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Week 09: Magnetoencephalography (MEG)

Magnetoencephalography measures the minute magnetic fields ($10-100\text{ fT}$) produced by synchronous intracellular neuronal currents. Unlike electrical potentials, magnetic fields pass undistorted through the skull. This week covers SQUIDs, magnetically shielded rooms, gradiometer coils, and source localization.

Interactive Demos & Activities

• MEG Simulator: From Dipoles to Source Estimates

  Full Recording Simulator · SQUIDs & OPMs, Shielding, SSP & SSS, ICA, Averaging & Inverse Methods

  Describe a recording in a small language (sensor array, sampling rate, filters, noise suppression and a stimulus timeline with trials and conditions) and record it from dipole sources in a spherical head with magnetometers, axial or planar gradiometers, or on-scalp OPMs. Compare the MEG field map with the EEG potential map of the same source to see the 90° rotation, the silence of radial and deep sources and the smearing of EEG by the skull. Switch environmental noise, line noise, the heart, blinks, muscle, dental work, SQUID jumps, a bad channel and head movement on and off, then the shielded room, gradiometers and reference sensors, SSP and SSS, filters, ICA and movement compensation. Watch an evoked field emerge as SNR grows with $\sqrt{N}$, coregister the head with an MRI, and compare dipole fitting, minimum norm and a beamformer on the same data.

• EEG vs. MEG: Radial vs. Tangential Dipole Sensitivity

  Comparative Biophysics · Sarvas Law & Cortical Sulci

  Directly test the fundamental biophysical difference between EEG and MEG: rotate a current dipole along a cortical fold (gyral crown vs. sulcal bank) to observe why radial dipoles produce zero external magnetic flux ($B_r = 0$) while tangential dipoles generate robust dipolar magnetic fields.

• Interactive SQUID Magnetometer vs. Gradiometer Simulator

  Signal Processing Simulator · Noise Cancellation

  Experience why gradiometer configurations (two opposing coils) are critical for measuring femtotesla neural signals: observe how uniform ambient magnetic noise is subtracted out while near-field cortical dipole signals are preserved.

• Vector Field & Curl Explorer

  Electromagnetic Principles · Biot-Savart Law

  Visualize vector fields and rotational curl ($\nabla \times \vec{B}$), reinforcing the right-hand rule and magnetic field lines generated by primary intracellular dendritic currents.

• MEG Modality Card Game

  Educational Card Game · SQUIDs & Source Localization

  Match SQUID, OPM and gradiometer sensors, the brain rhythms, shielding, signal space separation and source localization, and presurgical language mapping.

Topics Covered

• The Right-Hand Rule & Primary vs. Volume Currents: Why MEG is predominantly sensitive to tangential current dipoles in the sulci, whereas EEG detects both radial (gyral) and tangential sources.
• Cryogenic SQUIDs & Shielding: Liquid helium cooling (4 Kelvin), physical flux transformers, and multi-layer mu-metal magnetically shielded rooms (MSR).
• The Inverse Problem & Source Reconstruction: Helmholtz's ill-posed inverse problem, equivalent current dipoles (ECD), minimum norm estimation (MNE), and beamforming.