lectures week10 erp lecture

Event Related Potentials (ERPs)

John J. Curtin, Ph.D.University of Wisconsin, Madison

Overview

o Event-related potentials are patterned voltage changes embedded in the ongoing EEG that reflect a process in response to a particular event (e.g., visual or auditory stimuli)

o ERPs are measured from the same “raw data” (i.e., scalp electrical activity over time and space) as EEG

Time-locked Activity Extraction by Averaging

o Activity reflects both signal and “noise” o Signal: stimulus related processingo Noise: tonic background activity related to ongoing

processes (level of arousal, etc)

o The signal-related activity can be extracted because it is time-locked to the presentation of the stimulus

o Signal averaging is most common method of extracting the signal

o Sample EEG for ~1 second after each stimulus presentation & average together across like stimuli

o Time-locked signal emerges; noise averages to zero

What Does the ERP Reflect?

o May reflect sensory, motor, and/or cognitive events in the brain

o Reflect synchronous post-synaptic potentials of large neuronal populations engaged in information processing

After Lorente de Nó, 1947

What Does the ERP Reflect?

o Open field organizations (dendrites on one side, axon on other) summate. These include: most parts of cortex, parts of thalamus, cerebellum)

o Closed fields cancel each other out (e.g. midbrain nuclei)

What is an ERP Component?

o The presence of ERP components is reflected in the tendency of various segments of the ERP to covary with experimental manipulations

o Total ERP is an aggregate of numerous ERP components

Components are defined in 3 ways:1. Positive and negative peaks (min/max or average

approach)2. Aspects of ERP that covary across subjects,

manipulations and locations (PCA approach)3. Neural structures that generate the ERP (source

modeling approach)

What is an ERP Component?

o Peak may represent the sum of several functional (e.g., process) or structural sources

o Same brain structure may contribute to more than one portion of the ERP

o Different structures may produce activity that is functionally equivalent (e.g., homologous structures in left/right hemisphere)

Component is a "bump" or "trough"

Making Meaning from the Bumps

Pores o'er the Cranial map with learned eyes,Each rising hill and bumpy knoll decriesHere secret fires, and there deep mines of senseHis touch detects beneath each prominence.

Peaks Approach to Components

o Component identificationo Polarityo Latencyo Scalp locationo Sensitivity to experimental manipulations

o Most commonly, components are labeled by polarity (P or N) and latency (or index) at active recording site

o Quantifyingo Amplitude (min/max relative to baseline)o Area (average activity in window)o Latency

Component is a "bump" or "trough"

Early Components

o Waves I-VI represent evoked activity in auditory pathways and nuclei of the brainstem

o Early components <60-100 msec o Occur in obligatory fashion o Are called Exogenous = determined "outside"

organism

o Even subtle deviations in appearance may be indicative of pathology

Later ERP Components

o Highly sensitive to changes in State of organism Meaning of stimulus (NOT physical characteristics) Information processing demands of task

o Therefore termed Endogenous = determined “within" organism

o Not all components fit neatly into exogenous or endogenous categories

o Both obligatory but modulated by psychological factor (sometimes called “Mesogenous”; e.g. N100)

BUT…..

Evoked Vs Emitted ERP's

o Evoked are most commonly studied: occur in response to a physical stimulus

o Emitted potentials occur in absence of a physical stimulus (e.g., omission of item in sequence)

o Evoked can have both exogenous and endogenous components; emitted usually have only endogenous

Comparison to other “Windows on the Brain"

Comparison to other “Windows on the Brain"

o Very precise temporal resolution

o Spatial localization is more difficulto At the surface, activity of many functional synaptic

units recorded

o ERP's generated only by groups of cells that are synchronously activated in a geometrically organized manner

o Synchronous activation may occur in one or more than one location

o Yet localization is not impossible in conjunction with other techniques

Constructs in Search of Validation

o Determine antecedent conditions under which the ERP component appears (what manipulations affect component)

o Consider latency of ERP component relative to other components and behavior

o Develop hypotheses concerning functional significance of the "subroutine" underlying the ERP component

o Predict consequences of subroutine (behavior and other components)

o Validate empirically

Basic Signal Processing

Acquisition of Signal

o Precise temporal control over stimulus presentation necessary

o Individual stimuli are presented numerous times; ERP's generally do not habituate, unlike peripheral measures

o Concurrent with each stimulus, a signal/pulse must be sent to amplifiers to indicate time of stimulus onset

o A/D converter and sampling o Sampling either as pulse received, or it may be

continuously monitoredo Several pre-onset samples (to provide a baseline

for comparison); o Epoch length

o Epochs for like stimuli averaged together to create ERP for that set of stimuli

