THE STANDARD ELECTROENCEPHALOGRAM
Signal
The EEG is produced by a summation of excitatory and inhibitory
postsynaptic potentials produced in cortical gray matter. Because the electroencephalographic
signal is generated only by postsynaptic potentials and is much smaller than action
potentials recorded over nerves or from the heart, extreme care must be taken when
placing electrodes. The recording electrode impedances should be less than 5 kΩ
and matched to each other to permit clear electroencephalographic signals, which
may range from 5 to 500 mV. As electrode impedance increases above that value or
becomes mismatched, significant signal loss occurs, and background electrical "noise"
begins to obscure the electroencephalographic signal. Electrode impedance is kept
low by using gold cup electrodes with silver-silver chloride electrolyte gel placed
between the
Figure 38-1
International 10–20 system of electrode placement
for recording electroencephalograms and sensory evoked responses. (From
Hughes JR: EEG in Clinical Practice, 2nd ed. Newton, ME, Butterworth-Heinemann,
1994.)
scalp and the electrode. The electrode is held tightly to the scalp with collodion,
a biologic glue. Alternatively, subdermal needle electrodes may be used, particularly
when sterile application of an electrode close to a surgical field is necessary.
When electrodes are applied directly to the surface of the brain, impedance is minimized
by close electrode contact and saturation of the area with an electrolyte solution.
Electroencephalographic electrodes usually are placed according
to a mapping system that relates surface head anatomy to underlying brain cortical
regions. The placement pattern of recording electrodes is called a montage.
Use of a standard recording montage permits anatomic localization of signals produced
by the brain and allows development of normative electroencephalographic patterns
and comparison of recordings made at different times. The standard electroencephalographic
map is called the International 10–20 System for electroencephalographic electrode
placement ( Fig. 38-1
).
This system is a symmetric array of scalp electrodes placed systematically based
on the distance from the nasion to the inion and from the pretragal bony indentations
associated with both temporomandibular joints. Based on 10% or 20% of these distances,
recording electrodes are placed systematically over the frontal (F), parietal (P),
temporal (T), and occipital (O) regions at increasing distances from the midline.
Left-sided electrodes are assigned odd numbers, and right-sided electrodes are assigned
even numbers. Increasing numbers indicate an increasing distance from the midline.
Midline electrodes are designated with a lower-case z. Recording electrodes may
be referenced to other cephalic electrodes (i.e., bipolar recordings) or to electrodes
placed away from cortical areas (i.e., referential recordings). The standard diagnostic
EEG uses at least 16 channels of information,[7]
but intraoperative recordings have been reported using 1 to 32 discrete channels.
The intraoperative EEG is most commonly recorded from electrodes
placed on the scalp. Recordings may also be made from electrodes placed on the surface
of the brain (i.e., electrocorticography) or from microelectrodes placed transcortically
to record from individual neurons (e.g., during surgery for Parkinson's disease).
[8]
[9]
The electroencephalographic
signal is described using three basic parameters: amplitude, frequency, and time.
Amplitude is the size, or voltage of the recorded
signal and ranges commonly from 5 µV to 500 mV (compared with 1 to 2 mV for
the electrocardiographic signal). As neurons are irreversibly lost during the normal
aging process, electroencephalographic amplitude decreases with age. Frequency
can be thought of as the number of times per second the signal oscillates or crosses
the zero voltage line. Time is the duration of the
sampling of the signal; this is continuous and real time for the standard EEG but
is a sampling epoch (i.e., data over a given period) for the processed EEG.
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