Understanding Event-Related Potentials (ERPs) and Brain Source Analysis Techniques

2. The Event-Related Potential (ERP) • Embedded in the EEG signal is the small electrical response due to specific events such as stimulus or task onsets, motor actions, etc.

3. The Event-Related Potential (ERP) • Embedded in the EEG signal is the small electrical response due to specific events such as stimulus or task onsets, motor actions, etc. • Averaging all such events together isolates this event-related potential

4. The Event-Related Potential (ERP) • We have an ERP waveform for every electrode

5. The Event-Related Potential (ERP) • We have an ERP waveform for every electrode

6. The Event-Related Potential (ERP) • We have an ERP waveform for every electrode • Sometimes that isn’t very useful

7. The Event-Related Potential (ERP) • We have an ERP waveform for every electrode • Sometimes that isn’t very useful • Sometimes we want to know the overall pattern of potentials across the head surface • isopotential map

8. The Event-Related Potential (ERP) • We have an ERP waveform for every electrode • Sometimes that isn’t very useful • Sometimes we want to know the overall pattern of potentials across the head surface • isopotential map Sometimes that isn’t very useful - we want to know the generator source in 3D

9. Brain Electrical Source Analysis • Given this pattern on the scalp, can you guess where the current generator was?

10. Brain Electrical Source Analysis • Given this pattern on the scalp, can you guess where the current generator was? • Source Imaging in EEG/MEG attempts to model the intracranial space and “back out” the configuration of electrical generators that gave rise to a particular pattern of EEG on the scalp Duracell

11. Brain Electrical Source Analysis • EEG data can be coregistered with high-resolution MRI image Source Imaging Result Structural MRI with EEG electrodes coregistered

12. Intracranial and “single” Unit • Single or multiple electrodes are inserted into the brain • “chronic” implant may be left in place for long periods

13. Intracranial and “single” Unit • Single electrodes may pick up action potentials from a single cell • An electrode may pick up thecombined activity from several nearby cells • spike-sorting attempts to isolate individual cells

14. Intracranial and “single” Unit • Simultaneous recording from many electrodes allows recording of multiple cells

15. Intracranial and “single” Unit • Output of unit recordings is often depicted as a “spike train” and measured in spikes/second • Spike rate is almost never zero, even without sensory input • in visual cortex this gives rise to “cortical grey” Stimulus on Spikes

16. Intracranial and “single” Unit • Local Field Potential reflects summed currents from many nearby cells Stimulus on Spikes

17. Relationship between EEG / LFP / spike trains • All three probably reflect related activities but probably don’t share a 1-to-1 mapping • For example: there could be some LFP or EEG signal that isn’t associated with a change in spike rates. • WHY? Whittingstall & Logothetis (2009)

18. Synthesize the Big Picture

19. Synthesize the Big Picture

20. Lesion Studies • Logic of Lesion Studies: • damaged area plays a role in accomplishing whatever task is deficient after the lesion

21. Lesion Studies • Types of Lesions • Animal • Human

22. Lesion Studies • Animal Lesion Techniques • