Slide 1

What are we measuring with EEG and MEG ? Isabel Zlobinski & Xavier De Tige Slide 2 Introduction Neurophysiological background EEG MEG Slide 3 Introduction EEG and MEG are 2 functional cerebral imaging techniques that are closely related In both methods, the measured signals are generated by the same synchronized neuronal activity in the brain The main interest of M-EEG compared to other techniques TEMPORAL RESOLUTION Slide 4 Slide 5 The temporal resolution of M-EEG Follow the rapid changes in cortical activity Reflect ongoing signal processing in the brain Slide 6 Neurophysiological background Glial cells Structural support Metabolism Ions & NTT transport Myelin Neurons Information-processing units Slide 7 Grey matter cell bodies & dendrites White matter axones (myeline) Cortex & basal ganglia Slide 8 Like other cells, the neurons are surrounded by a membrane The membrane divides the tissue into intra- & extracellular compartments with different ions [ ] The difference in ions [ ] is maintained against their [ ] gradient by special proteins that pump selected ions Na+-K+ pump (3 Na+ out, 2 K+ in) Slide 9 The differences in ions [ ] & the permeability of the membrane for each ion Resting potential Goldman s equation The resting state of the neurons can be modified by Action PotentialsPostsynaptic Potentials AxonesSynaptic junctions + + + + + + + - - - - - - - - - - - - - - - - - - - Slide 10 Action Potentials DepolarizationHyperpolarization Repolarization Generated at the cell body/axone junction Slide 11 Action potentials - Generate 2 current dipoles = quadrupole parallel, equal intensity, opposite directions => 0 - Quadrupolar field decreases with distance as 1/r (compared to 1/r for dipolar field) - Duration = 1 ms temporal summation between neighbouring fibers difficult Not observable with M-EEG Slide 12 Postsynaptic Potentials Action Potentials Synaptic junctions mainly on cell body & dendrites Liberation of neurotransmitters Receptors Ion channels activated De- or hyperpolarization Action potential at the synaptic junction of the presynaptic neuron Slide 13 Acetylcholine or glutamate Activate Na+ and Ca++ channels Depolarization Excitatory PSP GABA Activate Cl- channels Hyperpolarization Prevents action potential generation Inhibitory PSP Summation of EPSP Action potential at the cell body/axon junction Slide 14 EPSP - Generate intracellular currents and extracellular currents - Generate (approximately) one current dipole - Dipolar fields decrease with distance as 1/r - Duration = 10 ms temporal summation between neighbouring fibers more effective A single EPSP produces a current dipole along the dendrite with a stenght of +/- 20 fA m Too small to be measured with M-EEG Are measured with M-EEG Slide 15 M-EEG see sources with strenght on the order of 10 nA m Cummulative summation of one million of synaptic junctions in a small region is required As apical dendrites of pyramidal neurons of the cortex tend to be perpendicular to the cortical surface Cummulative summation of EPSP in the same direction is more easily obtained with apical dendrites of pyramidal cells M-EEG signals are mainly produced by PSP generated at apical dendrites of pyramidal cells in the cortex Slide 16 What are we measuring with MEG ? Slide 17 M E G Magneto- -encephalo- -graphy Record magnetic fields generated by brain activity Slide 18 PSP induced intracellular currents (primary currents) and extracellular currents (secondary currents) Secondary currents yield potential differences on the scalp of the head that can be measured by EEG MEG measures magnetic fields induced mainly by primary currents Cummulative summation of PS primary currents of millions apical dendrites of pyramidal cells in one cortical area Generates a magnetic field measurable by MEG Slide 19 Primary currents Volume currents Induced magnetic field "Right Hand Law" Slide 20 Tangential currents will produce magnetic fields that are observable outside the head Radial currents will not produce magnetic fields outside the head MEG only detects tangential currents Slide 21 Slide 22 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 ) BUT Earth s magnetic field is about 0.5 mT Urban magnetic noise is about 1 nT to 1 T Noise is about a factor of 10 to 10 6 larger than the MEG signal Moving vehicules, moving elevators, radio, TV, powerlines, etc. The electrical activity of the heart, eye blinks also generate a field 2 to 3 order of magnitude larger than the signal from the brain Slide 23 We need very sensitive MEG sensors to pick up the brain magnetic fields SQUIDs MEG measurements need noise cancellation with extraordinary accuracy Design of the SQUID Magnetic shielded room Hardware and software Averaging Slide 24 Superconducting QUantum Interference Device To have their superconductive properties, the SQUIDs need to be maintained at-269 C The SQUIDs "translate" the magnetic field into an electrical current which is proportional to this field They are cooled in liquid He SQUIDs are sensitive to very low magnetic fields Slide 25 The different types of pick-up coils Magnetometers Axial and planar gradiometer CTF system Slide 26 19801995-2000 Whole-head sensors arrays which use 100 to 300 sensors at different locations Slide 27 Noise cancellation SQUID Design 1st order axial gradiometer This SQUID will only be sensitive to inhomogeneous changes of magnetic fields between the 2 coil Compensation coil compensates for variations in the background field Pick-up coil picks up the signal from the brain Background fields will be spatially uniform Shielded room Reduce the effect of external magnetic disturbances Slide 28 Hardware and softwares Use of reference A linear combination of the reference output is subtracted from the MEG primary sensor output Use of filters Low-pass filter, high pass filter 50-Hz filter, etc... Use of specific softwares Averaging of brain signals Slide 29 With MEG, you can make (as in EEG) : - Continuous acquisition of brain signals and study some events that appear randomly (Epileptic abnormalities, etc.) - Evoked response: averaged MEG signals that are synchronous with an external stimulus or voluntary motor event Slide 30 Slide 31 Slide 32 Slide 33 References : - Hmlinen et al., Reviews of Modern Physics, 1993 - Baillet et al., IEEE Signal Processing Magazine, 2001 - Jeremie Mattout PhD thesis - Murakami & Okada, J Physiol, in press