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Origin and technical aspects of the EEG
Origin of the EEG
The EEG records electrical activity from the cerebral cortex. Inasmuch as electrocortical activity is measured in microvolts (µV), it must be amplified by a factor of 1,000,000 in order to be displayed on a computer screen. Most of what we record is felt to originate from neurons, and there are a number of possible sources including action potentials, post-synaptic potentials (PSPs), and chronic neuronal depolarization. Action potentials induce a brief (10 ms or less) local current in the axon with a very limited potential field. This makes them unlikely candidates. PSPs are considerably longer (50–200 ms), have a much greater field, and thus are more likely to be the primary generators of the EEG. Long-term depolarization of neurons or even glia could also play a role and produce EEG changes.
In the normal brain an action potential travels down the axon to the nerve terminal, where a neurotransmitter is released. However, it is the synaptic potentials that are the most important source for the electroencephalogram. The resting membrane potential (electrochemical equilibrium) is typically –70 mV on the inside. At the post-synaptic membrane the neurotransmitter produces a change in membrane conductance and transmembrane potential. If the signal has an excitatory effect on the neuron it leads to a local reduction of the transmembrane potential (depolarization) and is called an excitatory post-synaptic potential (EPSP), typically located in the dendrites. Note that during an EPSP the inside of the neuronal membrane becomes more positive while the extracellular matrix becomes more negative. Inhibitory post-synaptic potentials (IPSPs) result in local hyperpolarization typically located on the cell body of the neuron. The combination of EPSPs and IPSPs induces currents that flow within and around the neuron with a potential field sufficient to be recorded on the scalp. The EEG is essentially measuring these voltage changes in the extracellular matrix. It turns out that the typical duration of a PSP, 100 ms, is similar to the duration of the average alpha wave. The posterior dominant rhythm (PDR), consisting of sinusoidal or rhythmic alpha waves, is the basic rhythmic frequency of the normal awake adult brain.
It is easy to understand how complex neuronal electrical activity generates irregular EEG signals that translate into seemingly random and ever-changing EEG waves. Less obvious is the physiological explanation of the rhythmic character of certain EEG patterns seen both in sleep and wakefulness. The mechanisms underlying EEG rhythmicity, although not completely understood, are mediated through two main processes. The first is the interaction between cortex and thalamus. The activity of thalamic pacemaker cells leads to rhythmic cortical activation. For example, the cells in the nucleus reticularis of the thalamus have the pacing properties responsible for the generation of sleep spindles. The second is based on the functional properties of large neuronal networks in the cortex that have an intrinsic capacity for rhythmicity. The result of both mechanisms is the creation of recognizable EEG patterns, varying in different areas of neocortex that allow us to make sense of the complex world of brain waves.
Technical Considerations
The essence of electroencephalography is the amplification of tiny currents into a graphic representation that can be interpreted. Of course, extracerebral potentials are likewise amplified (movements and the like), and these are many times the amplitude of electrocortical potentials. Thus, unless understood and corrected for, such interference or artifacts obscure the underlying EEG. Like the archeologist, the epileptologist seeks to fully understand artifacts in order to discern the truth. Later, we will discuss artifacts in detail and illustrate clearly their many guises. At this point we will consider the technical factors that are indispensable in obtaining an interpretable record.
Electrodes
Electrodes are simply the means by which the electrocortical potentials are conducted to the amplification apparatus. Essentially, standard EEG electrodes are small, non-reactive metal discs or cups applied to the scalp with a conductive paste. Several types of metals are used including gold, silver/silver chloride, tin, and platinum. Electrode contact must be firm in order to ensure low impedance (resistance to current flow), thus minimizing both electrode and environmental artifacts. For long-term monitoring, especially if the patient is mobile, cup electrodes are affixed with collodion (a sort of glue), and a conductive gel is inserted between electrode and scalp through a small hole in the electrode itself. This procedure maintains recording integrity over prolonged periods.
Other types of electrodes are available including plastic, as well as needle electrodes. In fact, new plastic electrodes are MRI compatible. Needle electrodes, which in the past were often used in ICUs, have been redeveloped and consist of a painless (really!) subdermal electrode.
Electrode Placement
Electrode placement is standardized in the United States and indeed in most other nations. This allows EEGs performed in one laboratory to be interpreted in another. The general problem is to record activity from various parts of the cerebral cortex in a logical, interpretable manner. Thanks to Dr. Herbert Jasper, a renowned electroencephalographer at the Montreal Neurological Institute, we have a logical, generally accepted system of electrode placement: the 10-20 International System of Electrode Placement ( Figure 1-1 ). The numbering has been slightly modified since the last edition to a 10-10 system ( Figure 1-2 ). The system was modified so that if additional electrodes are to be placed on the scalp, there is a logical numbering system with which to do so.
Both the 10-10 and the 10-20 system depend on accurate measurements of the skull, utilizing several distinctive landmarks. Essentially, a measurement of the skull is taken in three planes – sagittal, coronal, and horizontal. The summation of all the electrodes in any given plane will equal 100%. Electrodes designated with odd numbers are on the left; those with even numbers are on the right. Standard electrode designations and placement should be memorized during the student's first day of his or her elective ( Table 1-1 ).
