Evoked and Event-Related Potentials (ERP)Part 1 - Neurophysiological and Anatomical Basis

of Scalp-Recorded Activity

Craig E. Tenke & Jürgen Kayser

Division of Cognitive Neuroscience, New York State Psychiatric Institute, NY, NY

Department of Psychiatry, Columbia University College of Physicians & Surgeons, NY, NY

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Acronym ERP(wikipedia examples)

Event Related Potentials (ERP):Basics (Part 1)

27-Jan-2009

Outline

Part 1: Neurophysiological and Anatomical Basis of Scalp-Recorded Activityo Introduction: EEG measures synchronized neuronal activity (signal source)o The basic scientific method • Stimulation - Recording • Signal tracing/processing o Signal averaging: Evoked potentials (EPs) and event-related potentials (ERPs)• Theory of signal averaging• Animated examples• ERP applications• ERP topographies: Indicators of neuroanatomyo EEG biophysics• Volume Conduction and Ohm’s Law: Point generators• Vector formulation: A general model • Closed vs. open fieldso The cortical dipole: Direct evidence o A matter of scale: Micro- vs. macro- and intracranial vs. scalp electrodes

Part 2: Data acquisition and analysis: Conventions for scalp-recorded ERPs

o (details to be decided)

Part 1

Neurophysiological and Anatomical Basisof Scalp-Recorded Activity

Evoked and Event-Related Potentials (ERP)

What does EEG measure?

Changes in the extracellular potential corresponding to membrane polarization

Polarity reflects locationof cellular activity

Equivalent Current Dipolefrom apical EPSP

Equivalent Current Dipole from deep EPSP

Desynchronized PSP’s voltages cancel

“Closed Field” (No EEG at Scalp)

Importance of synchronized activity

Synchronized PSPs voltages add

“Open Field” (EEG at scalp)

Importance of synchronized activity

• when local neuronal activity is synchronized (time)

• when activity produces an open field (space)

• when activity at the recording site differs from the reference

Scalp-recorded EEG is measurable

Stimulation-Recording methods use timelocking

to synchronize activity

Stimulation-Recording Methods:Tracing signals through a “Black Box”

Signal Tracing:

Transit time:

Response Function:

To find System Properties

Compare Input Signal With Output Signal

e.g. synaptic delay across a sensory nucleus

Filtering and gain of output waveformNonlinear properties (e.g. flicker fusion)

Waveform latency, shape and topography e.g. cortical mapping

80 mm from stimulusThreshold Intensity Differential Threshold Differential Velocity

up is negative; right is reference

Mann 1997

Erlanger & Gasser

Compound Action Potential:A Stimulation-Recording Archetype

• time-locked electrical response of a neural system to an electrical or sensory signal

Averaged Evoked Potential

Evoked Potential (EP)

Schandry 1989

• average of time-locked EPs• EPs may vary considerably across

trials (averaging is generally necessary)

Examples of Averaged ERP

Auditory Oddball ERP(average)

Targets

Nontargets

Visual Hemifield ERP(average)

Kayser (2001) from Psychophysiology Lab website (http://psychophysiology.cpmc.columbia.edu)

• time-locked electrical response of a neural system to an electrical or sensory signal

Averaged Evoked Potential

Evoked Potential (EP)

Schandry 1989

• average of time-locked EPs• EPs may vary considerably across

trials (averaging is generally necessary)

Event-related Potential (ERP) • Generalized EP timelocked to a

stimulus, response, or informational event (e.g., missing stimulus in series of stimuli)

Applications of ERPs

1) Pathology:

3) Functional mapping:

Ramon y Cajal Mountcastle & Henneman(1952)

slowing or distortion of EP

parallel other neuroanatomical methods

2) Information processing(incl. perception, cognition)

ERP topographies have anatomical implications!

Topographies reflect macroscopic and microscopic anatomy

is directly proportional to

current I and inversely

related to distance (d)

EEG Biophysics:Volume Conduction and Ohm’s Law

Voltage Potential

V= I /R Voltage is directly proportional to current,and inversely related to resistance

For a point generator in a conductive medium, resistance is related to distance:

Tenke et al (1993))

4 electrodes

point source

As the generator becomes wider, the falloff becomes linear and shallow

EEG Biophysics:Volume Conduction implies Spatial Integration

EEG Biophysics:Complete Volume Conduction Model

Vector form of Ohm’s Law (proportionality of current flow and electric field vectors)

This fundamental relationship also underlies inverse models!Current Source Density (Im)

is a scalar

This is Poisson’s source equation relating current generators to voltage potentials

Tenke et al 1993

Problem: Neither current nor voltage are in this equation!

Simulated dipole laminae

50% inverted dipolesyield closed field

25% invertedyield open field

Field closure is quantitative,not qualitative

The Cortical Dipole

“Cortical Dipole” hypothesized from cortical projection cell asymmetry

Dep

th in

mm

(or

thog

onal

pe

netr

atio

n)

and supported superficial-to-deep polarity inversions

Intracortical profiles reveal complexity of processing

Within cortex field potential profiles reach maximum and invert in deeper layers

Tenke et al (unpublished))

A matter of scale:Intracranial recordings

ion channels,synaptic currents,

postsynaptic potentialsand unit discharges

(mV range)

Intracellular recordings

Extracellularrecordings

Local Field Potentials:postsynaptic potentials

and unit discharges

Selectivity for Local Activity!

Multicontact extracellularrecordings

Local Field Potentials:Summated PSPs, units

and multiunits•High-impedance electrodes•proximal to generators•far from external noise sources

A matter of scale:Surface and scalp recordings

Lower impedance electrodesLarger electrodes integrate over more tissueMore distant from generatorsFurther smearing by volume conduction

(smaller, composite signals)

Additional smearing(bone & skin)

Scalp recordings

Smaller amplitude compared to EOG, EKG etc. (uV range)

Proximity to EMG sources