can detect very rapid changes in brain activity, it can’t measure the precise locations of activity.

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