4 ms·

Neurons and muscles are electrically active, so an external electric field can weakly polarize a cell, and thereby alter its response to ongoing activity. They

Neurons and muscles are electrically active, so an external electric field can weakly polarize a cell, and thereby alter its response to ongoing activity. They're not magnetically active, but moving conductors in a magnetic field produce current, so it's not wildly implausible. Indeed, if you were a crab getting unexpectedly stimulated, sitting still would be one way to make it stop!

superkuh 5y ago
The rate of change in current, dI/dt, determines the rate of change in magnetic field and so induced voltage. In order to depolarize neurons in transcranial magnetic stimulation dI/dT in the magnetic coils placed directly above the tissue is about 2000-6000 amps per millisecond. Even a lot of AC power at low frequency in a single wire pair (or whatever) is not in any circumstances going to induce a voltage across or along a membrane or even in extracellular space that can effect the polarization of a neuron. So it is clear that changes in the magnetic field inducing voltages cannot be the mechanism of action in this case.
- mattkrause 5y ago
  TMS produces suprathreshold stimulation: it can take a neuron from its (passive) resting potential to spike threshold, all by itself. The numbers you give are correct for that. Weaker stimulation can't do that, but subthreshold stimulation can "nudge" the timing of spikes. In a living brain, neurons are constantly bombarded with synaptic input, and therefore sit closer to spike threshold and occasionally spike. Weak stimulation can therefore cause them to fire a bit sooner or later than they otherwise would. Since information is carried not just by the overall amount of firing, but also the spatiotemporal structure of that activity, these subthreshold changes can lead to overt changes in behavior. The recurrent nature of the brain can also amplify these effects. I know this because I've literally measured it in macaques and found that fields well below 1 V/m (produced by ±2 mA 5-40 Hz AC on the scalp) are enough to reorganize spike trains. Here's one of our papers: http://packlab.mcgill.ca/Krause%20et%20al%202019.pdf http://packlab.mcgill.ca/Krause%20et%20al%202019.pdf FWIW, I was surprised by these results--and deeply relieved when they replicated!
  - superkuh 5y ago
    How can you conflate your direct electrode transcranial electrical stimulation paper with non-contact magnetic field rate of change induced electrical fields? That's just unsupportable. You're not going to get anywhere near 1V/m across the brain tissues with even a kiloamp of low frequency (ie, <500 Hz) AC current non-contact. Rate of change matters for induced fields. Cool paper and impressive but the direct contact, part of the circuit, tES results don't translate to this context. The induced voltages (for the power cable) would be so far below the noise floor that not even arguments for stochastic resonance boosting above threshold make sense.

superkuh 5y ago

The rate of change in current, dI/dt, determines the rate of change in magnetic field and so induced voltage. In order to depolarize neurons in transcranial magnetic stimulation dI/dT in the magnetic coils placed directly above the tissue is about 2000-6000 amps per millisecond. Even a lot of AC power at low frequency in a single wire pair (or whatever) is not in any circumstances going to induce a voltage across or along a membrane or even in extracellular space that can effect the polarization of a neuron. So it is clear that changes in the magnetic field inducing voltages cannot be the mechanism of action in this case.

mattkrause 5y ago
TMS produces suprathreshold stimulation: it can take a neuron from its (passive) resting potential to spike threshold, all by itself. The numbers you give are correct for that. Weaker stimulation can't do that, but subthreshold stimulation can "nudge" the timing of spikes. In a living brain, neurons are constantly bombarded with synaptic input, and therefore sit closer to spike threshold and occasionally spike. Weak stimulation can therefore cause them to fire a bit sooner or later than they otherwise would. Since information is carried not just by the overall amount of firing, but also the spatiotemporal structure of that activity, these subthreshold changes can lead to overt changes in behavior. The recurrent nature of the brain can also amplify these effects. I know this because I've literally measured it in macaques and found that fields well below 1 V/m (produced by ±2 mA 5-40 Hz AC on the scalp) are enough to reorganize spike trains. Here's one of our papers: http://packlab.mcgill.ca/Krause%20et%20al%202019.pdf http://packlab.mcgill.ca/Krause%20et%20al%202019.pdf FWIW, I was surprised by these results--and deeply relieved when they replicated!
- superkuh 5y ago
  How can you conflate your direct electrode transcranial electrical stimulation paper with non-contact magnetic field rate of change induced electrical fields? That's just unsupportable. You're not going to get anywhere near 1V/m across the brain tissues with even a kiloamp of low frequency (ie, <500 Hz) AC current non-contact. Rate of change matters for induced fields. Cool paper and impressive but the direct contact, part of the circuit, tES results don't translate to this context. The induced voltages (for the power cable) would be so far below the noise floor that not even arguments for stochastic resonance boosting above threshold make sense.