Table 1-1
Standard electrode designations
| Left | Right | Electrode |
|---|---|---|
| Parasagittal/supra-sylvian electrodes | ||
| Fp1 | Fp2 | Frontopolar, located on the forehead – postscripted numbers are different than other electrodes in this sagittal line (3,4) |
| F3 | F4 | Mid-frontal |
| C3 | C4 | Central – roughly over the central sulcus |
| P3 | P4 | Parietal |
| O1 | O2 | Occipital-postscripted numbers are different from other electrodes in this sagittal line (3,4) |
| Lateral/temporal electrodes | ||
| F7 | F8 | Inferior frontal/anterior temporal |
| T7 | T8 | Mid-temporal – formerly T3, T4 |
| P7 | P8 | Posterior temporal/parietal – formerly T5,T6 |
| Other electrodes | ||
| Fz, Cz, Pz | Midline electrodes: Frontal, central and parietal. | |
| A1 | A2 | Earlobe electrodes. Often used as reference electrodes from contralateral side. Of note, they record ipsilateral mid-temporal activity. |
| LLC | RUC | Left lower canthus/right upper canthus (placed on the lower and upper outer corners of the eyes). These electrodes are used to detect eye movements and can help distinguish eye movements from brain activity. Sometimes designated LOC, ROC. |
How to measure for electrode placement
Sagittal plane: The sagittal measurement starts at the nasion (the depression at the top of the nose) over the top the head to the inion (the prominence in the midline at the base of the occiput). With a red wax pencil, mark the point above the nasion that is 10% of the total measurement (Fpz) and the point above the inion that also is 10% of the total (Oz). These locations are used as coordinates to help identify the other designated electrode destinations. Divide and mark the remaining 80% into four segments, each 20% of the total measurement. The first 20% point is Fz, the second Cz and the third Pz – the midline electrodes (z = zero). The final 20% is the distance between Pz and your point 10% above the inion (Oz). Thus, the total is 100% ( Figure 1-3A ).
Coronal plane: The coronal plane extends from the point anterior to the tragus (the cartilaginous protrusion at the front of the external ear) to the same point on the opposite side, making sure that the tape measure traverses the Cz point on the sagittal measurement. The intersection of the halfway (50%) points of the sagittal and coronal measurements is the location of the vertex and thus the Cz electrode. The first 10% points up from the tragus define T7 and T8, the mid-temporal electrodes. The next 20% points then define C3 and C4, the central electrodes. The remaining 20% segments represent the distance from C3 to Cz and Cz to C4 ( Figure 1-3B ).
Horizontal plane: The trickiest measurements are in the horizontal plane. The horizontal plane is generated with a measurement from Fpz to T7 to Oz on the left and from Fpz to T8 to Oz on the right. Fp1 and Fp2 are placed on either side of Fpz, both a distance of 5% of the total horizontal circumference from Fpz. Similarly, O1 and O2 are placed at a 5% distance of the total horizontal circumference from Oz. The distances from Fp1 to F7 to T7 to P7 to O1 on the left and from Fp2 to F8 to T8 to P8 to O2 on the right are all 10% of the total horizontal circumference ( Figure 1-3C ).
Finally, F3 and F4 are defined by the halfway points between F7 and Fz on the left and F8 and Fz on the right. Similarly, P3 and P4 are defined by the halfway points between P7 and Pz on the left and P8 and Pz on the right.
An observation: The F7 and F8 electrodes are probably placed too high for optimal definition of anterior temporal activity. Likewise, the P7 and P8 electrodes are probably too high for good definition of posterior temporal activity. Thus, it is possible to logically place additional electrodes (F9/F10, T9/T10, and P9/P10), which are placed 10% inferior to the standard (F7/8, T7/8, P7/8, respectively) electrodes. In some laboratories, these additional electrodes are routinely used.
In the 10-10 system, there are remaining electrode positions in the 10% intermediate lines between the existing standard coronal and sagittal lines. Best to look at Figure 1-2 while reading the next several sentences. Coronally, these electrode positions are named by combining the designation of the coronal lines anterior and posterior. For example, the coronal line between the parietal (P) and occipital (O) chain is designated PO. The only exception is in the first intermediate coronal line, which is named AF (anterior frontal) rather than FpF or FF. In the sagittal line, the same postscript numbers are used; for example, AF3, F3, FC3, C3, CP3, P3, and PO3. From the midline moving laterally the postscript begins at z followed by the numbers 1, 3, 5, 7, 9 on the left and 2, 4, 6, 8, 10 on the right. We now have the 10-10 system where each letter appears on only one coronal line and each postscripted number on a sagittal line ( except for Fp1/Fp2 and O1/O2). The 10-10 system locates each electrode at the intersection of a specific coronal (identified by the letter) and sagittal (identified by the number) line.
While the 10-10 system may sound ever so slightly complicated, in practice it is quite easily carried out. Nonetheless, there is nothing like actually measuring and placing the electrodes yourself under the guidance of an experienced EEG technologist. We recommend that all residents perform at least two to three supervised EEGs during their EEG rotations. Fellows should do more until they are confident in their ability to measure accurately and apply electrodes properly.
Potential Fields
Before discussing how we display the electrical information recorded by the electrodes, the reader should understand the concept of the potential field. The summation of IPSPs and EPSPs in a neuronal net creates electrical currents that flow in and around the cells. The flow of current creates a field that spreads out from the origin of an electrical event (such as a spike or slow wave), much the same as the concentric rings created on a glassy pond when one tosses a pebble onto its surface. Potential fields are usually oval in shape and may be quite restricted or very widespread. The field's effect diminishes as the distance from the source increases. This means that events producing maximal voltage on a particular electrode will affect adjacent electrodes as well, but to a lesser extent as the potential wanes from the point of origin ( Figure 1-4 ).
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