History Of The Electrophysiology Of The Brain

The existence of the electrical activity of the brain (i.e., the EEG) was discovered more than a century ago by Caton. After the demonstration that the EEG could be recorded from the human scalp by Berger in the 1920s, it was slowly accepted as a method of analysis of brain functions in health and disease. It is interesting to note that acceptance came only after the demonstration by Adrian and Mathews in the 1930s that the EEG, namely the alpha rhythm, was likely generated in the occipital lobes in man. The responsible neuronal sources, however, remained undefined until the 1970s, when it was demonstrated in dog that the alpha rhythm is generated by a dipole layer centered on layers IV and V of the visual cortex. It is not surprising that the mechanisms of generation and the functional significance of the EEG remained controversial for a relatively long time, considering the complexity of the underlying systems of neuronal generators and the involved transfer of signals from the cortical surface to the scalp due to the geometric and electrical properties of the volume conductor (brain, cerebrospinal fluid, skull, and scalp).

The EEG consists essentially of the summed electrical activity of populations of neurons, with a modest contribution of glial cells. Considering that neurons are excitable cells with characteristic intrinsic electrical properties and that interneuronal communication is essentially mediated by electrochemical processes at synapses, it follows that these cells can produce electrical and magnetic fields that may be recorded at a distance from the sources. Thus, these fields may be recorded a short distance from the sources [i.e., the local EEG or local field potentials (LFPs)], from the cortical surface (the electrocorticogram), or even from the scalp (i.e., the EEG in its most common form). The associated MEG is recorded usually by way of sensors placed at a short distance around the scalp.

In order to understand how the electrical and magnetic signals of the brain are generated, it is necessary to examine how the activity of assemblies of neurons is organized both in time and in space and which biophysical laws govern the generation of extracellular field potentials or magnetic fields.

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