Event Related Potentials (ERPs):
What can they tell us about
how we think?
Kim Sweeney
COGS1- Introduction to Cognitive Science
October 23, 2008
With thanks to Seana Coulson!
Overview
What
are ERPs?
ERP components:
– P300
– N400
– P600
What are they? When do they occur?
How
can we use these components to
investigate the workings of the mind?
Event Related Potentials (ERPs)
Electroencephalogram (EEG) invented 1928
Early recording set-up
Hans Berger
Human Subject
Electroencephalogram (EEG)
We record the EEG by
placing
electrodes on
the head
Info in raw EEG…
Could that work?
Neurons communicate with electrical signals
Summates
Cancels out
EEG monitors alertness
Event Related Potentials (ERPs)
Characteristics of ERP components
Polarity
– Is it a positive wave or a negative one?
Latency
– How long after stimulus presentation does it peak?
Amplitude
– How big is it?
Functional Significance
– What cognitive (or perceptual) activity is it sensitive
to?
– What makes it bigger or smaller?
What do ERPs reflect?
Sensory,
motor, and/or cognitive events in
the brain
Synchronous
activity of large populations
of neurons engaged in information
processing
What are ERPs?
ERPs are formed by averaging EEG time-locked
to the onset of stimuli that require cognitive
processing
ERPs represent electrical activity associated with
the processing of the stimuli
ERPs can be related to different kinds of
cognitive tasks, e.g. attention, memory, &
language comprehension
fMRI
functional Magnetic Resonance Imaging
ERPs
fMRI like time-lapse photography
fMRI
High Spatial Resolution
Low Temporal
Resolution
Sensitive to what sorts
of cognitive processes
active
Good for mapping the
brain
ERPs
Low Spatial Resolution
High Temporal
Resolution
Sensitive to what sorts
of cognitive processes
active
Good for studying
cognitive processes
that unfold rapidly in
time
fMRI
High Spatial Resolution
Low Temporal
Resolution
Sensitive to what sorts
of cognitive processes
active
Good for mapping the
brain
ERPs
Low Spatial Resolution
High Temporal
Resolution
Sensitive to what sorts
of cognitive processes
active
Good for studying
cognitive processes
that unfold rapidly in
time
P300- The Oddball Paradigm
P300- Amplitude. Surprise!
Beep. Beep.Beep.Beep.BOOP. Beep.Beep.
P300 amplitude is greater when stimuli are
important to the subject
___ counting
----- reaction time
….. feedback
P300- Latency
Synonyms of “prod” (20%) vs. other words(80%)
“David” (20%) vs. “Nancy” (80%)
Male names (20%) vs. Female names (80%)
The harder it is
to categorize a
stimulus, the
longer the
latency of the
P300
P300- more than a “reaction time”?
RT very useful in
psychological research
RT includes (at least!)
– stimulus evaluation
– response selection/execution
RT delayed by task difficulty,
e.g.
– ‘noise’ in stimulus
– response incompatibility
P300 latency affected by
noise, but not response
incompatibility!
P300 Component
ERP
component sensitive to probability
and importance (or personal relevance) of
a stimulus.
May
reflect the updating of a ‘schema’
(model) of the world, incorporating novel
information.
What about unexpected language?
He
spread
socks
bread
warm
with
the
P300 to incongruous (nonsense) endings?
Surprising stimulus
N400
Professor Hillyard
Professor Kutas
words, pictures, sounds
Kutas & Hillyard, 1980
N400 measures processing difficulty
a) N400 to anomalies in the middle
of sentence as well as the end
b) sensitive to typicality
c) to written & spoken words, and
to pictures
d) smaller if a word is repeated (or
if it’s primed)
e) smaller for frequent words than
for rare
f) In sensible sentences, N400 big
for first words, smaller for later
ones
Is N400 language-specific?
I take my coffee with
cream and…
dog.
sugar.
N400 to words vs. pictures
Similar type of ERP
response
Different topography
suggests slightly
different brain areas
active
How does the brain “do” language?