Artifact and its Reduction

Sourceso Eye movement and eyelid movemento EMG in head and necko Movemento Electrical activity of the hearto Inattention

Solutions?o Environment and task parameterso Discard epochs with artifact (loss of data and bias)o Filtering (overlap with signal)o Correction algorithms

Signal to Noise Problem

o EEG is on order of + 50 microvolts

o ERPs (that we are interested in) are on order of 2 – 20 microvolts

o Often want to detect differences of 1-2 microvolts

Solutions include:o Signal averagingo Filteringo Pattern recognition techniques (cross

correlation, Woody filter)

Assumptions of Averaging Methods

o Signal and noise (in each epoch) sum linearly together to produce the recorded waveform for each epoch (not some peculiar interaction)

o The evoked signal waveshape attributable solely to the stimulus is the same for each presentation (discuss latency jitter)

o The noise contributions can be considered to constitute statistically independent samples of a random process

Benefit of Averaging

S/Nave N = sqrt(N) * S/Nsingle trial

P3 = 20 microvoltsEEG = 50 microvolts

S/N = 20/50

If have thirty trials thenS/N = (20 * 5.5)/50 = 110/50

An Important Limitation of Averaging

o The signal averaging method of reducing noise means that we do not have access to single trial data.

o Therefore, it is difficult to look at within subject variation of ERP with other measures (e.g., behavior) using averaging techniques

Filtering and its Influence on the ERP

o Despite many trials and averaging, some noise may remain in the averaged waveform

o If you are only interested in later & slower components, then a low-pass filter may be of interest

Same ERP filtered with 12.5 (black), 8 (red) , and 5 (green) Hz Low Pass FIR Filter

Same ERPs overlaid; note amplitude attenuation in P3 amplitude with stricter filters

Flanker Task

Designo 400 trials of modified Flanker paradigm (2 blocks w/break)o Stimuli are HHHHH; SSSSS; HHSHH; SSHSSo Flankers are congruent (HHHHH) or incongruent (SSHSS)o Targets are hi frequency (80%) or lo frequency (20%)

ERP components o Stimulus locked (N2, P3)o Response locked (ERN)

Trigger codes o 3 digit number (ABC)o A= block (0-2)o B=target frequency (1=frequent)o C= Flanker congruency (1=congruent)

A Data Reduction Processing Stream

1. Merge task and EEG data (correct trials; RT windows; always keep archive of original file)

2. Filter (filter settings)

3. Ocular artifact correction (continuous vs. epoched; filtered)

4. Stimulus-locked epoching (Epoch length)

5. Baseline correction (baseline length; role of variable ITI)

6. Artifact rejection (algorithmic vs. user initiated)

7. Form condition average waveforms

8. Scoring components (Min/Max vs. Average; Determination of window; Peak to peak vs. Baseline to peak)

Specific ERP components and their applications

Construct Validity of P300 (P3, P3b)

o First observed by Sutton, Braren, Zubin, & John (1965)

o Thought to reflect stimulus evaluation and categorization

o Also thought to represent “context updating”

o Johnson's model is P3 Amplitude = f[T x (1/P + M)] where o P = probability of occurrence, o M = Stimulus meaningo T = amount of information transmitted

Aspects of the Model

Probabilityo The P300 is observed in variants of the "oddball

paradigm"o The rare stimulus almost invariantly elicits a P3: largest

at parietal, then central, and then frontal siteso Subjective probability

Stimulus meaningo Actually composed of three dimensions

Task complexity Stimulus complexity Stimulus value

Information Transmission (proportion 0 to 1)

Overall Stimulus Probability

Local Probability

Stimulus Meaning

Stimulus complexityo Complex (interesting?) visual stimuli produce larger P3 [e.g.,

Verbaten, Roelofs, Sjouw, & Slangen, 1986]o Words elicit larger P3 than more simple visual stimuli [Johnson,

