Intro to Psycholinguistics - Research Methods

7/29/2019 Intro to Psycholinguistics - Research Methods

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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

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Intro to Psycholinguistics - Research Methods