Physiological Foundations of Quantitative EEG Analysis - Quantitative EEG Analysis Methods and Clinical Applications - page 9

In standard EEG terminology, synchrony is a qualitative term normally indicat-
ing sources that are approximately phase locked with small or zero phase offsets;
sources then tend to add by linear superposition to produce large scalp potentials. In
fact, the term desynchronization is often used to indicate EEG amplitude reduction,
for example, in the case of alpha amplitude reduction during cognitive tasks. The
term coherent refers to the standard mathematical definition of coherence, equal to
the normalized cross spectral density function and a measure of phase locking. With
these definitions, all synchronous sources (small phase lags) are expected to produce
large coherence estimates, but coherent sources may or may not be synchronous
depending on their phase offsets.

1.5 Scalp Potentials Generated by the Mesosources

Nearly all EEGs are believed to be generated by cortical sources [2]. Supporting rea-
sons include: (1) cortical proximity to scalp, (2) the large source-sink separations
allowed by cortical pyramidal cells (see Figure 1.4), (3) the ability of cortex to pro-
duce large dipole layers, and (4) various experimental studies of cortical and scalp
recordings in humans and other mammals. Exceptions include the brainstem
evoked potential , where the angle brackets indicate a time average, in

this case over several thousand trials needed to extract brainstem signals from sig-
nals due to cortical sources and artifact.
We here view the mesosource function or dipole moment per unit volume P ( r , t )
as a continuous function of cortical location r , in and out of cortical folds. The func-
tion P ( r , t ) forms a dipole layer (or dipole sheet) covering the entire folded
neocortical surface. Localized mesosource activity is then just a special case of this
general picture, occurring when only a few cortical regions produce large dipole
moments, perhaps because the microsources s ( r , t ) are asynchronous or more ran-
domly distributed within most columns. Or more likely, contiguous mesosource
regions P ( r , t ) are themselves too asynchronous to generate recordable scalp poten-
tials. Again, the qualitative EEG term synchronous indicates approximate phase
locking with near zero phase lag; source desynchronization then suggests reductions
of scalp potential amplitude. In the case of the so-called focal sources occurring in
some epilepsies, the corresponding P ( r , t ) appears to be relatively large only in
selective (centimeter-scale) cortical regions.
Potentials

〈 V ( r i , r j , t ) 〉

( r , t ) at scalp locations r due only to cortical sources can be
expressed as the following integral over the cortical surface:

Φ

() ( )( )()

∫∫

Φ r , t = G r r , ′ ⋅ P r ′ , t dS r ′ (1.5)

S

S

If subcortical sources contribute, (1.5) may be replaced by a volume integral over
the entire brain. All geometric and conductive properties of the volume conductor
are accounted for by the Green's function G S ( r , r ′ ), which weighs the contribution of

the mesosource field P ( r and the location of the

recording point r on the scalp. Contributions from different cortical regions may or
may not be negligible in different brain states. For example, source activity in the

′ , t ) according to source location r ′