Pfefferbaum, & Kopell, 1985; Kutas et al., 1977]

Stimulus valueo Stimuli associated with reward [Jenness, 1972; Johnston, 1979]o Target statuso Stimuli associated with punishment [Curtin et al., 2001]o Interesting [Homberg, Grumewald, and Netz, 1984]

Task complexityo Count vs. passive listeno Predict vs. count

Information Transmission

o Ratio of information received over total information in the signal

o Often divided into equivocation and allocation of attentional resources

o Equivocation represents the information loss during stimulus presentation due to a posteriori uncertainty about having correctly perceived the evento Evoked vs. emitted P3 [Ruchkin & Sutton, 1978]o as the detectability of a signal stimulus improved, the P3 amplitude

to that stimulus as increased [Hillyard, Squires, Bauer, & Lindsay, 1971]

o inverse relationship between P3 amplitude and the size of the memory set in Sternberg memory paradigm [Hoffman, Simons, & Houck, 1983]

Information Transmission

o Selective attention paradigms: Instructed to attend or make task relevant [Kramer et al., 1983, 1985, Roth et al., 1976]

o Dual task or divided attention: P3 elicited by task relevant stimuli is directly proportional to the relative allocation of resources to these stimuli [Isreal et al. 1980]o P3 to secondary stimuli proportional to difficulty and priority of

primary task [Hoffman et al., 1985; Kramer et al., 1983, 1985; Strayer & Kramer,

1990]o In studies measuring p3 to stimuli in competing tasks, the

amplitude of the P3 elicited by the two tasks has been observed to be reciprocal, indicating that as more resources are dedicated to the processing of one task, fewer resources remain for the processing of the competing task [Strayer & Kramer, 1990]

Allocation of attention applies to paradigms in which information is lost due to inattention rather than uncertainty about stimulus occurrence

Isreal et al., 1980

Isreal, J. B., Chesney, G. L., Wickens, C. D., & Donchin, E. (1980). P300 and tracking difficulty: Evidence for multiple resources in dual-task performance. Psychophysiology, 17(3), 259-273.

P3 Latency

o An index of processing time, independent of response requirements (i.e. stimulus evaluation time)o RT measures confounds the two

o Correlation between P3 latency and RT. Correlation is greater if accuracy is emphasized over speed (Kutas et al., 1977, Science, 197, 792-795)

o McCarthy & Donchin (1981)

o Duncan Johnson, & Kopell (1981)

McCarthy & Donchin (1981)

o Manipulated stimulus evaluation and response compatibility

o Stimulus evaluation: Words RIGHT or LEFT embedded in matrix of #’s (easy) or letters (hard)

o Response compatibility: Respond with same or opposite hand as indicated by word

McCarthy, G., & Donchin, E. (1981). A metric for thought: A comparison of P300 latency and reaction time. Science, 211(4477), 77-80.

• Both manipulations had additive effects on RT

• P300 latency delayed when discriminability more difficult

• Response compatibility had no effect on P300 latency

• Note amplitude reduction as function of noise--information transmission)

Duncan Johnson, C. C., & Kopell, B. S. (1981). The Stroop effect: Brain potentials localize the source of interference. Science, 214, 938-940.

Duncan Johnson, & Kopell (1981)

o Used P3 latency to determine source of interference in Stroop effect

o Standard Stroop with color naming and word reading of congruent, neutral and incongruent stimuli.

o Also manipulated hue discriminability

o Expected pattern of response timeso P3 latency was 21 ms slower for word readingo However, congruency did not affect P3 latencyo Hue discriminability did affect P3 latency

The P3a

o The P3a was first observed in Squires, Squires, and Hillyard, 1975

o P3-like component with a frontal maximum and occurs to improbable stimuli in the "to-be-ignored" class of stimuli; a novelty response

How Many P3s?

o The Classic P3/P300/P3bo Parietal Central Maximumo Largest when stimuli rare and task-relevant

o The P3a (Courchesne et al., 1975; Squires et al., 1975).o More anterior scalp distributiono Slightly earlier latencyo Responsive to rare, unexpected, unattended

stimulio Response when no template is available for the

stimulus

From Simons et. al, 2001• Squires task was tones,

frequent and infrequent that were attended and not attended

• Courchesne task was visual (text: “two” “four” and common and unique visual scenes

