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Research Methods
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Two major types of methods:
Electrophysiological:EEG (electroencephalogram),
MEG (magnetoencephalogram) - directmeasures of brain activity
Haemodynamic:PET (Positron Emission Tomography),fMRI (functional Magnetic ResonanceImaging) - indirect measures of brain
activity
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Other methods:
TMS - Transcranial Magnetic Stimulation
DTI - Diffusion Tensor Imaging(measures the flow of water throughthe white matter, thus enabling us to
record activity within it)
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Experimental design
Blocked - stimuli presented in blocks
Randomised - stimuli presented in
random order
Combined - usually randomised blocks
of stimuli
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Electrophysiological measures
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Electrophysiological methods -Intracranial EEG
Electrodes inserted into the brain of apatient
Advantages: gold standard for localisinggenerating structures, excellenttemporal resolution
Disadvantages: limited samplingdictated by clinical concerns, brains areabnormal
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Electrophysiological methods -EEG
EEG reflects the postsynaptic potentials
We measure the activity at the scalp
EEG recordings can be analysed in two ways:frequency analysis, ERP analysis
ERP = Event Related Potentials
EEG signal is averaged time-locked tostimulus onset across many trials resulting inERPs
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EEG
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Electrode configuration
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EEG signals
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Raw EEG
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ERPs
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ERPs and language
Components that reflect language processing:ELAN/LAN, N400, P600/SPS
ELAN - Early Left Anterior Negativity -morphosyntax
LAN - Left Anterior Negativity - morphosyntax
N400 - reflects semantic integration of anykind
P600/SPS (Syntactic Positive Shift) - reflectssyntactic integration
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ELAN/LAN
Appear over the left hemisphere
ELAN reflects processing of word categories; appearsas a response to the violation of word category rules
latency: 100 - 200ms from the place we measure
LAN - response to the violation of morphosyntacticrules (verb-subject agreement, gender agreement,case agreement, tense agreement)
latency - 300 - 500ms
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N400
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N400
part of the response to every word
related to the clause probability
elicited by any potentially meaningful stimulus sensitive to: semantic relations, context, memory
largest to pronounceable non-words
larger to less frequent words
larger early than late in the sentence obtained also to world knowledge violations
(Hagoort, 2003)
reflects the ease of semantic integration
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P600/SPS
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P600/SPS
Response to violation of various syntacticrules
BUT also to syntactically ambiguoussentences - gardenpath sentences (Thewoman persuaded to answer the door)
the syntactic complexity of the sentence can
influence the amplitude of P600
it seems that P600 reflects impossibility ofintegration, and not integration itself
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ERP maps - N400
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ERPs
Advantages: non-invasive, randomiseddesign, excellent temporal resolution
(milliseconds), cheep Disadvantages: poor spatial resolution
(the inverse problem)
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Electrophysiological methods -MEG
Measures magnetic fields generated bysynchronised postsynaptic currents in
dendrites of pyramidal neuronesAssumptions: generators are
distributed, signal is generated by the
grey matterAnatomically constrained MEG - we
combine MEG with fMRI
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MEG
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MEG - sensors
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MEG
Whole head coverage - 100 - 300sensors
Magnetically shielded room
Measures only activity in the sulci -insensitive to the radial component of
the signal!
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MEG - raw data
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MEG maps - N400
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Anatomically constrained MEG- visual stimuli
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MEG
Advantages: non-invasive, excellenttemporal resolution (milliseconds),
randomised design Disadvantages: poor spatial resolution,
very expensive!
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Haemodynamic methods - PET
A radioactive tracer is injected into theblood stream
Detects pairs of gamma rays emitted bythe tracer - collision of a positron andelectron
Invasive!
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PET
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PET
12 scans per subject
temporal resolution: 40 seconds
time between two scans: 10 minutes
63 slices
slice distance: 2.4mm
costs:1,300 euro per subject
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PET image
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PET
Advantages:
metal not a problem
little noise
relatively insensitive to movement
no signal loss in the inferior temporallobe
enables us study overt speechproduction!
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PET
Disadvantages:
invasive
radioactivity
poor temporal resolution
only blocked design!
expensive
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Haemodynamic methods -fMRI
Relies on the differences inparamagnetic properties of oxy- and
deoxyhaemoglobin or the BOLD (BloodOxygen Level Dependant) signal
the signal comes from a higher ratio of
oxygenated haemoglobin - blood flowincreases in active brain regions
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fMRI - 3T
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fMRI - extremely sensitive tomovement, thus:
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fMRI
30 - 40 slices; resolution: 3.5mm
combined with structural MRI to achieve
better resolution temporal resolution - at best 3-5 sec., usually
around 15-30; cannot be improved - wemeasure blood flow which has a steady rate
PROBLEM - baseline!
PROBLEM - threshold!
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fMRI - raw data
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fMRI - computed image
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fMRI - Pseudowords vs. Words
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fMRI - blind to certain regions
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fMRI - experimental design
In the past - only blocked design (poortemporal resolution)
BUT now - event related design(randomisation) - the signal is linearlyadditive so we can apply certain methods andresolve it; peak comes after about 6 sec; still
you have to have random periods of rest ofrandom length (2-8) seconds
it takes 20 sec for the signal to go back to thebaseline
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Event related design
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fMRI
Advantages: good spatial resolution,non-invasive
Disadvantages: poor temporalresolution, blind to certain regions suchas inferior temporal cortex, expensive,
noise, intracorporeal metal, sensitive tomovement
M h d d d
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Methods - advantages anddisadvantages
Method Temporal
resolution
Spatial
resolution
Invasive Expensive
Inracranial
EEG
excellent gold
standard
Yes Yes
EEG excellent very poor No No
MEG excellent poor No Yes
PET extremely
poor
very good Yes Yes
fMRI very poor almostexcellent
No Yes
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Which Method to Use?
Depends on the question you want toask!
When - electrophysiological methods Where - haemodynamic methods
When and Where - combine two
methods from different groups