EEG and Brain Connectivity: A Tutorial - Bio-Medical Instruments, Inc.

that the total population of synaptic generators of the EEG are the
summation of : 1- a synchronous generator (M) compartment and, 2- an
asynchronous generator (N) compartment in which the relative contribution
to the amplitude of the EEG is A = M N . This means that synchronous
generators contribute much more to the amplitude of EEG than
asynchronous generators. For example, assume 10 5 total generators in
which 10% of the generators are synchronous or M = 1 x 10 4 and N = 9 x
10 4 4
4
then EEG amplitude = 10 9x 10 , or in other words, a 10% change in
the number of synchronous generators results in a 33 fold increase in EEG
amplitude (Lopez da Silva, 1994). Blood flow studies of intelligence often
report less blood flow changes in high I.Q. groups compared to lower I.Q.
subjects (Haier et al, 1992; Haier and Benbow, 1995; Jausovec and
Jausovec, 2003). Cerebral blood flow is generally related to the total
number of active neurons integrated over time, e.g., 1 – 20 minutes
(Yarowsky et al, 1983: 1985; Herscovitch, 1994). In contrast, EEG
amplitude as described above is influenced by the number of synchronous
generators much more than by the total number of generators and this may
be why high I.Q. subjects while generating more synchronous source current
than low I.Q. subjects often fail to show greater cerebral blood flow
(Thatcher et al, 2006).
3- What is Volume Conduction and Connectivity?
The EEG has a dual personality. One personality are the electrical
fields of the brain which operate at the speed of light where dipoles
distributed in space turn on and off and oscillate at different amplitudes and
frequencies. Paul Nunez’s book “Electrical Fields of the Brain”, Oxford
Univ. Press, 1981 is an excellent text especially in regard to the electrical
personality of the EEG. The other personality of the EEG is the source of
the electrical activity which is an excitable medium, much like a forest fire
in which the fuel at the leading edge of the fire results in a traveling wave
with ashes left behind representing a long duration refractory period.
Hodkin and Huxley wrote the fundamental excitable medium equations of
the brain in 1952 for which they later received the Nobel prize. The
excitable medium of the brain are the axons, synapses, dendritic membranes
and ionic channels that behave like “kindling” at the leading edge of a
confluence of different fuels and excitations. As mentioned previously, the
majority of the cortex about 80% is excitatory with recurrent loop
connections yet there is no epilepsy in a healthy brain. How is such
stability possible with such an abundance of positive feedback? The answer