evoked and event-related potentials (erp) part 1 - neurophysiological and anatomical basis
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27-Jan-2009. Event Related Potentials (ERP): Basics (Part 1). Evoked and Event-Related Potentials (ERP) Part 1 - Neurophysiological and Anatomical Basis of Scalp-Recorded Activity Craig E. Tenke & Jürgen Kayser - PowerPoint PPT PresentationTRANSCRIPT
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