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I see this fact stated in many places that discuss the basis of the MEG signal but it's never been made completely clear why MEG can't distinguish IPSPs and EPSPs. Please could someone elaborate?

Thanks

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Two reasons:

First, currents create a magnetic signal that is measured by MEG; from the perspective of the MEG, there isn't any meaning to whether ions are moving into cells or out of them, or whether the ions moving are positive or negative, just that there is some net charge moving a particular direction. Same for EEG. If you can connect a particular waveform to a particular event in the brain through other means, though, you may be able to infer that a given signal is primarily excitatory or inhibitory, but not directly from MEG itself. The Wikipedia article on MEG talks a bit about how this all works and how some level of synchrony is necessary for MEG to work: https://en.wikipedia.org/wiki/Magnetoencephalography Mostly, it works because of the orientation of neurons and flow within dendrites rather than across membranes. Here's a paint-quality diagram:

Current flow

On the left, positive ions enter a dendrite (black arrows). This causes a net positive flow down the dendrite (red solid arrow). On the right, negative ions enter a dendrite somewhere else (black arrows). This causes a net positive flow in the same direction. MEG is measuring synchronous red solid arrow current flow over many many of these parallel dendrites.

Second, most IPSPs in the brain are shunting inhibition; they don't create much net current or voltage change at all, rather they help to maintain membrane potential near the chloride reversal which is near a typical rest potential.

In any event, the things that MEG or EEG measure are typically rhythmic brain activity, and we know this sort of activity results from a combination of inhibition and excitation anyways. So, while it's true that one can't directly measure EPSPs or IPSPs, that's not at all what these techniques measure. It's sort of like saying that a scale can't tell you how tall you are.

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