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Summary Task 4 - MEG, EEG, oscillations & social cognition
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Summary of Task 4 of Methods of Cognitive Neuroscience
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TASK 4: MEG, EEG, OSCILLATIONS &SOCIAL COGNITION
INTRODUCTION INTO MAGNETOENCEPHALOGRAPHY (MEG)
HOW DOES MEG WORK?
Synaptic activity leads to small magnetic fields perpendicular to electrical current
Non-invasive
Recorded with neuromagnetometer – positioned around outside of the head
Underlying electrical activity deduced by mathematical modelling
MEG traces can be recorded & averaged over a series of trails to obtain event-related fields
(ERFs)
Magnetic fields detected using superconducting quantum interference devices (SQUIDs)
Placed at various points on the surface of the scalp
Magnetic fields must be sampled over a range of locations distribution of electrical
currents inside brain can be calculated accurately
Magnetometers – cover whole scalp & complete magnetic-field pattern can be measured
simultaneously
Newest one – 306 SQUIDs at 102 measuring sites
Each of 102 sensors measures in x, y & z directions
Immersed in liquid helium at -169°C & positioned close to the head
Preventing contamination with fields from e.g., power lines measurements inside a
room made from several layer of aluminium & mu-metal
Iron & nickel – high magnetic permeability external magnetic fields are “trapped” in it,
shielding room inside
All magnetic materials are forbidden inside the shielded room
Direction of magnetic flux outside head determined by direction of current within
group of neurons , according to right-hand rule of electromagnetism
CALCULATING THE SOURCE OF MAGNETIC FIELDS
Inverse problem – brain is spherical & active areas can be adequately represented by
single / multiple current dipoles
Computer makes initial guess to where dipoles might be & then calculates external
magnetic field that these dipoles would produce
Compares computed field to measured field
Repeats calculation with dipoles at different positions until calculated &
experimental results match
2 or more regions of brain active – measured magnetic field depends on position & strength
of dipoles & extent to which neurons in different regions fire at the same time
, Minimum current estimate technique
Gives most probable distribution of currents in brain, calculated according to
concept of minimum norm
Advantage: used without making any specific assumptions about way in which
currents are distributed
ADVANTAGES & LIMITATIONS
Same temporal resolution as ERPs BUT better spatial resolution
Magnetic fields are not distorted as they pass through the brain, skull, scalp
Limitations
Current flow needs to be parallel to surface of the skull (recorded neurons usually
within sulci)
Magnetic fields generated by brain are extremely weak room that is
magnetically shielded from all external magnetic fields
Room for improvement in signal-to-noise ratio
Increased by placing SQUID sensors closer to brain BUT difficult, because they have to be
at liquid-helium temperatures at all times
MEG VS. EEG
MEG EEG
Based on dipolar currents, measure the same neuronal currents
Very good temporal resolution
Better spatial resolution – skull & scalp do Skull & scalp distort electrical potential
not distort magnetic fields
Currents have to be tangential to brain Better at detecting currents that originate
surface – all other currents cancel each deep inside the brain / are radially
other out oriented
Cheaper
DATA ACQUISITION & SIGNAL ANALYSIS IN EEG
Important to adhere to standardised electrode locations – distance of 2-3cm between
electrodes required
Improved special resolution with high-density recordings (64-128 electrodes
enough)
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