Application to biomedical signals - Practical Biomedical Signal Analysis Using MATLAB - page 101

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Application to biomedical signals
4.1 Brain signals: local field potentials (LFP), electrocorticogram
(ECoG), electroencephalogram (EEG), and magnetoen-
cephalogram (MEG), event-related responses (ERP), and
evoked fields (EF)
Electroencephalogram (EEG) is a record of the electric signal generated by the
cooperative action of brain cells, or more precisely, the time course of neuronal ex-
tracellular field potentials generated by their synchronous action. EEG recorded in
the absence of stimuli is called spontaneous EEG; a brain electric field generated as
a response to external or internal stimulus is called an event-related potential (ERP).
EEG can be measured by means of electrodes placed on the scalp or directly on the
cortex. In the latter case it is called an electrocorticogram (ECoG); lately also iEEG
(intracranial EEG) abbreviation is used. Electric fields measured intracortically with
electrodes implanted in the brain structures were named local fields potentials (LFP).
The same electrodes (if small enough) can also record action potentials (spikes). The
amplitude of EEG of a normal subject in the awake state, recorded with the scalp
electrodes, is 10-100 μ V. In case of epilepsy the EEG amplitudes may increase by
almost an order of magnitude. Scalp potentials are largely independent of electrode
size which is due to severe space averaging by volume conduction between brain and
scalp. Intracranial potentials amplitude depend on electrode size (smaller electrode—
higher potential) and may vary in size over four orders of magnitude.
Variable electric field generates a magnetic field which follows from the Maxwell
equations. The recording of the magnetic field of the brain is called a magnetoen-
cephalogram (MEG). The amplitude of the MEG is less than 0.5 picotesla (pT) and
its frequency range is similar to that of the EEG. Since electric and magnetic fields
are orthogonal, radial sources (dipoles oriented perpendicular to the skull) are bet-
ter visible in EEG and tangential sources in MEG. Radial sources, due to geometry,
hardly contribute to MEG, but the advantage of this technique is that the magnetic
field is weakly influenced by the structures of the head. Figure 4.1 shows schematic
representations of the electric and magnetic fields generated by a current dipole.
Applications of EEG. EEG found application in the investigation of the informa-
tion processing in the brain and in medical diagnosis. In particular it is helpful in
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