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Event Related Potentials (ERPs):Event Related Potentials (ERPs):
What can they tell us about What can they tell us about
how we think?how we think?
Kim SweeneyKim Sweeney
COGS1COGS1-- Introduction to Cognitive ScienceIntroduction to Cognitive Science
October 23, 2008October 23, 2008
With thanks to Seana Coulson!
OverviewOverview
�� What are ERPs?What are ERPs?
�� ERP components:ERP components:
–– P300P300
–– N400N400
–– P600P600
�� What are they? When do they occur?What are they? When do they occur?
�� How can we use these components to How can we use these components to
investigate the workings of the mind?investigate the workings of the mind?
Electroencephalogram (EEG) invented 1928Electroencephalogram (EEG) invented 1928
Early recording set-up Human Subject
Hans Berger
Electroencephalogram (EEG)Electroencephalogram (EEG)
�� We record the EEG by We record the EEG by
placing placing
electrodes on electrodes on
the headthe head
Info in raw EEG…Info in raw EEG…
Characteristics of ERP componentsCharacteristics of ERP components
�� PolarityPolarity
–– Is it a positive wave or a negative one?Is it a positive wave or a negative one?
�� LatencyLatency
–– How long after stimulus presentation does it peak?How long after stimulus presentation does it peak?
�� AmplitudeAmplitude
–– How big is it?How big is it?
�� Functional SignificanceFunctional Significance
–– What cognitive (or perceptual) activity is it sensitive What cognitive (or perceptual) activity is it sensitive
to?to?
–– What makes it bigger or smaller?What makes it bigger or smaller?
What do ERPs reflect?What do ERPs reflect?
�� Sensory, motor, and/or cognitive events in Sensory, motor, and/or cognitive events in
the brainthe brain
�� SynchronousSynchronous activity of large populations activity of large populations
of neurons engaged in information of neurons engaged in information
processingprocessing
What are ERPs?What are ERPs?
�� ERPs are formed by averaging EEG timeERPs are formed by averaging EEG time--locked locked
to the onset of stimuli that require cognitive to the onset of stimuli that require cognitive
processingprocessing
�� ERPs represent electrical activity associated with ERPs represent electrical activity associated with
the processing of the stimulithe processing of the stimuli
�� ERPs can be related to different kinds of ERPs can be related to different kinds of
cognitive tasks, e.g. attention, memory, & cognitive tasks, e.g. attention, memory, &
language comprehensionlanguage comprehension
fMRIfMRI ERPsERPs
�� Low Spatial ResolutionLow Spatial Resolution
�� High Temporal High Temporal
ResolutionResolution
�� Sensitive to what sorts Sensitive to what sorts
of cognitive processes of cognitive processes
activeactive
�� Good for studying Good for studying
cognitive processes cognitive processes
that unfold rapidly in that unfold rapidly in
timetime
�� High Spatial ResolutionHigh Spatial Resolution
�� Low Temporal Low Temporal
ResolutionResolution
�� Sensitive to what sorts Sensitive to what sorts
of cognitive processes of cognitive processes
activeactive
�� Good for mapping the Good for mapping the
brainbrain
fMRIfMRI ERPsERPs
�� Low Spatial ResolutionLow Spatial Resolution
�� High Temporal High Temporal
ResolutionResolution
�� Sensitive to what sorts Sensitive to what sorts
of cognitive processes of cognitive processes
activeactive
�� Good for studying Good for studying
cognitive processes cognitive processes
that unfold rapidly in that unfold rapidly in
timetime
�� High Spatial ResolutionHigh Spatial Resolution
�� Low Temporal Low Temporal
ResolutionResolution
�� Sensitive to what sorts Sensitive to what sorts
of cognitive processes of cognitive processes
activeactive
�� Good for mapping the Good for mapping the
brainbrain
P300 amplitude is greater when stimuli are P300 amplitude is greater when stimuli are
important to the subjectimportant to the subject
___ counting
----- reaction time
….. feedback
P300P300-- LatencyLatency
Synonyms of “prod” (20%) vs. other words(80%)“David” (20%) vs. “Nancy” (80%)
Male names (20%) vs. Female names (80%)
�� The harder it is The harder it is
to categorize a to categorize a
stimulus, the stimulus, the
longer the longer the
latency of the latency of the
P300P300
P300P300-- more than a “reaction time”?more than a “reaction time”?
