Neural Electromagnetics

NEM responses are governed by the same physical processes that give rise to electric and magnetic fields in other systems. The vector currents set up by potential differences along cellular processes give rise to an electric field aligned with the current and an orthogonal magnetic field that encircles the current element. The sense (polarity) of the magnetic flux is predicted by the right-hand rule, which also predicts the direction of current flow induced in a coil penetrated by the flux. In a simple medium, charge is neither created nor destroyed; thus, all currents flow in closed loops. These relationships are summarized in Fig. 1.

Longitudinal, intracellular current flowing in the neuron during activation is matched by return current flowing through the extracellular space. These currents flow passively throughout the entire volume conductor, driven by the spatial potential gradient and limited by the resistance of the medium. In a spherical volume conductor, the magnetic contributions of these volume return currents integrate to zero, so that the measurement is dominated by the coherent currents flowing within neurons. The magnetic field is not strongly affected by the properties of biological tissue. EEG depends on the measurement of potential distributions set up by charge migration within the electric field and are critically dependent on the properties of the volume conductor. The potentials observed at the head surface arise from volume or capacitance currents that penetrate the highly resistive skull.

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