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The ERP Boot Camp

Isolating Components with Difference Waves

and The Boot Camp

Experiment

w1,1

w2,1

w3,1

w1,2w2,2

w3,2

w1,3

w2,3

w3,3

C1

C2

C3

E1

Difference Waves

C1 C2

C3E2

E1

E3

If a single component varies across conditions, it can be isolated by means of a difference wave

For this to work, the conditions must be so similar that only one component varies across conditions

E2

E3

Isolating the LRP

Left Hemisphere(Ipsi)

Right Hemisphere(Contra)

Right Hemisphere(Ipsi)

Left Hemisphere(Contra)

LRP = Contra minus ipsi, averaged over left & right hands

Smulders & Miller (2010)

Isolating the LRP

LRP = Contra minus ipsi, averaged over left & right hands

Smulders & Miller (2010)

Isolating the LRP

Left Hemisphere(Ipsi)

Right Hemisphere(Contra)

Right Hemisphere(Ipsi)

Left Hemisphere(Contra)

Nonlateralized components eliminated by subtraction

Smulders & Miller (2010)

Isolating the LRP

Left Hemisphere(Ipsi)

Right Hemisphere(Contra)

Right Hemisphere(Ipsi)

Left Hemisphere(Contra)

Overall hemisphere differences eliminated by subtraction(RHem more positive than LHem for both hands)

Isolating the LRP

Left Hemisphere(Ipsi)

Right Hemisphere(Contra)

Right Hemisphere(Ipsi)

Left Hemisphere(Contra)

Overall hand differences eliminated by subtraction(RHand more positive than LHand for both hemispheres)

Smulders & Miller (2010)

Isolating the LRP

LRP provides a pure measure of the relative activation of correct vs incorrect response at

each moment in timeSmulders & Miller (2010)

The N2pc Component

N2pc Difference Waves

The C1 Wave

Mangun, Hillyard, & Luck (1993)

C1 can be isolated with Upper minus Lower difference wave

Isolating the P3 Wave• P3 amplitude depends on the probability of a task-defined

stimulus category, not the probability of a physical stimulus• P3 probability effect cannot begin until after categorization

occurs• Rare-minus-frequent difference wave isolates processes that

occur after perception and categorization• P3 latency indexes “stimulus evaluation time”

P3 Difference Waves

Shortcomings of Difference Waves

• May not isolate a single component- Example: Rare-Frequent yields P2, N2, P3

• If Cond1–Cond2 difference wave is reduced, cannot distinguish between smaller Cond1 and larger Cond2- For most components, need to look at original waveforms

as well as difference waves (not for N2pc/LRP)- Ask Emily how we solved this problem for LRP

• Changes in latency between Cond1 and Cond2 produce an apparent difference in amplitude- Not a problem if you view difference wave as simply

meaning a difference in the time course of amplitude

The MONSTER Paradigm

• Goal- Efficiently measure the speed and integrity of a broad set of specific neurocognitive processes- First step in determining the nature of deficits in a given

patient group- Possibly useful for diagnosis of disease subtypes (e.g., in

multiple sclerosis)

• Use orthogonal subtractions to isolate specific ERP components that reflect well-characterized neurocognitive processes- Same data divided different ways to isolate different

components

Stimuli and Task

Task: Attend to black or whitePress left for A (p = .80 or p = .20)Press right for B (p = .20 or p = .80)Left/Right and .80/.20 are counterbalanced

Stim: Duration = 200 msSOA = 1500 ± 150 ms2 blocks of 512 stimuli

Stimuli and Task

C1: Upper Checks minus Lower Checks

N2pc: Contra target minus Ipsi target

P3: Rare minus Frequent

LRP: Contra to response hand minus Ipsi to response hand

These subtractions are orthogonal -- same 1024 trials divided into different pairs of subsets and then subtracted

Recording with Biosemi ActiveTwo• Active Electrodes-

- Each electrode has a small, built-in preamplifier connected to an Ag/AgCl tip

- Doesn’t actually amplify; creates an ultra low impedance signal that is noise resistant

- Down side: Expensive to buy and replace- Fill electrode holders with gel and plug in electrodes- Flat electrodes for mastoids, eyes, etc.

Recording with Biosemi ActiveTwo

• Single-Ended (not differential) recording- Record electrical potential between “active” and CMS

(“common mode sense” -- analogous to ground)- Reference digitally, off line

• DRL (“driven right leg”) electrode- Think of it as a power source for the other electrodes

• High impedance• Digitization

- Separate A/D converter for each electrode- Output is fiber optic cable, which connects to a box near

the computer (safe, not an antenna for line noise)- Sampling rate = 1024 Hz (must be power of 2)- Built-in hardware low-pass anti-aliasing filter- No high-pass filter (DC recording)

Electrode Sites

LRP

N2pc

C1

P3