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) Event Related Potentials (ERP): Basics (Part 1) 27-Jan-2009

2. 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)

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

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 apical EPSP Equivalent Current Dipole from deep EPSP

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

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

8. 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

9. Stimulation-Recording methods use timelocking to synchronize activity

10. 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 waveform Nonlinear properties (e.g. flicker fusion) Waveform latency, shape and topography e.g. cortical mapping

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

12. 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)

13. 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)

14. 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)

15. 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)

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

17. 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))

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

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! Current Source Density ( I m ) 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!

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

21. The Cortical Dipole “Cortical Dipole” hypothesized from cortical projection cell asymmetry Depth in mm (orthogonal penetration) 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))

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

23. A matter of scale: Surface and 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) Scalp recordings Smaller amplitude compared to EOG, EKG etc. (uV range) Proximity to EMG sources