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

1. 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 Division of Cognitive Neuroscience, New York State Psychiatric Institute, NY, NY Department of Psychiatry, Columbia University College of Physicians & Surgeons, NY, NY • 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) Acronym ERP (wikipedia examples)

2. Outline • Part 1: Neurophysiological and Anatomical Basis of Scalp-Recorded Activity • Introduction: EEG measures synchronized neuronal activity (signal source) • The basic scientific method • Stimulation - Recording • Signal tracing/processing • Signal averaging: Evoked potentials (EPs) and event-related potentials (ERPs) • Theory of signal averaging • Animated examples • ERP applications • ERP topographies: Indicators of neuroanatomy • EEG biophysics • Volume Conduction and Ohm’s Law: Point generators • Vector formulation: A general model • Closed vs. open fields • The cortical dipole: Direct evidence • A matter of scale: Micro- vs. macro- and intracranial vs. scalp electrodes • Part 2: Data acquisition and analysis: Conventions for scalp-recorded ERPs • (details to be decided)

3. Evoked and Event-Related Potentials (ERP) Part 1 Neurophysiological and Anatomical Basis of Scalp-Recorded Activity

4. What does EEG measure? Changes in the extracellular potential corresponding to membrane polarization

5. Polarity reflects location of cellular activity Equivalent Current Dipole from deep EPSP Equivalent Current Dipole from apical EPSP

6. Importance of synchronized activity Desynchronized PSP’s  voltages cancel “Closed Field” (No EEG at Scalp)

7. Importance of synchronized activity Synchronized PSPs  voltages add “Open Field” (EEG at scalp)

8. 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

9. Stimulation-Recording methods use timelocking to synchronize activity

10. Stimulation-Recording Methods: Tracing signals through a “Black Box” To find System Properties With Output Signal Compare Input Signal Transit time: 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

11. Compound Action Potential: A Stimulation-Recording Archetype up is negative; right is reference Erlanger & Gasser Threshold Intensity Differential Threshold Differential Velocity Mann 1997 80 mm from stimulus

12. 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)

13. Examples of Averaged ERP Visual Hemifield ERP (average) Auditory Oddball ERP (average) Nontargets Targets Kayser (2001) from Psychophysiology Lab website (http://psychophysiology.cpmc.columbia.edu)

14. 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)

15. 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)

16. ERP topographies have anatomical implications! Topographies reflect macroscopic and microscopic anatomy

17. 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 current I and inversely related to distance (d) Tenke et al (1993))

18. EEG Biophysics: Volume Conduction implies Spatial Integration 4 electrodes point source As the generator becomes wider, the falloff becomes linear and shallow

19. 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! Problem: Neither current nor voltage are in this equation! Current Source Density (Im) is a scalar This is Poisson’s source equation relating current generators to voltage potentials Tenke et al 1993

20. Field closure is quantitative, not qualitative Simulated dipole laminae 25% inverted yield open field 50% inverted dipoles yield closed field

21. “Cortical Dipole” hypothesized from cortical projection cell asymmetry The Cortical Dipole Intracortical profiles reveal complexity of processing Within cortex field potential profiles reach maximum and invert in deeper layers Depth in mm (orthogonal penetration) and supported superficial-to-deep polarity inversions Tenke et al (unpublished))

22. A matter of scale: Intracranial recordings Intracellular recordings Extracellular recordings Multicontact extracellular recordings ion channels, synaptic currents, postsynaptic potentials and unit discharges (mV range) Local Field Potentials: postsynaptic potentials and unit discharges Local Field Potentials: Summated PSPs, units and multiunits Selectivity for Local Activity! • High-impedance electrodes • proximal to generators • far from external noise sources

23. A matter of scale: Surface and scalp recordings Scalp recordings Lower impedance electrodes Larger electrodes integrate over more tissue More distant from generators Further smearing by volume conduction (smaller, composite signals) Additional smearing (bone & skin) Smaller amplitude compared to EOG, EKG etc. (uV range) Proximity to EMG sources