Electroencephalogram (EEG) is the records of the brain electrical activity. Neural activity is manifested by the electrical signals known as graded potentials and action potentials. The electrical activity in the brain’s neurons, particularly those in the cortex near the surface of the brain, can be recorded from the outside of the head. Electrodes, which are wires attached to the head by a salty paste that conducts electricity, pick up electrical signals generated in the brain and transmit them to a machine that records them as the electroencephalogram (EEG). The majority of the electrical signal recorded in the EEG originates in the pyramidal cells of the cortex. The processes of these large cells are oriented perpendicularly to the brain’s surface, and the EEG records postsynaptic potentials in their dendrites. While we often think of electrical activity in neurons in terms of action potentials, action potentials do not usually contribute directly to the EEG. Rather, EEG patterns are largely due to graded potentials, in this case summed postsynaptic potentials in the many hundreds of thousands of brain neurons that underlie the recording electrodes.
EEG patterns are waves, albeit complex ones, with large variations in both amplitude and frequency. The wave’s amplitude, measured in microvolts, indicates how much electrical activity of a similar type is going on beneath the recording electrodes at any given time. If the amplitude is high, this indicates that many neurons are being activated simultaneously. In other words, it indicates the degree of synchronous firing of whichever neurons are generating the synaptic activity. If, on the other hand, amplitude is low, this indicates that these neurons are firing asynchronously. The amplitude may range from 0.5 to 100 microvolts. Note that EEG amplitudes are about 1000 times smaller than the amplitude of an action potential. The wave’s frequency indicates how often the wave cycles from its maximal amplitude to its minimal amplitude and back. The frequency is measured in hertz (Hz, or cycles per second) and may vary from 1 to 40 Hz or higher. Four distinct frequency ranges are found in EEG patterns. In general, lower EEG frequencies indicate less responsive states, such as sleep, whereas higher frequencies indicate increased alertness. As we will see, one stage of sleep is an exception to this general relationship.