EEG (Electroencephalography) is a non-invasive technique in which the electrical activity of the brain is recorded at the level of the scalp. The resulting encephalogram presents the changes in electrical activity as a function of time. The electrical activity is recorded by means of electrodes that are positioned over the scalp surface. The number of électrodes used can range from 32 to 256.
In an EEG experiment the volunteer carries out a specified task during recording so that fluctuations in electrical activity (measured in microvolts, mV) over time can be measured as a function of the defined experimental conditions.
What exactly does EEG measure ?
There are two main types of electrical activity assciated with neurons :
- Action Potentials
- Postsynaptic Potentials
Action Potentials are discrete voltage spikes that travel along a neuron’s axon to the axon terminals, where neurotransmitters are released. Surface electrodes cannot generally detect action potentials due to their timing and the physical arrangement of the axons. Action Potentials can only be recorded if individual neurons fire at exactly the same time causing the voltages of the action potentials to sum. However, as neurons rarely fire at exactly the same time and also because axons are relatively random orientation of axons, action potentials generally cancel each other out. Action potentials have a very short duration, about 1 millisecond (ms).
Postsynaptic Potentials (PSPs) arise when neurotransmitters bind to receptors on the membrane of the postsynaptic cell, causing ion channels to open or close and leading to a graded change in potential across the cell membrane. PSPs are longer in duration than action potentials, lasting up to tens or even hundreds of milliseconds. They are generally confined to the dendrites of a neuron and occur instantaneously. As dendrites are oriented in parallel along the cortical sheet, these characteristics of PSPs mean that they can summate and are, thus, thought to largely contribute to the signal measured by EEG.
Electrical activity is recorded by the EEG system as a time-dépendent signal. It is possible to study signal changes as a function of time, or to carry out a spectral decomposition of the signal and look at its frequency components and their evolution over time. Because the electrical activity is captured by a number of electrodes positioned over the scalp surface, it is possible to look at the spatial distribution of the activity and how the topography changes over time. With the application of techniques used in Magnetoencephalography (MEG) it is possible to analyse the underlying neural generators of the scalp-level electrical activity; a technique known as source reconstruction.