Figures
Figure 1
EEG signals are generated by the transmembrane ion currents in the pyramidal neurons (cortical layers IV-V). The black ellipsoids symbolize the volume conduction of the return currents in the tissue between the generator (red arrow) and the recording EEG electrode (cup containing conductive paste) on the scalp. The box to the right is a schematic representation of the local field potentials in the cortical generator. Excitatory post-synaptic potentials (EPSP, in green) have extracellular negativity at the synapse (active sink) due to the influx of Na+ ions, and extracellular positivity at the passive source, due to the compensatory current. Inhibitory post-synaptic potentials (IPSP, in red) have extracellular positivity (active source) due to the influx of Cl- or efflux of K+ ions, and extracellular negativity at the passive sink, due to the compensatory currents.
Figure 1
Figure 2
Schematic drawing showing the current flow generated by cortical sources (depicted here as green ellipsoids, at the center of the red cross-hair) with radial (A) and tangential (B) orientation. A) When the cortical source is on the convexity (radial orientation), the return currents generated along the apical dendrites demonstrate the direction shown in the figure, causing a relatively circumscribed area of negative potentials on the scalp, overlying the source. The rest of the scalp has low-amplitude, diffuse positive potentials. (B) When the cortical source is in the wall of a sulcus (tangential orientation), the return currents are parallel with the surface, as shown in the figure. This results in two areas with opposite polarity on the scalp. The negative polarity is in the direction of the cortical surface of the generator. The color scale for the arrows indicates the polarity: red=positive, blue=negative, and yellow=the transition between them.
Figure 2
Figure 3
The standard 25-electrode array of the International Federation of Clinical Neurophysiology.
Figure 3
Figure 4
Anatomical landmarks and measuring of the 10% and 20% distances for standard electrode placement. (A) Coronal plane: the distance is measured between the preauricular points (T9 and T10). (B) Sagittal plane: the distance is measured from nasion to inion. The electrodes highlighted in yellow represent the inferior temporal chain (“low row”).
Figure 4
Figure 5
Schematic representation of the EEG amplifier.
Figure 5
Figure 6
Each EEG channel is displayed as an oscilloscope: time runs from left to right and voltage is plotted on the vertical axis. By convention, in the EEG, negative values are presented towards the top of the graph and positive values towards the lower part of the graph on the vertical axis.
Figure 6
Figure 7
EEG montages in which scalp electrodes are used as reference. (A) Referential montage (to Cz). (B) Longitudinal bipolar montage. (C) Transversal bipolar montage.
Figure 7
Figure 8
Schematic representation of EEG traces recorded from a signal generated by a cortical source with radial orientation. (A) Longitudinal bipolar montage. (B) Common average montage. Color code for the topographic distribution on the scalp: blue=negative, red=positive potentials, and yellow=transition between them.
Figure 8
Figure 9
Schematic representation of EEG traces recorded from a signal generated by a cortical source with tangential orientation (the anterior wall of the central sulcus). (A) Longitudinal bipolar montage. (B) Common average montage. Color code for the topographic distribution on the scalp: blue=negative, red=positive potentials, and yellow=transition between them.
Figure 9
Figure 10
Voltage maps showing topographic distributions of negative (blue) and positive (red) potentials, generated by cortical sources with radial (A) and tangential (B) orientation.
Figure 10
Figure 11
Voltage maps corresponding to signals generated in different regions of the left temporal lobe ([A] basal; [B] polar; [C] lateral). In the upper row, the cortical areas generating the signals are highlighted in red. In the lower row, the arrows show the location and the orientation of the cortical sources: tangential in (A) and (B), radial in (C).
Figure 11
Tables
Authors
1 Departments of Clinical Neurophysiology, Aarhus University Hospital, Aarhus and Danish Epilepsy Centre, Dianalund; Department of Clinical Medicine, Aarhus University, Aarhus, Denmark
2 Department of Neurology, Beth Israel Deaconess Medical Center, Harvard University, Boston, MA, USA
* Correspondence: Sándor Beniczky
Danish Epilepsy Centre Filadelfia,
Visby Allé 5,
4293 Dianalund,
Denmark
Electroencephalography (EEG) is the most commonly used functional investigative method in epilepsy. The goal of this educational review paper is to summarize the most important aspects related to the biophysical phenomena of EEG signal generation and the technical features that a clinician needs to understand in order to read and interpret EEGs. We explain the EEG electrodes and recording arrays, amplifiers, filters, analogue-to-digital conversion and signal display. We describe the advantages and disadvantages of the different montages and the indications for the various types of EEG recordings and provocative maneuvers. We explain how to use topographic maps to estimate the source of the cortical generator.
This work is licensed under a
Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License