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9 Cards in this Set
- Front
- Back
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MEG Definition
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Measures magnetic field produced by neural activity in the brain. MEG detects pyramidal cells of the cortex.
Magnetic field of brain very weak - need very sensitive detectors and way of suppressing magnetic background |
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SQUIDS
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Detectors are called SQUIDS - Superconducting Quantum Interference Device
Cooled by liquid helium (scanner needs about 300) |
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Suppressing Magnetic Background
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Shielding (scanner in magnetically shielded room)
Gradient Measures (closer the magnetic source is to SQUID the more rapidly the M field will decrease) Software Processing (used to remove external M field) |
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Using MEG
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Temporal Analysis (EVP) - similar to EEG but with two types of measurement gradiometers, magnetometers
Spatial Analysis (Dipole Fitting - computer calculates the location of a magnetic dipole that would give rise to recorded magnetic fields.) |
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Differences between MEG and EEG
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EEG = Gyri vs. MEG = Sulci
MEG can only detect magnetic dipoles not oriented radially thus it cannot typically detect activity at the surface of the brain and also has difficulty detecting activity at centre of the brain (both due to radial dipoles) |
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Advantage of MEG over EEG
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MEG detects fewer sources is often able to localise them more precisely than EEG as skull and meninges distort magnetic fields less than EEG signals. Still only small spatial improvement as you have to use Dipole Modelling (fMRI still better option).
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fMRI Technique Summary
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Poor temporal resolution (max temporal R = 2 Secs)
Good spatial resolution (3x3x3 mm3) Good at determining which brain areas active Bad at determining when brain areas become active Very expensive to own and run but safe |
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EEG/MEG Technique Summary
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Great temporal resolution (on the order of a few MS) but uncertain spatial resolution
Can be used to localise brain activity (to within 5mm) as inverse problem they are unreliable - MEG better at this Great at determine when a brain is activated Bad at determining which brain areas activated EEG cheap ($10 per session) vs. MEG ($1000) |
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TMS Technique Summary
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Virtual lesion
Great at determining if a brain area is required for a task AND for determining when that brain area is required. Cannot use to scan brain for survey work to work out what areas are involved Some safety concerns, quite cheap to buy and free to use |
