Biomedical Engineering Reference
In-Depth Information
EEG
30
μ
Cerebrum
4 seconds
Thalamus
AMP
2
Cerebellum
Brain stem
2468 012
14
16
Frequency (C/S)
(a)
(c)
Dendrites
Synapses
Cell
body
Current
lines
Action
potential
Synaptic
potential
Axons
(b)
Figure 1.1
(a) The human brain. (b) Section of cerebral cortex showing microcurrent sources due to
synaptic and action potentials. Neurons are actually much more closely packed than shown, about
10
5
neurons per square millimeter of surface. (c) Each scalp EEG electrode records space averages
over many square centimeters of cortical sources. A 4-second epoch of alpha rhythm and its corre-
sponding power spectrum are shown. (
From:
[2]. © 2006 Oxford University Press. Reprinted with
permission.)
near-sinusoidal voltage oscillations (alpha rhythms) in awake, relaxed subjects with
eyes closed. Early finding that opening the eyes or performing mental calculations
often caused substantial reductions in alpha amplitude have been verified by mod-
ern studies. Unfortunately, it took more than 10 years for the scientific community
to accept these scalp potentials as genuine brain signals. By the 1950s, EEG technol-
ogy was viewed as a genuine window on the mind, with important applications in
neurosurgery, neurology, and cognitive science.
This chapter focuses on the fundamental relationship between scalp recorded
potential
V
(
r
i
,
r
j
,
t
), which depends on time
t
and the electrode pair locations (
r
i
,
r
j
),
and the underlying brain sources. In the context of EEG, brain sources are most con-
veniently expressed at the millimeter (mesoscopic) tissue scale as current dipole
moment per unit volume
P
(
r
,
t
).
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