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I study cognitive neuroscience and I periodically run into physics related questions in the context of neuroimaging technologies.

My question specifically refers to electric and magnetic fields that can be measured by electroencephalography (EEG) and Magnetoencephalography (MEG), respectively.

One interesting difference between the EEG and MEG signal is that unlike the electric field, the magnetic field is unimpeded by differing conductances across brain, skull, scalp and other tissues. I was wondering if somebody could explain what differences between the two fields account for these phenomena.

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Although it is possible to shield from magnetic fields by re-routing them through material with high magnetic permeability, all magnetic fields must terminate at the opposite pole. That's why there are no magnetic monopoles in existence. Once a magnetic field is created, it is not stopped by living tissue.

Electric fields, on the other hand, are easily shielded by almost anything, including living tissue. Living tissue with varying conductivity may act more or less like a Faraday Cage (http://en.wikipedia.org/wiki/Faraday_cage).

The lines of force of a magnetic field generally are less affected than electric fields by the electrical conductivity of materials through which the magnetic field passes. This may be why the MEG signal is unimpeded by electrical conductances across living tissue.

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    $\begingroup$ The electric field in an EEG is not stationary and does not arise from static charges. Answer needs substantial revision. $\endgroup$ – Rob Jeffries Nov 3 '18 at 13:08
  • $\begingroup$ @RobJeffries : Thank you. I revised the answer to eliminate superfluous material about static electric charges. $\endgroup$ – Ernie Nov 5 '18 at 16:34
  • $\begingroup$ OK, but you now have that "magnetic fields must terminate in an opposite charge", which isn't the case since no magnetic charges exist. You are also claiming that the passage of (time variable) magnetic fields is unaffected by the conductivity of materials, but this may not be true (e.g. induction heating). You need to combine this statement with some discussion of the (low?) frequency of the signals. $\endgroup$ – Rob Jeffries Nov 5 '18 at 16:56
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    $\begingroup$ @RobJeffries : I will work with these points and revise the answer. Thank you. $\endgroup$ – Ernie Nov 7 '18 at 1:02

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