Recorded visual evoked potentials from 18 normal college students performing in a visual discrimination task. Ss counted the number of presentations of the numeral 4 which was interposed rarely and randomly within a sequence of tachistoscopically flashed background stimuli (numeral 2s). Intrusive, task-irrelevant (not counted) stimuli were also interspersed in the sequence of 2s; these stimuli were of 2 types: simples, which were easily recognizable, and novels, which were completely unrecognizable. The simples and the counted 4s evoked posteriorly distributed P3 waves (latency 380-430 msec) while the irrelevant novels evoked large, frontally distributed P3 waves (latency 360-380 msec). These large, frontal P3 waves to novels were also preceded by large N2 waves. Findings indicate that the P3 wave is not a unitary phenomenon but should be considered in terms of a family of waves, differing in their brain generators and in their psychological correlates.

Courchesne et al., 1975

Courchesne, E., Hillyard, S. A., & Galambos, R. (1975). Stimulus novelty, task relevance and the visual evoked potential in man. Electroencephalography and Clinical Neurophysiology, 39(2), 131-143.

Conducted 2 experiments with a total of 12 adults to make direct comparisons between the late-positive waves evoked by shifts in ongoing trains of tones in conditions of active attention (counting) vs those of nonattention (reading). 2 distinct late-positive components of the scalp-recorded auditory evoked potential were identified which differed in their latency, scalp topography, and psychological correlates. The earlier component (latency about 240 msec) was elicited by infrequent, unpredictable shifts of either intensity or frequency in a train of tone pips whether the S was ignoring or actively attending to the tones. The later component (mean latency about 250 msec) occurred only when the S was attending to the tones; it was evoked by the infrequent, unpredictable stimulus shifts, regardless of whether the S was counting that stimulus or the more frequently occurring stimulus

Squires, N. K., Squires, K. C., & Hillyard, S. A. (1975). Two varieties of long-latency positive waves evoked by unpredictable auditory stimuli in man. Electroencephalography & Clinical Neurophysiology, 38(4), 387-401.

Squire et al., 1975

Sources of P3

o Likely distributed

o Candidates found in:o bilaterally in the anterior superior temporal gyruso inferior and middle frontal gyruso inferior and superior parietal lobuleso anterior and posterior cingulateo thalamuso Caudateo Amygdala/hippocampal complexo Insulao Among others!

Method

Participants

o 48 social drinkers in 2 beverage conditions Alcohol (0.08%) and No-alcohol

Measures

o Startle responseo Event related potentials (focused on P3)o Task performance (response time)

Curtin et al., (2001). Psychological Science

Trial Structure

Startle Shock Button press

S1 ! S2 ^

300ms 1400ms 300ms 200ms 300ms

S1 Threat-focus: Animal/Body-part Divided attention: Animal/Body-part or Animal/Body-part

Method

• 24 blocks of trials (20 trials per block)– 8 Threat-focus blocks– 16 Divided attention blocks

Block Structure

Next Block: SHOCK Only

Read each word as it is presentedShocks to animal words

!!!SHOCK!!!

Next Block: TASK & SHOCK

Press button quickly to square after GREEN wordDo not press button after RED word

Shocks to ANIMAL words

!!!SHOCK!!!

10 of 10 responses credited in this block

Fear Potentiated Startle

o In threat focus, no sig. difference in FPS between beverage groups

o In divided attention, FPS sig. reduced in ALC group

Attention to Shock Cues

-20

-15

-10

-5

0

5

10

15

20

25

-200 0 200 400 600 800 1000 1200 1400 1600

Time (ms)

Pz

(mic

rov

olt

s)

CUE+CUE-

Threat Cue Processing

Threat Cue Processing

o In threat focus, no sig. difference in P3 differentiation between beverage groups

o In divided attention, P3 sig. reduced in ALC group

Task Performance

165

190

215

240

265

CUE- CUE+

CUE Type

Rea

ctio

n ti

me

No-Alcohol

Alcohol

o Threat cue related interference with task performance is reduced in alcohol group

Intoxicated Behavior

o Aggressiono Sexual and other risk-taking activitieso Increased self-disclosureo Driving while intoxicatedo “Loss of control” drinking

o Deficits in response inhibition paradigms Go/No-Go Go/Stop Stroop

Situations in which these behaviors occur are characterized by “response conflict”

