Physiological Foundations of Quantitative EEG Analysis - Quantitative EEG Analysis Methods and Clinical Applications - page 19

would like to understand better the complex nonlinear dynamics-associated cogni-
tion and behavior, but even the origin of the 100-ms timescale associated with the
10-Hz alpha rhythm remains controversial. Many view the human brain as the pre-
eminent complex system, although one may argue that the human social system
consisting of 6 10 9 interacting brains is far more complex [8]. In any case, the

physiological origins of cortical dynamics can be expected to challenge many future
generations of scientists.
With this perspective in mind, I tend to favor more emphasis on relatively sim-
ple questions about brain dynamics based on the general rule that a person must evi-
dently learn to crawl before he can walk. Thus, I conclude this chapter with a short
summary of several “simple” theoretical issues associated with EEG dynamics.
Three basic questions can be expressed as follows:

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1. Which spatial scale(s) of tissue determine the dominant timescales observed
in EEGs? For example, do alpha or gamma oscillations originate from single
neurons, from millimeter-scale networks, from the entire cortex, or can they
be generated simultaneously at multiple scales?
2. How important are interactions across spatial scales?
3. How important are cortical global boundary conditions? And, do global
boundary conditions provide an important top-down influence on
small-scale dynamics as typically observed in physical systems including
chaotic systems [4]?

To distinguish the various theories of large scale cortical dynamics, I have sug-
gested the label local theory to indicate mathematical models of cortical or
thalamocortical interactions (feedback loops) for which corticocortical propaga-
tion delays are assumed to be zero. The underlying timescales in these theories are
typically postsynaptic potentials (PSP) rise and decay times. Thalamocortical net-
works are also “local” from the viewpoint of scalp electrodes, which cannot distin-
guish purely cortical from thalamocortical networks. Finally, these theories are
“local” in the sense of being independent of global boundary conditions dictated by
the size and shape of the cortical-white matter system.
By contrast, I use the label global theory to indicate mathematical models in
which delays in corticocortical fibers forming most of the white matter in humans
provide the important underlying timescales for the large spatial scale EEG dynam-
ics recorded by scalp electrodes. Periodic boundary conditions are generally essen-
tial to global theories because the cortical-white matter system of each hemisphere
is topologically very close to a spherical shell. One global theory [2, 4, 18, 33] that
follows the mesoscopic excitatory synaptic action field Ψ e ( r , t ) has achieved some

predictive value in electroencephalography despite its neglect of most network
effects. This “toy brain” is presented first as a plausible entry point to more realistic
theory in which cell assemblies play a central role in cognition and behavior. Sec-
ondly, I conjecture that global synaptic action fields may act (top-down) on local
networks in a manner analogous to human cultural influences on social networks,
thereby providing a possible solution to the so-called binding problem of brain sci-
ence [2, 8, 18]. Several recent theories of neocortical dynamics include selected