�� RT very useful in RT very useful in
psychological researchpsychological research
�� RT includes (at least!)RT includes (at least!)
–– stimulus evaluationstimulus evaluation
–– response selection/executionresponse selection/execution
�� RT delayed by task difficulty, RT delayed by task difficulty,
e.g.e.g.
–– ‘noise’ in stimulus‘noise’ in stimulus
–– response incompatibilityresponse incompatibility
�� P300 latency affected by P300 latency affected by
noise, but noise, but notnot response response
incompatibility!incompatibility!
P300 ComponentP300 Component
�� ERP component sensitive to probability ERP component sensitive to probability
and importance (or personal relevance) of and importance (or personal relevance) of
a stimulus.a stimulus.
�� May reflect the updating of a ‘schema’ May reflect the updating of a ‘schema’
(model) of the world, incorporating novel (model) of the world, incorporating novel
information.information.
Hesocksspreadthewarmbreadwith
What about unexpected language?What about unexpected language?
P300 to incongruous (nonsense) endings?P300 to incongruous (nonsense) endings?
Surprising stimulusSurprising stimulus
N400 measures processing difficultyN400 measures processing difficulty
�� a) N400 to anomalies in the middle a) N400 to anomalies in the middle of sentence as well as the endof sentence as well as the end
�� b) sensitive to typicalityb) sensitive to typicality
�� c) to written & spoken words, and c) to written & spoken words, and to picturesto pictures
�� d) smaller if a word is repeated (or d) smaller if a word is repeated (or if it’s primed)if it’s primed)
�� e) smaller for frequent words than e) smaller for frequent words than for rarefor rare
�� f) In sensible sentences, N400 big f) In sensible sentences, N400 big for first words, smaller for later for first words, smaller for later onesones
Is N400 languageIs N400 language--specific?specific?
�� I take my coffee with I take my coffee with
cream and…cream and…
sugar.
dog.
N400 to words vs. picturesN400 to words vs. pictures
�� Similar type of ERP Similar type of ERP
responseresponse
�� Different topography Different topography
suggests slightly suggests slightly
different brain areas different brain areas
activeactive
How does the brain “do” language?How does the brain “do” language?
I like to eat frosting. Iliketoeatfrosting.
How does the brain “do” language?How does the brain “do” language?
I like to eat frosting.
IliketoI like to eat frosting.
Is music like language?Is music like language?
�� Familiar vs. Unfamiliar MelodiesFamiliar vs. Unfamiliar Melodies
�� End with…End with…
–– Congruent NoteCongruent Note
–– Diatonic Note Diatonic Note (wrong note, same key)(wrong note, same key)
–– NonNon--Diatonic Note Diatonic Note (out (out
of key)of key)
�� P300 sensitive to P300 sensitive to
–– How ‘bad’ the note isHow ‘bad’ the note is
–– How ‘good’ the listener is How ‘good’ the listener is
Besson & Faita, 1995
Familiar Unfamiliar
Musicians
Non Musicians
Is the P600 languageIs the P600 language--specific?specific?
�� YesYes
�� Language is innateLanguage is innate
–– Dedicated brain Dedicated brain
mechanisms for mechanisms for
learning grammarlearning grammar
�� Dedicated brain Dedicated brain
regions are devoted regions are devoted
to processingto processing
–– Semantics (meaning)Semantics (meaning)
–– Syntax (grammar)Syntax (grammar)
�� NoNo
�� Ability to learn Ability to learn language derives from language derives from generalgeneral learning learning mechanismsmechanisms
�� Brain response to Brain response to grammatical errors is grammatical errors is a subset of the brain a subset of the brain response to response to improbable eventsimprobable events
Is the P600 languageIs the P600 language--specific?specific?
