Brain Activity Measurement

Neurophysiological Basis

It is widely accepted that most of the time both excitatory and inhibitory postsynaptic potentials simultaneously are present in the pyramidal cells of the upper and middle cortical layers. Usually they are in balance without releasing considerable action potentials. It is assumed that this is particularly true when a module became charged without immediate output. A negative potential on the surface is measured because excitatory synapses are predominant in upper layers (negative interstitium in the upper layers). The release of action potentials (negative interstitium far below) will change the dipole causing a positive potential.

Basic Activity

Negative and positive potentials in EEG alternate with main fluctuations within about 0.1 s (equivalent to around 10 Hz). Dominant frequencies in the range of about 8–12 Hz are called EEG alpha. Alpha is observed in awake but resting subjects without demanding memory load. Alpha is generated by burst activity produced by loops between thalamic nuclei and the related cortical areas in case of attenuated stimulation. A lower portion of the alpha band (8–10 Hz) is discussed as reflecting attenuation of cortical activity during mental load while attending stimuli actively, for example in a time series resulting in partial loss of feature-related activation. The upper portion of alpha seems to be closer related to a more general attenuation of mental load mainly in processing stimuli, even by exogenous stimulation. Frequencies of 12–14 Hz (EEG spindles) seem to be indicative of active suppression of sensory stimuli during sleep.

Frequency 4–8 Hz (EEG theta) is discussed as indicative for extension of receptive fields, for example in coarse classification of stimuli. Theta is found to be increased during drowsiness and undirected memory search (flight of thought) as well as during top down or effortful processes causing directed memory search. The latter findings gave rise to the view that theta reflects [Page 146]involvement of hippocampal memory functions. Theta power can be found in posterior locations as well as above the premotor cortex indicating activated wide motor concepts. In learning response concepts, frontal theta is increased in good learners compared to poor learners.

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