What are we measuring in EEG and MEG? Methods for Dummies 2007 Matthew Longo

Basic Logic Electrical activity of neurons produces currents spreading through the head. These currents reach the surface of the scalp, in the form of voltage changes and magnetic fields, both of which can be measured non- invasively. Measured voltage changes at the scalp are called the electroencephologram (EEG). Measured magnetic fields at the scalp are called the magnetoencephologram (MEG).

Electrical Activity in the Cortex Action Potential –Presynaptic –Axonal –Generally notmeasurable by EEG/MEG Excitatory postsynaptic potential (EPSP) –Postsynaptic –Dendritic –Measurable by EEG/MEG

Apical dendrites are oriented in parallel along the cortical sheet Apical dendrites thought to contribute strongest signals measurable with EEG/MEG Axons are more randomly located, resulting in currents from presynaptic action potentials cancelling each other out Postsynaptic electrical activity (EPSP) sums, creating large “dipole”

EEG and MEG Signals EPSPs of parallel dendrites in cortical columns creates: –Primary current (what we want to know about) –Secondary/volume currents Measured by EEG Influenced by intervening tissue –Magnetic field perpendicular to primary current Measured by MEG Unaffected by intervening tissue

Effects of Dipole Alignment

Spatial Resolution Single synapse on dendrite contributes ~20 fA-m (femto = = one quadrillionth) Empirical observations suggest EEG/MEG signals are typically ~ 10 nA-m (nano = = one millionth) Therefore, typical EEG/MEG signals reflect summed activity of ~ 500,000 – 1,000,000 neurons ~ 1-5 mm 2 of cortex forms lower bound of spatial resolution In practice, the inverse problem further limits the ability to spatially pinpoint EEG/MEG signals.

MEG Signals MEG measures the fluctuations of frequency (Hz) and amplitude (T) of the brain magnetic signal 10 fT ( ) to about several pT ( ) Earth’s magnetic field ~.5 mT Requires: –Preposterously sensitive magnetometer (SQUID) –Shielding from external noise

The SQUID Superconducting Quantum Interference Device (SQUID)

Pros and Cons of EEG & MEG EEGMEG Signal magnitudeLarge signal (10 mV), easy to detect Tiny signal (10 fT), difficult to detect CostCheapExpensive What does signal index?Measures secondary (volume) currents Measures fields generated by primary currents Signal purityAffected by skull, scalp, etc.Unaffected by skull, scalp, etc. Temporal Resolution~ 1 ms Spatial Localization~ 1 cm~ 1 mm Experimental FlexibilityAllows some movementRequires complete stillness Dipole OrientationSensitive to tangential and radial dipoles Sensitive only to tangential dipoles

Further Reading Baillet et al. (2001). Electromagnetic brain mapping. IEEE Signal Processing Magazine. Del Gratta et al. (2001). Reports on the Progress of Physics, 64, Hämäläinen et al. (1993). Review of Modern Physics, 65, Murakami & Okada. (2006). Journal of Physiology, 575.3, Nunez & Silberstein. (2000). Brain Topography, 13,