Second edition

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Normal EEG
The electrical activity recorded by the EEG arises from the
apical dendrites of cortical pyramidal neurons (Humphrey
1968; Purpura and Grundfest 1956). Although the electrical
activity associated with an action potential is too brief
to be recorded on an EEG (i.e., lasting less than 1 ms),
activity derived from both inhibitory and excitatory postsynaptic
potentials lasts much longer (from 15 to 200 ms)
and it is this activity that is reflected in the EEG (Humphrey
1968). The electrical activity arising from one neuron is
obviously too weak to affect the surface electrodes, so it is
upon the summed activity of numerous neurons that the
EEG depends. Furthermore, it must be borne in mind that
abnormal electrical activity occurring deep below the cortex
may not ‘reach’ the scalp electrodes (Cooper et al. 1965)
and thus certain deep lesions, such as lacunar infarcts, may
not cause any abnormality on the EEG although have profound
clinical consequences (MacDonnell et al. 1988).
Electroencephalographic activity may or may not be
rhythmic and it appears that rhythmicity occurs secondary
to the activity of the thalamus, which acts like a pacemaker
or ‘conductor’, exerting rhythmic control over the cortical
‘orchestra’, and bringing large groups of neurons into synchrony
(Dempsey and Morrison 1942; Steriade et al. 1990).
This dependence of cortical neurons upon the thalamus
for rhythmic firing was demonstrated by experiments in
which the destruction of the thalamus abolished rhythmic
cortical activity (Jasper 1949).
The EEG consists of various waves that may differ in
terms of morphology, amplitude, and duration. Thus, in
terms of morphology, an individual wave may be monophasic,
diphasic, triphasic, or polyphasic, depending on how many
times the ‘baseline’ is crossed by the wave in question.
Amplitude is measured in microvolts from the crest to the
trough of the wave: customarily, amplitudes under 20 μV
are considered low, those between 20 and 50 μV, medium,
and those over 50 μV, high (some electroencephalographers
will, however, rather than using this absolute scale,
consider the amplitude of a given wave relative to the overall
amplitude of background activity: thus, if the background
activity were generally of 60 μV, a 30-μV wave, using this
relative scale, might be considered low). It is therefore critical
that the electroencephalographer specifies whether an
absolute or a relative scale is being used when reporting
amplitude. The duration of the wave is measured in milliseconds:
waves lasting less than 70 ms are referred to as
‘spikes’ and those lasting from 70 to 200 ms as ‘sharp
waves’; those lasting for over 200 ms are spoken of either as
‘slow waves’ or simply ‘waves’.
Waves may be isolated or recurrent. If recurrent, their
frequency is reported in cycles per second (Hz): by convention,
frequencies less than 4 Hz are termed ‘delta’, those
from 4 to under 8 Hz ‘theta’, those from 8 to 13 Hz ‘alpha’,
and those over 13 Hz as ‘beta’ waves. Some electroencephalographers
also use the terms ‘slow’ and ‘fast’, ‘slow’
referring to both delta and theta activity (i.e., anything
1.5 Electroencephalography 23
under 8 Hz) and ‘fast’ referring to any activity in the beta
range (i.e., over 13 Hz). Recurrent activity may also be rhythmic
and regular in occurrence, or arrhythmic and irregular.
The EEG will normally have a recognizable background
activity that is more or less persistent and similar throughout
the recording. Upon this background, one may at
times see isolated events that, for one reason or another,
stand out from the background, such events being referred
to as ‘transients’. Transients may, in turn, consist either of
an isolated spike or wave, or a ‘complex’ of two or more of
these. Complexes themselves are further described in terms
of whether they are isolated or recurrent, and if recurrent,
whether they recur irregularly or regularly. ‘Spindles’
comprise a specific type of transient complex, consisting of
a group of rhythmic waves that gradually increase in
amplitude, and then just as gradually decrease.
The normal adult awake EEG, as seen during relaxed
wakefulness with the eyes closed, contains an alpha rhythm
and a beta rhythm. These two terms must not be used
loosely: for example, although much EEG activity may
occur in the alpha frequency, the activity must fulfill certain
other criteria to qualify as an alpha rhythm. In a minority
of individuals, a mu rhythm may also be seen.
The alpha rhythm consists of more or less regular sinusoidal
activity, ranging in amplitude from 20 to 60 μV
(averaging about 50), occurring in the alpha range and
most prominent posteriorly. The alpha rhythm is generally
‘blocked’ by eye opening, being replaced by lower amplitude,
and faster/irregular activity. Although the frequency
of the alpha rhythm is the same on each side, the actual
waves themselves are generally out of phase. Further, there
is also generally an amplitude difference between the two
sides, with the left side alpha being of lower amplitude than
the right. Generally, this amplitude differential is no more
than 20 percent; however, the range of normal here is wide,
with some normal individuals having differentials up to 50
percent. The alpha rhythm is best seen in a state of relaxed
wakefulness with the eyes closed. In a small minority of
cases, variants of the alpha rhythm may occur (Goodwin
1947), wherein the frequency of the sinusoidal activity is
either in the 4- to 5-Hz range (‘slow’ alpha variant) or
16- to 20-Hz range (‘fast’ alpha variant). These variants
represent ‘harmonics’ of the more typical alpha rhythm.
The beta rhythm consists of bilateral beta activity of an
amplitude of 30 μV or less, seen best anteriorly, which is
blocked unilaterally by contralateral tactile stimulation,
movement, or merely an intention to move. Although the
waves are generally out of phase, the frequency is bilaterally
symmetric. An amplitude variance of up to 35 percent
from side to side is considered normal. Beta activity is
often increased by sedatives such as benzodiazepines and
barbiturates (Brown and Penry 1973; Frost et al. 1973;
Greenblatt et al. 1989).
The mu rhythm represents another normal type of EEG
activity, one that is not seen as routinely as the alpha or beta
rhythms and is present in only about 10 percent of normal
adults. The mu rhythm consists of theta or alpha activity