Thatcher's reply on Gamma Oscillation: Master or Slave
In Brain Topopgraphy the EEG Gamma frequencies (30 – 80 Hz) are given a whole issue in Vol. 22 No. 1. This issue shows that there are serious artifact contamination problems in the Gamma frequency due to muscles and saccades of the eyes. These studies confirm that Gamma frequencies > 30 Hz are not technically easy because of the low pass filter characteristics of the dura and skull and muscle artifact is larger amplitude than EEG gamma.
The Gamma frequency range has no hard and specific frequency definition, except that voltage dependent resonance of pyramidal neurons ends at or near 20 Hz and intracellular studies show that inhibitory neuron bursting is responsible for EEG frequencies in the 20 Hz to 200 Hz range (Buzsaki, 2006).
The study by Schroeder and Lakatos "The Gamma Oscillation: Master or Slave? Is a must read. There is a excitement about the higher frequencies of the EEG, i.e., > 20 Hz but most importantly is the fact that the lower frequencies determine the burst frequency of the higher frequencies, that is, the EEG is like the ocean where the higher frequencies ride on top of the slower waves. The EEG slow waves represent large surges of excitability in the form of ions crossing neural membranes, a little like a stampede and the higher frequencies are synchronized by the low frequency ionic surge and neurons fire action potentials during the depolarizing phase of the slower ionic waves. Thus, it is the slow waves that are the "master" and the "slave" is Gamma. Below is a Rapid share hyperlink to Schroeder and Lakatos's article:
Abstract The idea that gamma enhancement reflects a state of high neuronal excitability and synchrony, critical for active brain operations, sets gamma up as a ``master'' or executor process that determines whether an input is effectively integrated and an effective output is generated. However, gamma amplitude is often coupled to the phase of lower frequency delta or theta oscillations, which would make gamma a ``slave'' to lower frequency activity. Gamma enslavement is productive and typical during rhythmic mode brain operations; when a predictable rhythm is in play, low and mid-frequency oscillations can be entrained and their excitability fluctuations of put to work in sensory and motor functions. When there is no task relevant rhythm that the system can entrain to, low frequency oscillations become detrimental to processing. Then, a continuous (vigilance) mode of operation is implemented; the system's sensitivity is maximized by suppressing lower frequency oscillations and exploiting continuous gamma band oscillations. Each mode has costs and benefits, and the brain shifts dynamically between them in accord with task demands.
http://rapidshare.com/files/384204539/Gamma_Oscillation-Master_or_Slave.pdf
Enjoy - Bob