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The ERP Boot Camp

What Are ERPs and What Are

They Good For?

Overview

• A bit of history

• Review: Basic neurophysiology & electricity

• Neural origins of ERPs

• Comparison of ERPs with other techniques

Walter, W. G., Cooper, R., Aldridge, V. J., McCallum, W. C., &

Winter, A. L. (1964). Contingent negative variation: An electric

sign of sensorimotor association and expectancy in the

human brain. Nature, 203, 380-384.

The Dawn of History

• 1964: Gray Walter and the CNV

No Task: Click Only

No Task: Flashes Only

No Task: Click followed by flashes

Task: Press button when flashes start

CNV

Which Way is Up?

René Descartes

Sutton, S., Braren, M., Zubin, J., & John, E. R. (1965).

Evoked potential correlates of stimulus uncertainty.

Science, 150, 1187-1188.

Scenario:

Cue stimulus indicating whether click

or flash was likely

Delay of 3-5 seconds: Subject

guesses whether stimulus will be

click or flash

Click or flash occurs

The Birth of the P300

• 1965: Sutton, Braren, Zubin, & John

P300

Sound-Elicited

ERPs

Light-Elicited

ERPsP300

The Next 4 Decades

• 1970s- Development and standardization of methods

- Characterization of P3 and application to psychopathology

- Selective attention (Hillyard, Näätänen, Harter)

• The 1980s- Kutas & Hillyard (1980) discover N400

- ERPology transitions into cognitive neuroscience

- Multichannel recordings (16+ electrodes) and BESA

• The 1990s- fMRI takes off, leading to predictions of the demise of ERPs

The Next 4 Decades

• 1970s- Development and standardization of methods

- Characterization of P3 and application to psychopathology

- Selective attention (Hillyard, Näätänen, Harter)

• The 1980s- Kutas & Hillyard (1980) discover N400

- ERPology transitions into cognitive neuroscience

- Multichannel recordings (16+ electrodes) and BESA

• The 1990s- fMRI takes off, leading to predictions of the demise of ERPs

- ERPs try to become a neuroimaging technique

- More components: LRP, N2pc, ERN, N170, etc.

• The 2000s- Time-frequency analyses become mainstream

- The ERP Boot Camp!

Some Basics of Neuroscience

• Resting membrane potential- -70 mV on inside of cell

• Action potentials- Triggered when membrane potential goes sufficiently positive

- Starts at axon hillock and travels down axon

- Rarely contributes to scalp ERPs

• Postsynaptic potentials (PSPs)- Neurotransmitter binds with receptor, opens ion channels

- Excitatory: Positive charges move into cell

- Inhibitory: Negative charges move into cell

- The origin of most ERPs

Some Basics of Electricity

• Current (I for Intensity; Amperes)- Movement of charges across space (coulombs per second)

- Like rate of water coming out of a hose (liters per second)

• Voltage (E for Electromotive Force; Volts)- Potential for charges to move

- Like water pressure

• Resistance (R; Ohms [Ω])- Resistance to movement of charges

- Like having a skinny or blocked hose segment

• Impedance (Z)- Resistance to the flow of alternating current (AC)

- Combines resistance, capacitance, and inductance

Some Basics of Electricity

• Power (Watts) = EI (typically proportional to E2)

• Ohm’s Law: E = IR- If resistance increases and current is constant, voltage

increases!

- If you keep total water flow constant but constrict part of the hose, the pressure increases

Some Basics of Electricity

• Electricity follows the path of least resistance

Overall R < lowest individual R Overall R = sum of individual Rs

Some Basics of Electricity

• Measuring Electrode Impedances

Measuring between E1 and E8 gives

you the sum of E1 and E8; which

impedance is high?

Measuring between E1–E7 (in parallel)

and E8 gives you the sum of E8 and less

than the lowest of E1–E7

Some Basics of Electricity

• Induction- If you pass an electrical current through a conductor, a magnetic

field will run around it (right-hand rule)

- If you pass a magnetic field across a conductor, an electrical current is induced in the conductor

Some Basics of Electricity

• AC is “Alternating Current”- Changes fairly rapidly over time

- Line current (50 or 60 Hz)

- EEG

• DC is “Direct Current”- Fairly constant “offset” in voltage

- Batteries

- Skin potentials

- In discussions of amplifiers, DC can also mean “Direct Coupled” (as opposed to capacitively coupled)

• Conveniently, this means that the amplifier can amplify DC (direct current) signals

Where Do ERPs Come From?Cortical pyramidal cell (basic input-output

cell of cerebral cortex)

Excitatory transmitter released on apical

dendrites causes positive charges to flow

into dendrites

Net negative on outside of cell

Current flows through cell, completing the

circuit

Polarity reverses with inhibitory transmitter

Polarity reverses with PSP on cell body

and basal dendrites

Polarity at scalp also depends on

orientation of the cortical surface and

position of reference electrode

Where Do ERPs Come From?

