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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.

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what are we measuring in eeg and meg

What are we measuring in EEG and MEG?

Methods for Dummies 2007

Matthew Longo

basic logic
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
Electrical Activity in the Cortex
  • Excitatory postsynaptic potential (EPSP)
    • Postsynaptic
    • Dendritic
    • Measurable by EEG/MEG
  • Action Potential
    • Presynaptic
    • Axonal
    • Generally notmeasurable by EEG/MEG
slide5
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
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
spatial resolution
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 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
The SQUID
  • Superconducting Quantum Interference Device (SQUID)
further reading
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.