I like to eat frosting.
frosting.
like
to
I
eat
How does the brain “do” language?
I like to eat frosting.
I like to eat frosting.
like
to
I
How does the brain “do” language?
DeLong, Urbach and Kutas, 2005
Is music like language?
Familiar vs. Unfamiliar Melodies
Familiar
End with…
Unfamiliar
Musicians
– Congruent Note
– Diatonic Note
(wrong note, same key)
– Non-Diatonic Note
(out
Non Musicians
of key)
P300 sensitive to
– How ‘bad’ the note is
– How ‘good’ the listener is
Besson & Faita, 1995
Besson et al., 2002
Is the P600 language-specific?
Yes
Language is innate
– Dedicated brain
mechanisms for
learning grammar
Dedicated brain
regions are devoted
to processing
– Semantics (meaning)
– Syntax (grammar)
No
Ability to learn
language derives from
general learning
mechanisms
Brain response to
grammatical errors is
a subset of the brain
response to
improbable events
P300
Is the P600 language-specific?
In one part of experiment,
80% of sentences
grammatical, 20%
ungrammatical
– Ungrammatical surprising
In another, 80% of
sentences ungrammatical,
20% grammatical
– Grammatical surprising
Coulson, King, & Kutas, 1998
Language ERP Components
N400
Sensitive to the
difficulty of
understanding the
meaning of a word
P600
Sensitive to the
grammaticality of
words in sentences
P600 related to P300
Brain areas sensitive
to grammaticality
similar to those
sensitive to
probability
P600 related to P300
Unusual grammatical
events dealt with like
other sorts of
surprising events
The brain is a complicated place
McGurk Effect
“ba…da…ga…va…tha”
Sound-Induced Illusory Flash
what you see
what you hear
“beep”
“beep beep”
what you perceive
If what you hear can influence what you see…
And what you see can influence what you hear…
Perhaps what you feel might influence some
other aspect of cognition?
Some effects of mood on behavior
People
in a positive mood…
– Greater life satisfaction (Schwarz & Clore, 1983)
– More likely to help, more generous (Isen, 1970)
– Better judgments of neutral possessions (Isen, 1978)
– More likely to ‘think outside the box’ (Isen, 1987)
– More likely to focus on global rather than local
features (Fredrickson & Branigan, 2005)
Some effects of mood on behavior
People
in a positive mood…
– More words from longer word (Smith & Larsen, 1989)
– More (unusual) associations (Isen, 1985)
– Broader, more inclusive categories (Isen,
Isen, ‘92, Murray, ‘90)
– Better at Remote Associates Task (Isen, 1987)
worm, shelf, end… BOOK
Thinking when feeling “positive”
Fredrickson & Branigan, 2005
Semantic Relationships and the N400
Sensitive to semantic processing
Sensitive to semantic categorization
“desk”
_
“kiwi”
+
“apple”
0
200
400
600
800 ms
Pineapple
Eagle
Apple
FRUIT
Banana
Desk
Orange
Tomato
Kiwi
Mood affects
CATEGORIZATION
The N400 is sensitive to semantic
CATEGORIZATION
The amplitude of the N400
reflects the degree of difficulty
of the integration process
In CATEGORIZATION
we would expect to
see effects of mood
on the amplitude of
the N400
Semantic Relationships and the N400
A kind of fruit…
“desk”
_
“kiwi”
+
“apple”
10
female participants
Musical mood induction
Participants read sentences and performed
a category judgment task
A kind of fruit: pineapple
10 Female Participants- Positive Mood
10 Female Participants- Negative Mood
Context Matters
Summary
ERPs – electrical brain
activity associated with
processing particular types of
stimuli
– Elicited by surprising events
– Related to updating model of
world in memory
N400 component
–
–
ERPs reflect post-synaptic
potentials
P300 component
ERPs sensitive to different
kinds of cognitive processes
Elicited by meaningful stimuli
Measures processing difficulty
P600 component
– Elicited by ungrammatical
stimuli
– Related to P300 component
Behavioral products- including ERPs- are interesting because they tell us
something about the underlying processes!
Thank you!