To be recorded at a distance, large numbers of neurons

must have similar voltage fields

Equivalent Current Dipole

Where Do ERPs Come From?

Scalp-recorded potentials are

possible only for layered

structures with consistent

orientations

Primarily cerebral cortex

What about:

Cerebellum?

Brainstem nuclei?

Superior colliculus?

Possible role of radial gliaLocal Field Potentials

Open Field Closed Field

Where Do ERPs Come From?

Voltages spread through the brain by “volume conduction”

Nearly speed of light

Voltage everywhere except at positive-negative transition

Skull causes lateral spread (like spraying hose on cardboard)

Magnetoencephalography (MEG)

Magnetic fields travel around electrical dipoles

The skull is transparent to magnetism -- less blurring

Deep and radial dipoles are invisible from outside the head

w1,1

w2,1

w3,1

w1,2

w2,2

w3,2

w1,3

w2,3

w3,3

C1

C2

C3

E1

The Superposition Problem

C1 C2

C3

E2

E1

E3

Voltage at an electrode at time t is a

weighted sum of all components that

are active at time t

There is no foolproof way to recover the

underlying components from the

observed waveforms

E2

E3

What are ERPs Good For?

• Reaction time for the 21st Century

- Continuous measure of processing between S and R

• Determine whether an experimental manipulation

influenced Process A or Process B

- Which ERP component was affected?

• Identifying multiple neurocognitive processes

- A given behavioral effect is often accompanied by multiple ERP

effects (different components)

- Easy to identify multiple processes contributing to behavior

• Covert monitoring of processing

- Processing can be measured under conditions that do not involve a

behavioral response (or from subjects who cannot easily be trained

to respond)

- Did the brain do something that was not evident in behavior?

What are ERPs Good For?

• Link to the brain

- Under some conditions, neural systems can be identified

- But people often draw unwarranted conclusions about

underlying neural processes from ERPs

- Many researchers desperately want ERPs to be like fMRI or

single-unit recordings, but they are not

- “Those English boys want to play the blues so bad—and they

DO play it so bad” — Sonny Boy Williamson

- “Those ERPers want to study the brain so bad—and they DO

study it so bad” — Sonny Boy Luck

ERPs Are Bad When…

• You desire certainty about the neuroanatomical locus of an

effect

• You are interested in activity that is not time-locked to a

sudden, observable event

• You cannot collect large numbers of trials in each critical

condition

- Long period of time between trials

- Severe adaptation of response over trials

- Huge number of different control conditions

- Need to surprise subjects

• Subjects make frequent head or mouth movements during

the time period of interest

- Speech is particularly bad (tongue has strong dipole)

Comparison of Techniques

An Example Experiment

Stimuli

Left standards (p = .4)

Left deviants (p = .1)

Right standards (p = .4)

Right deviants (p = .1)

Duration = 100 ms; SOA = 300-500 ms

Conditions

Attend left (press for left deviants)

Attend right (press for right deviants)

Also…

Maintain fixation (verify with EOG)

(based on many experiments from the Hillyard lab)

An Example Experiment

Comparison of Attended and Ignored Standards

Things to notice:

Same stimuli; different psychological

conditions

Time 0 is stimulus onset

Assumption: Early in time means early in

information processing sequence

Conclusion: Attention influences sensory

gain at early stage

“Upper bound” on onset time

Hard to test this with behavioral

experimentsTime relative to stimulus onset

O1/O2

An Example Experiment

Things to notice:

Standard/Deviant comparison not perfectly controlled

Ignored deviants may be detected by a system that does not

generate an ERP

Pre-History

• 1929: The Berger Rhythm

• 1934/35: Confirmation by the labs of Adrian, Jasper, and

Davis

• 1939: Single-trial ERPs reported by Pauline & Hallowell

Davis

• 1940-1960: Sensory ERPs recorded with primitive

methods

• 1962: First publication of computer-averaged ERPs by

Galambos

Basic EEG

Raw EEG

Delta (1-3 Hz)

Slow Wave Sleep

Theta (4-7 Hz)

Non-REM Sleep

Basic EEG

Alpha (8-12 Hz)

Awake, Relaxed

(Zoning)

Beta (12-25 Hz)

Mentally Active

Gamma (25+ Hz)

Local Synchrony?