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

synchronized cortical neurons produce the electricity called the
electroencephalogram and the generators are largely located near to the
electrode location with approximately 50% of the amplitude produced
directly beneath the recording electrode and approximately 95% within a 6
cm radius (Nunez, 1981; 1995). Unrelated distant sources produce lower
amplitude potentials by volume conduction that add or substract at a zero
phase difference between the source and the surface sensors. Locally
synchronized neurons are connected to distant groups of neurons (3 cm to 21
cm) via cortico-cortical connections (Braitenberg, 1978; Schulz and
Braitenberg, 2002) and are connected to localized clusters or populations of
neurons that exhibit significant phase differences or delays due to axonal
conduction velocities, synaptic rise times, synaptic locations and other
neurophsyiological delays that can not be produced by volume conduction
which is defined at Phase Difference = 0. Connectivity is defined as the
magnitude of coupling between neurons, independent of volume conduction.
This is because in this paper we are interested in the synchronous coupling
and de-coupling of local and long distance populations of neurons that add
together and give rise to the rhythmic patterns of the EEG seen at the scalp
surface (i.e., dynamic connectivity). Much has been learned about brain
function in the last few decades and EEG biofeedback to control robotic
limbs coupled with PET and fMRI cross-validation of the location of the
sources of the EEG shows that the future of quantitative EEG or QEEG is
very bright and positive because of the reality of the neurophysics of the
brain and high speed computers. 3-dimensional EEG source localization
methods have proliferated with ever increased spatial resolution and crossvalidation
by fMRI, PET and SPECT. Understanding measurements of
coupling between populations of neurons in 3-dimensions using 3-
Dimensional Source analysis such as by Michael Scherg, Richard
Greenblatt, Mark Pflieger, Fuchs, Roberto Marqui-Pascual and others in the
last 20 years. An easily applied “Low Resolution Electromagnetic
Tomography” is one of the better localization methods although it does offer
resolutions of only 3 – 6 cm, but nonetheless, much better than the
alternative of zero 3-dimensional resolution that conventional EEG provides
Pascual-Marqui, 1999; Pascual-Marqui et al, 2001; Thatcher et al, 1994;
2005a; 2005b; 2006; Gomes and Thatcher, 2001). As emphasized by many,
it is critical to understand how widely distant regions of the brain
communicate before one can understand how the brain works. It is in
recognition of the importance of understanding brain connectivity especially
using explicit and step by step methods that the present paper was
undertaken. We attempt to use hand calculator simplicity when ever