What is Quantitative EEG
David A. Kaiser, Ph.D.
Rochester Institute of
Technology
What determines the characteristic rhythms of the EEG?
Moruzzi and Magoun (1949) were the first to shed light on the origin of
EEG
rhythms. Working in cats they determined that stimulating the reticular
formation of the brainstem aroused the animal behaviorally and any dispersed
high-amplitude EEG rhythms at the cortex. This work eventually led to a general
physiological model of rhythmic activity (Andersen & Andersson, 1968;
Steriade et al. 1990). According to the model, isolated thalamocortical neurons
fire rapidly at their own pace due to metabolic characteristics but in an
intact brain, a sheath of cells known as the reticular thalamic nucleus (RTN)
inhibits intrinsic or random firing of thalamocortical neurons and unites
individual discharges into simultaneous volleys. These volleys propagate
to the cortex and synchronize pyramidal cell activity, whose synchronization
can be detected at the scalp as high-amplitude oscillations (e.g., alpha
bursts, sleep spindles). Corticothalamic feedback influences these volleys by
inhibiting RTN's inhibitory action so that neuronal ensembles may break free of
reticular thalamic influence and fire in response to specific processing
demands. When this occurs, large slow waveforms (theta, alpha) are replaced
by faster frequencies of lower amplitude (beta, gamma), a process
originally called alpha blocking and now called EEG desynchronization.
Desynchronization may be localized to a single electrode as uncommitted
cortical areas remain "idling" or synchronized, or it may involve
several brain areas or electrodes (Pfurtscheller, 1992; Sterman et al. 1994).
Regional patterns of simultaneous desynchronization and synchronization
characterize
specific cognitive and behavioral states (Pfurtscheller & Klimesch,
1990) and it is bymeasuring the mix of slow and fast rhythms across the head
that we identify the nature and extent of cortical engagement.
How is QEEG Related to Human Behavior?
EEG as a crude measure of mental state ( eg eyes open (attention, alert,
aroused or eyes closed (inattention and inactivity) has been documented
extensively and over a considerable period of time.
Behavioral and mental states such as mathematical processing, reading,
or relaxed wakefulness are assumed to be distinct and uniform in nature,
consisting of similar perceptual and cognitive operations whenever they occur.
It is also assumed that distinct mental operations present distinct EEG and
biochemical profiles which are reproduced reliably whenever a task or mental
state occurs. These assumptions lay the foundation to functional MRI as well as
EEG assessment and are the rationale for EEG normalization
training.
The most reliable finding in EEG research occurs when an individual
resting with eyes closed opens his or her eyes in a well-lit room: alpha
blocking occurs. The alpha rhythm is replaced by fast low-amplitude waveforms,
or beta rhythm. (When eyes are opened in a dark room, alpha blocking does not
generally occur; Bohdanecky et al., 1984.) The degree and localization of
blocking or desynchronization is associated with stimulus intensity,
complexity, novelty, and meaningfulness (Gale & Edwards,1983; Berlyne &
McDonnell, 1965; Baker & Franken, 1967; Boiten, Sergeant, & Geuze,1992;
Gevins & Schaffer, 1980). Topographic analysis reveals whether
EEGdesynchronization is nonspecific (many or all sites) or selective (few
sites). Nonspecific arousal is modulated by drugs, drowsiness, drive, and time
of day, whereas sensory and strategic demands activate specific brain areas
such as parietal and occipital cortex to visual stimulation and temporal cortex
to acoustic stimulation (e.g., Grillon & Buchsbaum, 1986; Pfurtscheller,
Maresh, & Schuy, 1977; Chapotot, Jouny, Muzet,
Buguet, & Brandenberger, 2000).
Table 4. Cortical gyrus below each electrode position, based on Mokotoma
et al
|
LOBE
|
GYRUS
|
BRODMANN AREA
|
SITE (left/right)
|
|
Frontal
|
Superior
|
10
|
Fp1/2
|
|
|
Inferior
|
47
|
F7/8
|
|
|
Medial
|
9
|
F3/4
|
|
|
Medial
|
8
|
Fz
|
|
|
Precentral
|
6
|
C3/4
|
|
|
Superior
|
6
|
Cz
|
|
Temporal
|
Medial
|
21
|
T3/4
|
|
|
Medial
|
37
|
T5/6
|
|
Parietal
|
Inferior
|
7
|
P3/4
|
|
|
Precuneus
|
7
|
Pz
|
|
Occipital
|
Medial
|
19
|
O1/2
|
Figure 3. International 10-20 system for electrode placement on the
scalp.
Coherence and Co modulation
Coherence analysis
quantifies phase consistency between signals and comodulation analysis
quantifies
magnitude consistency (Goodman, 1957; Kaiser, 1994). Two signals are
said to be
coherent when their phase relationship is stable, even if signals are
entirely out of phase
with each other. Two signals are said to comodulate when their magnitude
relationship is
stable, regardless of absolute difference between signals.
Development before an adult rhythm at 10 Hz is established
(Niedermeyer, 1987).
The alpha rhythm emerges as a slow 3-4 Hz rhythm in infancy, and it
takes a decade
Table 5. Rhythm Maturation: Alpha & Sleep Spindle Frequency Range by
Age Group
(modified from Niedermeyer, 1987)
|
Rhythm
|
Newborn
|
Infant
|
Toddler
|
Preschooler
|
Preteen
|
|
Alpha
|
Not present
|
4-6
|
5-8
|
7-9
|
9-10
|
|
Sleep spindle
|
Not present
|
12-14
|
12-14
|
12-14
|
12-14
|
BooksPfurtscheller G, &
Lopes da Silva FH. (1999). Event-related EEG/MEG synchronization and
desynchronization: basic principles. Clinical Neurophysiology, 110, 1842-57.
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Endnotes:
