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 = 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 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 (10-15) to about several pT (10-12)

• 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 & MEGEEG MEG

Signal magnitude Large signal (10 mV), easy to detect

Tiny signal (10 fT), difficult to detect

Cost Cheap Expensive

What does signal index? Measures secondary (volume) currents

Measures fields generated by primary currents

Signal purity Affected by skull, scalp, etc. Unaffected by skull, scalp, etc.

Temporal Resolution ~ 1 ms ~ 1 ms

Spatial Localization ~ 1 cm ~ 1 mm

Experimental Flexibility Allows some movement Requires complete stillness

Dipole Orientation Sensitive 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, 1759-1814.

• Hämäläinen 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.