�� In one part of experiment, In one part of experiment,
80% of sentences 80% of sentences
grammatical, 20% grammatical, 20%
ungrammaticalungrammatical
–– Ungrammatical surprisingUngrammatical surprising
�� In another, 80% of In another, 80% of
sentences ungrammatical, sentences ungrammatical,
20% grammatical20% grammatical
–– Grammatical surprisingGrammatical surprising
Coulson, King, & Kutas, 1998
Language ERP ComponentsLanguage ERP Components
�� N400N400
�� Sensitive to the Sensitive to the
difficulty of difficulty of
understanding the understanding the
meaningmeaning of a wordof a word
�� P600P600
�� Sensitive to the Sensitive to the
grammaticality of grammaticality of
words in sentenceswords in sentences
P600 related to P300P600 related to P300
�� Brain areas sensitive Brain areas sensitive
to to grammaticalitygrammaticalitysimilar to those similar to those
sensitive to sensitive to
probabilityprobability
�� P600 related to P300P600 related to P300
�� Unusual grammatical Unusual grammatical
events dealt with like events dealt with like
other sorts of other sorts of
surprising eventssurprising events
The brain is a complicated placeThe brain is a complicated place
�� ““baba……dada……gaga……vava……thatha””
�� ““beep”beep”
�� “beep beep”“beep beep”
McGurk Effect
Sound-Induced Illusory Flash
what you hear what you perceivewhat you see
�� If what you hear can influence what you see…If what you hear can influence what you see…
�� And what you see can influence what you hear…And what you see can influence what you hear…
�� Perhaps what you Perhaps what you feelfeel might influence some might influence some
other aspect of cognition?other aspect of cognition?
Some effects of mood on behaviorSome effects of mood on behavior
�� People in a positive mood…People in a positive mood…
–– Greater life satisfaction Greater life satisfaction (Schwarz & (Schwarz & CloreClore, 1983), 1983)
–– More likely to help, more generous More likely to help, more generous ((IsenIsen, 1970), 1970)
–– Better judgments of neutral possessions Better judgments of neutral possessions ((IsenIsen, 1978), 1978)
–– More likely to ‘think outside the box’ More likely to ‘think outside the box’ ((IsenIsen, 1987), 1987)
–– More likely to focus on global rather than local More likely to focus on global rather than local
featuresfeatures ((Fredrickson & Branigan, 2005)
Some effects of mood on behaviorSome effects of mood on behavior
�� People in a positive mood…People in a positive mood…
–– More words from longer word More words from longer word (Smith & Larsen, 1989)(Smith & Larsen, 1989)
–– More (unusual) associations More (unusual) associations ((IsenIsen, 1985), 1985)
–– Broader, more inclusive categories Broader, more inclusive categories ((IsenIsen, , ‘92‘92, Murray, ‘90) , Murray, ‘90)
–– Better at Remote Associates Task Better at Remote Associates Task ((IsenIsen, 1987), 1987)
�� worm, shelf, end… BOOKworm, shelf, end… BOOK
�� Sensitive to semantic processingSensitive to semantic processing
�� Sensitive to semantic categorizationSensitive to semantic categorization
“apple”
“kiwi”
“desk”
_
+
0 200 400 600 800 ms
Semantic Relationships and the N400Semantic Relationships and the N400
The N400 is sensitive to semantic CATEGORIZATION
Mood affectsCATEGORIZATION
The amplitude of the N400 reflects the degree of difficultyof the integration process
In CATEGORIZATIONIn CATEGORIZATION
we would expect to we would expect to
see effects of mood see effects of mood
on the amplitude of on the amplitude of
the N400the N400
Semantic Relationships and the N400Semantic Relationships and the N400
“apple”
A kind of fruit…
“kiwi”
“desk”
_
+
�� 10 female participants10 female participants
�� Musical mood inductionMusical mood induction
�� Participants read sentences and performed Participants read sentences and performed
a category judgment taska category judgment task
SummarySummary
�� ERPs ERPs –– electrical brain electrical brain activity associated with activity associated with processing particular types of processing particular types of stimulistimuli
�� ERPs reflect postERPs reflect post--synaptic synaptic potentialspotentials
�� ERPs sensitive to different ERPs sensitive to different kinds of cognitive processeskinds of cognitive processes
�� P300 componentP300 component–– Elicited by surprising eventsElicited by surprising events
–– Related to updating model of Related to updating model of world in memoryworld in memory
�� N400 componentN400 component–– Elicited by meaningful stimuliElicited by meaningful stimuli
–– Measures processing difficultyMeasures processing difficulty
�� P600 componentP600 component–– Elicited by ungrammatical Elicited by ungrammatical
stimulistimuli
–– Related to P300 componentRelated to P300 component
Behavioral products- including ERPs- are interesting because they tell us something about the underlying processes!