Which of the following statements about the nature of MEG signals is CORRECT?

Question

BlackTom AI · Accepted Answer

Actional potentials are less likely to contribute to MEG signals because of their short durations.

Relevant knowledge points and concepts:
- MEG measures magnetic fields generated mainly by synchronous intracellular postsynaptic currents (primary currents) in the apical dendrites of pyramidal neurons. These currents produce net current dipoles when many aligned neurons are activated together.
- Postsynaptic potentials (PSPs) last on the order of ~10–100 ms, allowing temporal summation and measurable low-frequency magnetic fields. Action potentials (APs) are brief (~1 ms) and often produce locally balanced, biphasic currents that cancel out at the population level.
- Tissue conductivity (skull, scalp) strongly distorts electric fields measured by EEG but has negligible effect on magnetic fields measured by MEG. However, the orientation of the neural current relative to the sensors matters: tangential currents to the scalp are detected best by MEG.

Step-by-step reasoning:
1. Compare durations: typical AP duration ≈ 1 ms; PSP duration ≈ 10–100 ms. A 1 ms event has spectral power concentrated around ~1 kHz, while a 10–100 ms event has power below ~100 Hz. MEG sensors and SNR are much better for the lower-frequency, sustained fields from PSPs.
2. Consider spatial summation: For a measurable MEG signal, thousands to hundreds of thousands of similarly oriented, synchronously active neurons are needed. PSPs, being longer and more synchronous across populations, produce coherent dipoles; APs’ rapid, often asynchronous currents cancel.
3. Magnetic fields pass through skull/scalp without substantial distortion, so MEG reflects the true magnetic field geometry but remains sensitive to current orientation.

Why the correct answer is correct:
- The short duration and often bipolar/current-neutral nature of APs greatly reduce their contribution to the low-frequency, synchronous magnetic fields MEG detects. PSPs dominate MEG signals.

Why the other options are incorrect:
- Option 1: The numeric claim about 100,000 neurons is misleading and the reasoning is wrong—MEG sensitivity is due to PSPs, not action potentials.
- Option 2: Magnetic signals are not significantly distorted by skull/scalp; distortion is an EEG issue.
- Option 4: Orientation does affect MEG strength; tangential sources are detected better, so the statement is false.

Conclusion: The correct statement is "Actional potentials are less likely to contribute to MEG signals because of their short durations." This matches the provided answer.

Which of the following statements about the nature of MEG signals is CORRECT?Single choice

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