EEG and MEG Flashcards
What is EEG Based on
Small changes of voltage in the scalp that are caused by electrical currents by neurons in the brain.
How does EEG detect electrical signals
in order of micro volts
detectable by placing electrodes in electrical contact with scalp
What is Magnetoenceplalography based in
Detetction of small magnetic firlds which are induced by electrical current in the brain
How are the magnetic fields detectable and how are they feasable
require extremely sesnitive magnetic fields dectors
only feasible with Superconducting Quantum Interference Devices (squid)
How many electrodes are placed for EEG
How many coils are placed for MEG
32-128 ELECTRODES
300 radially oriented oick up coils
Which has a higher spatial resolution and why
MEG but both have poor spatial resolution due to field cancellations.
But Electric potentials are distorted by inhomogeneous conductivity whereas magnetic is not
What effects can be recorded by MEG and EEG
- Spontaneous rhythms
- Phase locked evoked effect
- Time-locked but not phase-locked induced effect
Spontaneous rhythms
Spontaneous rhythms (sometimes called brain oscillations or brain waves) occur naturally in the brain and do not require any sensory stimulation of the subject.
When is Delta frequenct most prominent?
Slowest of all spontaneous brain activity, the delta rhythm is most prominent in deep sleep.
When is Theta frequenct most prominent?
spontaneous activity in the theta band is associated with sleep
spontaneous activity in the theta band is associated with sleep
When is Alpha frequenct most prominent?
Most prominent in awake and relaxed subjects, alpha waves are blocked by sensory stimulation
When is Beta frequenct most prominent?
motor and visual cortexes
When is Gamma frequenct most prominent?
active processing of information in the cortex
What is Evoked Effects
measured in response to the presentation of some external stimulus to the subject.
How is the evoked effect in an experiment
present the same stimulus to subject a large number of times
Recorded MEG AND EEG THEN AVERAGED
What is the Induced Effect
to stimulus-induced changes in spontaneous rhythms.
Difference between induced and evoked effect
Induced are not phase-locked to stimulus so averaging would cause loss of induced effect.
How must induced effect be calculated
EM signal misy be squared to obtain oscillatory power in frequency band
Averaging power across trials will result in observable induced effects.
What are dendrites
Extention of soma which receive stimuli from other cells
What are Pyramidal neurons
Dendrites orientated parallel to each other and perpendicular to the cortical surface.
What are Stellate neurons
Dendrites are symmetrically distributed around the soma.
How does current flow in neurons in the braon
In pyramidal neurons - Nirmal to cortical surface
In stellate neurons - isotropics - produces little electromagnetic field at a distance
What is post synaptic currents
In the resting state, there are imbalances in ion concentrations across neuron
Cells communicate via chemical interaction
Termination of action potential at pre-synaptic cell causes neurotransmitter release across synaptic cell.
Change in local membrane permeability enables sodium ions to rush into the cell and give rise to primary current directed along dendrite of a neuron
Voltage change is post synaptic potentials.
What is action potential
the leading edge of depolarisation followed by the trailing edge of repolarisation
How is the electrical potential from EEG induced
potentials associated with postsynaptic events and is a measure of volume currents
Why is it important for signals to be more sensitive ti signals which are different at different electrodes than those which are the same.
Electrical signals from brain vary from electrode to electrode
but electrical signals from noise is generally uniform.
Whar is the principle of common mode rejection
signals which are the same at two electrodes are largely attenuated than signals which are different at two electrodes
What are EEG recording strategies
EEG measures potential differences between two electrodes connected to opposite ends of differential amplifier
Recirding depends on position of individual electrodes and how they’re paired
What are the two types of EEG recording sstrategies
- Referential / unipolar strategy
- Bipolar Strategy
What is Unipolar strategy
Advantages and Disadvantages
- p.d is determined with resoect to single common electrode
ADVANTAGES
- comparisons between signal amplitudes recorded at different electrodes can be made
- inferences about source of electrical signal can be made as electrode recording is presumed to be nearest sources
DISADBANTAHES
- Source location is not related to the electrode with max signal amplitude as ref cannot be placed in truly inactive part of brain
What is Bipolar strategy
Advantages and Disadvantages
- p.d measured between adjancent electrode which are normally paired in a chain
- electrodes common to two pairing are connected to possite sides of differential amplifier in each pairing
ADVANTAGES
- . If the source of the current vector causing the cortical activity is normal to the skull surface then this is an excellent way to locate the source of brain activity.
DISADVANTAGES
- , if the source is at an oblique angle to the skull then the voltage maxima and the source may be far from one another making source localisation with this information alone impossible.
What is the MEG Forward problem
Given a known current distribution in the brain, can we compute the magnetic field distribution
outside the head?
Solved by Maxwells equations
don’t do this for eeg due to inhomogeneous conductivity
MEG Forward problem; neuronal current approximations
1 - Spherically symmetric conductor approximation - head is modelled as single conducting sphere with homogenous conductivity
2 - Current dipole approximation - at a distance B -field results from large number of synchronised post synaptic current resulting from single current dipole
Assumption 1 is reasonable, though more advanced models based on an MRI of a participant are
available. Assumption 2 is reasonable provided the region of brain activated is small.
Whatis lead fields
N-dimenstional vector containing B-fields measured at each N MEG sensor in response to single current dipole source if unit amplitude ar predetermined locations and orientations in brain