Cognitive Neuroscience of Attention

o Attention is a broad construct

o Multiple functions include (Posner, 1995):

Maintenance of alert state

Sensory orienting (attentional “spotlight”)

Executive function

o Different neural sub-systems responsible for these functions

NE pathways from locus coeruleus

Posterior system* for attentional engagement/disengagement and moving (parietal lobe, pulvinar, superior colliculus)

Anterior executive attention system (PFC, ACC, SMA)

* Visual attention

Cognitive Control

When is it needed?

o Overcome prepotent response (response conflict)

o Behavioral inhibition is needed (also a form of RC)

o Correct or respond to errors

o Novel (vs. practiced) tasks

Cognitive Control

Effortful activation and allocation of cognitive resources in the selection and processing of task-relevant information for purposes of maximizing performance on tasks involving high difficulty, complexity, interference, or novelty.

What does it do?

o Guide, coordinate, and update behavior in a flexible fashion

o Biases processing of information in favor of task-relevant stimuli and responses

o Establishes the appropriate stimulus-response mapping

Components of Cognitive Control

Evaluative component:

o Responsible for monitoring the need for control (“action monitoring”)

o Signaling when adjustments in control are necessary

o Anterior cingulate cortex (ACC) may be underlying neural system

Regulative component:

o Responsible for activation/implementation of control related processes

o Prefrontal cortex (PFC) may be underlying neural system

Stroop Methods

Design

o 48 participants in alcohol (BAL = 0.080%) or no-alcohol

o 432 trials of standard Stroop paradigm

o Color naming and word reading tasks (blocked)

o Congruent, neutral and incongruent conditions (equal frequencies)

Stroop Methods

Design


o 432 trials of standard Stroop paradigm

o Color naming and word reading tasks (blocked)

o Congruent, neutral and incongruent conditions (equal frequencies)

Dependent variables

o Task performance (error rate and response time)

o Posterior P3 [Attentional orienting/switching]

o Anterior N450 [evaluative control]

o Negative Slow Wave [NSW; regulative control]

Error Rate

CongruentNeutralIncongruent

Per

cen

t er

ror(

%)

0.0

2.5

5.0

7.5

10.0

Color Name Word Read Color Name Word Read

No-Alcohol Alcohol

Alcohol increased error rate for incongruent trials during color-naming

Response Time

CongruentNeutralIncongruent

Re

sp

on

se t

ime

(m

s)

400

500

600

700

800

Color Name Word Read Color Name Word Read

No-Alcohol Alcohol

Alcohol increased response time for incongruent trials during color-naming

Posterior/Parietal P3

Anterior N450

N450

N450

• Phasic nature

• Latency

• Magnitude pattern

• Source localization (Liotti et al., 2000)

Anterior Negative Slow Wave (NSW)

N450

NSW

NSW

• Relatively tonic nature

• Timing of activity


• Topography

• (West & Alain, 2000)

Anterior Negative Slow Wave (NSW)

NSW

NSW

• Relatively tonic nature

• Timing of activity


• Topography

• (West & Alain, 2000)

Flanker Methods

Design


o 400 trials of modified Flanker paradigm (4 blocks w/break)

o Stimuli are HHHHH; SSSSS; HHSHH; SSHSS

o Flankers are congruent (HHHHH) or incongruent (SSHSS)

o Targets are hi frequency (80%) or lo frequency (20%)

Dependent variables

o Task performance (response time and error rate)

o Error related negativity (ERN)

o N2c

Characteristics of the ERN

o A sharp, negative-going deflection of up to 15 V in amplitude.

o Onsets shortly after the onset of EMG activity (or behavioral response) associated with making an error.

o This is a response locked ERP

o Peaks approximately 100-150 ms after EMG evidence of a subject initiating an incorrect response.

o It is largest over the front and middle of the scalp (i.e., at Fz and Cz) and is non-lateralized.

o Dipole localization studies of the ERN suggest that it is generated by medial frontal structures, most likely the anterior cingulate cortex (ACC).

