Physiological and Physical Basis of Functional Brain Imaging 6. EEG/MEG Kâmil Uludağ, 20. November 2007
Course schedule 1. Overview 2. fmri (Spin dynamics, Image formation) 3. fmri (physiology) 4. fmri (arterial spin labeling, other MRI methods, summary, references) 5. NIRS 6. E/MEG (electric and magnetic physiology) 7. E/MEG (reconstruction) 8. PET 9. Multi-modal imaging 10. Open prepared discussion of imaging technologies
Neurovascular coupling rcbf MR Imaging Stimulus Neuronal activity CMRO 2 Blood Oxygenation metabolism Electro- and magneto encophalography (E/MEG) measure mass synchronized neuronal activity
Spatial and temporal resolution
Neuronal activity Ionic exchange across the neuron and Action potentials create currents and magnetic fields
Ionic processes neurons are surrounded by a membrane intra- & extracellular compartments with different ions special proteins pump selected ions Na+-K+ pump (3 Na+ out, 2 K+ in) ost energy consuming process in the body
Postsynaptic potentials (PSP) Action Potentials Action potential at the synaptic junction of the presynaptic neuron Release of neurotransmitters Binding at Receptors Ion channels activated De- or hyperpolarization
Neuronal activity Generated at the cell body/axone junction Depolarization Hyperpolarization Repolarization
Excitation & Inhibition Acetylcholine or glutamate Activate Na+ and Ca++ channels GABA Activate Cl- channels Depolarization Excitatory PSP Summation of EPSP (excitatory PSP) Action potential at the cell body/axon junction Hyperpolarization Prevents action potential generation IPSP (inhibitory PSP)
EPSP and EEG EPSP are measured with M-EEG - Generate intracellular currents and extracellular currents - Generate (approximately) one current dipole - Dipolar fields decrease with distance as 1/r² - Duration = 10 ms 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 Mass neuronal activity needed
Pyramidal Neuron as Dipole Dipole character of pyramidal cells adapted from Ritter
Pyramidal cells 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 E/MEG signals are mainly produced by EPSP generated at apical dendrites of pyramidal cells in the cortex no significant contribution of other neuron types (e.g. stellate cells)
EPSP and IPSP dipoles Depending on location of EPSP and IPSP, different scalp EEG are generated adapted from Ritter
Primary and secondary currents 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
Cortical structure Cortical structure determines what is measurable and what not E.g. currents on the opposite sulcus cancel each out Spatial blurring and attenuation of signals, especially prominent in EEG Magnetic field
Cortical structure (2) Radial currents will not produce magnetic fields outside the head MEG only detects tangential currents
Summary
Noise is about a factor of 10³ to 10 6 larger than the MEG signal
SQUID Superconducting QUantum Interference Device SQUIDs are sensitive to very low magnetic fields The SQUIDs "translate" the magnetic field into an electrical current which is proportional to this field To have their superconductive properties, the SQUIDs need to be maintained at-269 C They are cooled in liquid He
MEG instrument Short set-up time Magnetic shielded room Hardware and software Averaging
EEG instrument Cap Amplifiers Electrodes Gel
E/MEG artifacts Physiological fluctuations create unwanted E/MEG signals
10-20 system Anatomical landmarks are designed by the 10-20 system
EEG instrument Differential signals are measured
Sampling rate Typically several 100 Hz
Brain oscillations Brain state is characterized by oscillations at certain frequencies Gamma (γ) activity > 30 Hz Beta (β) activity > 13 Hz Alpha (α) activity 8-13 Hz Theta (Θ) activity 4-7 Hz Delta (δ) activity < 4 Hz Spiking activity
ERP and source localization Trial-locked averaging yields event-related potentials (ERPs).
Time-frequency analysis Makeig et al., 2004
ERP conventions Standard ERP deviations named N (negative) and P (positive) and number or time of appearance (e.g. N1 = N75)
Phase resetting ERP components are created by phase-resetting of oscillations or by evoked responses
Displaying the results
Fetal MEG Fetal MEG as a niche application (although fetal MRI also possible)
ERP components Each component has unique meanings: e.g. N75 input, P300 attention & novelty Intense research to explore basis of component generation
Spatio-temporal evolution of ERPs Unique spatio-temporal evolution of ERP Dale, 2000
ERP fingerprint for diseases Diseases are correlated to typical pattern of ERP
Summary Only EPSP are measured with E/MEG mostly synchronous pyramidal cells some neurons (e.g. stellate cells) don t give rise to any signals signal cancellation due to gyrus/sulcus structure spatial blurring due to conductivity MEG does not measure tangential magnetic fields Oscillations and ERPs are specific to brain processes
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