27-Jan-2009
Evoked
Event
and Related
Event-Related
Potentials
Potentials
(ERP):(ERP)
Basics (Part
Part 1 - Neurophysiological
and1)
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
Acronym ERP
(wikipedia examples)
• towns (Geography: in southern France, Germany, Netherlands)
• enterprise resource planning (Information Technology)
• Ethernet Ring Protection (Information Technology)
• erotic role-playing (Sexuality)
• Economic Report of the President (Economics)
• effective refractory period (Medicine: cardiac cycle)
• Estonian Reform Party (Politics)
• Electronic Road Pricing (Politics: toll-collection scheme in Singapore)
• European Recovery Program (History: Marshall Plan)
• European Radio Project (Communication: European Radio Network)
• exposure and response prevention (Psychology: cognitive-behavioral treatment method)
• event-related potential (Physics: an electrophysiological response to an internal or external stimulus)
Outline
Part 1: Neurophysiological and Anatomical Basis of Scalp-Recorded Activity
o 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 neuroanatomy
o EEG biophysics
• Volume Conduction and Ohm’s Law: Point generators
• Vector formulation: A general model
• Closed vs. open fields
o 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)
Evoked and Event-Related Potentials (ERP)
Part 1
Neurophysiological and Anatomical Basis
of Scalp-Recorded Activity
What does EEG measure?
Changes in the extracellular potential
corresponding to membrane polarization
Polarity reflects location
of cellular activity
Equivalent Current Dipole
from apical EPSP
Equivalent Current Dipole
from deep EPSP
Importance of synchronized activity
Desynchronized PSP’s  voltages cancel
“Closed Field” (No EEG at Scalp)
Importance of synchronized activity
Synchronized PSPs  voltages add
“Open Field” (EEG at scalp)
Scalp-recorded EEG is measurable
• when local neuronal activity is synchronized (time)
• when activity produces an open field (space)
• when activity at the recording site
differs from the reference
Stimulation-Recording methods
use timelocking
to synchronize activity
Stimulation-Recording Methods:
Tracing signals through a “Black Box”
To find System Properties
Compare Input Signal
Transit time:
With Output Signal
e.g. synaptic delay across a sensory nucleus
Response Function: Filtering and gain of output waveform
Nonlinear properties (e.g. flicker fusion)
Signal Tracing:
Waveform latency, shape and topography
e.g. cortical mapping
Compound Action Potential:
A Stimulation-Recording Archetype
up is negative; right is reference
Mann 1997
Erlanger & Gasser
Threshold Intensity
Differential Threshold
80 mm from stimulus
Differential Velocity
Schandry 1989
Evoked Potential (EP)
• time-locked electrical response of a
neural system to an electrical or
sensory signal
Averaged Evoked Potential
• average of time-locked EPs
• EPs may vary considerably across
trials (averaging is generally
necessary)
Examples of Averaged ERP
Auditory Oddball ERP
(average)
Visual Hemifield ERP
(average)
Nontargets
Targets
Kayser (2001) from Psychophysiology Lab website (http://psychophysiology.cpmc.columbia.edu)
Schandry 1989
Evoked Potential (EP)
• time-locked electrical response of a
neural system to an electrical or
sensory signal
Averaged Evoked Potential
• 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:
slowing or distortion of EP
2) Information processing
(incl. perception, cognition)
3) Functional mapping:
parallel other neuroanatomical methods
Ramon y Cajal
Mountcastle & Henneman(1952)
ERP topographies have anatomical implications!
Topographies reflect macroscopic and microscopic anatomy
EEG Biophysics:
Volume Conduction and Ohm’s Law
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:
Voltage Potential
is directly proportional to
I and inversely
related to distance (d)
current
Tenke et al (1993))
EEG Biophysics:
Volume Conduction implies Spatial Integration
4 electrodes
point
source
As the generator becomes wider,
the falloff becomes linear and shallow
EEG Biophysics:
Complete Volume Conduction Model
Vector form of Ohm’s Law
(proportionality of current flow
and electric field vectors)
Problem: Neither current nor voltage
are in this equation! This fundamental relationship
Current
Source
Density
(Im)
also underlies
inverse
models!
is a scalar
This is Poisson’s source equation relating
current generators to voltage potentials
Tenke et al 1993
Field closure is quantitative,
not qualitative
Simulated dipole laminae
50% inverted dipoles
yield closed field
25% inverted
yield open field
Depth in mm (orthogonal penetration)
The Cortical Dipole
Intracortical profiles
reveal complexity of
processing
Within cortex field
potential profiles
reach maximum
and invert in deeper
layers
“Cortical Dipole”
hypothesized from
cortical projection
cell asymmetry
and supported
superficial-to-deep
polarity inversions
Tenke et al (unpublished))
A matter of scale:
Intracranial recordings
Intracellular
recordings
Extracellular
recordings
Multicontact extracellular
recordings
ion channels,
Local Field Potentials:
synaptic currents,
postsynaptic potentials
postsynaptic potentials
and unit discharges
and unit discharges
(mV range)
Selectivity for Local Activity!
•High-impedance electrodes
•proximal to generators
•far from external noise sources
Local Field Potentials:
Summated PSPs, units
and multiunits
A matter of scale:
Surface and scalp recordings
Scalp recordings
Lower impedance electrodes
Additional smearing
Larger electrodes integrate over more tissue
(bone & skin)
More distant from generators
Smaller amplitude compared
Further smearing by volume conduction
to EOG, EKG etc. (uV range)
(smaller, composite signals)
Proximity to EMG sources