EEG basics

EEG, electroencephalography, is the recording of voltages from the brain. In special circumstances, the recording can be done directly from the brain surface, but normally electrodes on the scalp are used.

We will not go into details of electrode placings and the pairing of electrodes for recording. The electrodes are usually fairly numerous, and placed in a symmetrical pattern.

The voltage amplitudes are small, typically in the range of tens of microvolts. They are thought to be caused by synchronized activity in very large numbers of synapses in the cerebral cortex. The "centre of synchronization" is somewhere deep in the thalamus or brain stem, but EEG gives only indirect information about those regions.

The figure below tries to illustrate how the apical dendrites of pyramid cells could generate an EEG voltage, if a large group of them are simultaneously excited. The group to the left are not excited at the moment, and the extracellular excess of positive charge is maximal. The group to the right are depolarized by the synapses, decreasing the charge separation across the membrane. Since the electrodes are extracellular, it is easy to understand that there will be a voltage between them, the left one being positive. A moment later, the situation might be reversed.

The EEG curves are classed according to "rhythm". There are four recognized rhythms:

Alpha rhythm: fairly regular but waxing and waning, with dominant frequency 8 - 13 Hz. It typically occurs over the occipital region when the person is awake but relaxed and content.

Beta rhythm: to call this a "rhythm" is really too polite. It is low-amplitude, high-frequency and irregular. It is the typical EEG of the awake adult. It also occurs in REM sleep.

Theta rhytm. Typical frequency 3-7 Hz, higher amplitude than alpha.

Delta rhythm. Typical frequency 0.5 - 2 Hz, even higher amplitude.

Theta and delta rhythms occur in sleep, in small children and in various pathological conditions.

Below is a simulation, intended to give you a general idea of how the EEG looks in these rhythms. You can choose your rhythm in the menu. There is a marker in the curve for every second. Depending on your computer and browser, the curve may or may not develop in real time, ie the second markers actually come at second intervals. The smoothness of the procedure also varies.

Next: Evoked potentials

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