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

Basic LogicElectrical 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 CortexAction PotentialPresynapticAxonalGenerally notmeasurable by EEG/MEG

Excitatory postsynaptic potential (EPSP)PostsynapticDendriticMeasurable by EEG/MEG

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

EEG and MEG SignalsEPSPs of parallel dendrites in cortical columns creates:Primary current (what we want to know about)Secondary/volume currentsMeasured by EEGInfluenced by intervening tissueMagnetic field perpendicular to primary currentMeasured by MEGUnaffected by intervening tissue

Effects of Dipole Alignment

Spatial ResolutionSingle synapse on dendrite contributes ~20 fA-m (femto = 10-15 = one quadrillionth)Empirical observations suggest EEG/MEG signals are typically ~ 10 nA-m (nano = 10-9 = one millionth)Therefore, typical EEG/MEG signals reflect summed activity of ~ 500,000 1,000,000 neurons~ 1-5 mm2 of cortex forms lower bound of spatial resolutionIn practice, the inverse problem further limits the ability to spatially pinpoint EEG/MEG signals.

MEG SignalsMEG measures the fluctuations of frequency (Hz) and amplitude (T) of the brain magnetic signal10 fT (10-15) to about several pT (10-12)Earths magnetic field ~ .5 mTRequires:Preposterously sensitive magnetometer (SQUID)Shielding from external noise

The SQUIDSuperconducting Quantum Interference Device (SQUID)

Pros and Cons of EEG & MEG

Further ReadingBaillet et al. (2001). Electromagnetic brain mapping. IEEE Signal Processing Magazine.Del Gratta et al. (2001). Reports on the Progress of Physics, 64, 1759-1814. Hmlinen et al. (1993). Review of Modern Physics, 65, 413-497.Murakami & Okada. (2006). Journal of Physiology, 575.3, 925-936.Nunez & Silberstein. (2000). Brain Topography, 13, 79-96.

EPSP: a temporary depolarization of postsynaptic membrane potential caused by the flow of positively charged ions into the postsynaptic cell Magnetic fields are also created by volume currents, but approximately cancel and sum to zero. Assuming a spherical head, they would precisely sum to zero.