History of the ERN

o Discovered independently by two separate research groups around the early ‘90s

o Gehring, Goss, Coles, Meyer, and Donchin (1993). Psych ScienceError-Related Negativity (ERN)

o Falkenstein, Hohnsbein, Hoorman, & Blanke (1991). Electroencephalography & Clinical NeurophysiologyError Negativity (Ne)

ERN

What Might This Reflect?

o Activation within a system responsible for detecting performance errors

o Compensatory action related to correcting errors

o A response conflict monitoring system

ERN from Flanker

-8

-4

0

4

8

12

16

-500 -400 -300 -200 -100 0 100 200 300 400 500 600 700 800 900 1000

mic

rovo

ltsCorrect TrialsError Trials

Curtin, unpublished data

Methodology in ERN Studies

o Categorization tasks:Bird SpoonRobin Robin

o Eriksen Noise-Compatibility or Flanker TaskHHHHH SSSSS HHSHH SSHSS

o Go-No/GO with choice reaction time

o and many others

ERN: What does it index?

o The ACC and Pre-Frontal Cortex interact to form an executive or supervisory cognitive control system responsible for action monitoring and (when needed) compensatory action

o ACC activation reflects action monitoring processes

o PFC activation reflects compensatory processes (selective attention; corrective behavior)


o Detection or awareness of an error that occurs when a mental representations of the appropriate/intended response with those of the actual response.

o ERNs occur primarily following errors and generally not following correct responses (though can be observed on correct trials)

o ERNs are larger when accuracy is emphasized over speed

o Demonstration of ERN co-variation with certainty of error (e.g., Scheffers & Coles, 2000)


True Accuracy

Correct Correct Incorrect Incorrect

Perceived Accuracy

“Sure Correct”

“Don’t Know”

“Don’t Know”

“Sure Incorrect”

Information about the Target Stimulus

Sufficient Insufficient Insufficient Sufficient

Comparison of Appropriate Response to Actual Response

Full Match

Partial Mismatch

Partial Mismatch

Full Mismatch

o Scheffers and Coles (2000). JEP: Human Perception and Performance

o Used Eriksen Tasko Recorded ERNo After each trial, subjects rated perceived accuracy


0

1

2

3

4

5E

RN

(m

icro

volts

)

Young Age-Matched

PFC

Group

Correct

Incorrect

Gehring and Knight (2000), Nature Neuroscienceo Eriksen Tasko ERNs for:

PFC Damaged Individuals (Mean Age = 69) Healthy Age-Matched (Mean Age = 70) Healthy Young Adults(Mean Age = 24)

o PFC did not correct or use less force but did slow


o The ERN reflects detection of conditions that may predispose one to making errors (e.g., conflicts such as response competition).

o ACC activity might be considered conflict-monitoring rather than error-monitoring.

o Conflict is believed to occur when simultaneous activity occurs in both the correct and incorrect response channels.


o Errors most probable when conflict exists

o ERN is largest during partial errors that are corrected

o An ERN appears even on correct trials if the correct response is subsequently reversed (Gehring et al., 1993)

o Some ERN even on correct trials that are not reversed (and covaries with conflict manipulations)

o Increased ACC activity observed through brain imaging techniques in tasks in which response competition is high

Flanker, Target, & Block Effects

Block

1 2 3 4

Res

po

nse

tim

e (m

s)

390

420

450

480

510

Flanker Congruency

Res

po

nse

tim

e (m

s)

390

420

450

480

510

Congruent Incongruent

Target Frequqncy

Res

po

nse

tim

e (m

s)

390

420

450

480

510

Frequent Infrequent

Beverage and Flanker Effect

Res

po

ns

e ti

me

(ms

)

400

425

450

475

500

525

Flanker Congruency

Re

sp

on

se

Tim

e (

ms

)

400

425

450

475

500

525

Congruent Incongruent

Alcohol

No-Alcohol

Beverage and Target Frequency Effect

Res

po

ns

e ti

me

(ms

)

400

425

450

475

500

525

No-Alcohol

Target Frequency

Re

sp

on

se

Tim

e (

ms

)

400

425

450

475

500

525

Frequent Infrequent

Alcohol

Error Related Negativity (ERN)

No-alcohol/ErrorAlcohol/ErrorNo-alcohol/CorrectAlcohol/Correct

ERN in OCD

And amplitude of ERN correlates with Symptom severity (correlation magnitude ~.50); Gehring et al. (2000)

www.brainwavescience.com

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