Download What are we measuring in EEG and MEG?

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 noninvasively.
• 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 & MEG
EEG
MEG
Signal magnitude
Large signal (10 mV), easy to Tiny signal (10 fT), difficult
detect
to detect
Cost
Cheap
What does signal index?
Measures secondary (volume) Measures fields generated by
currents
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
Expensive
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.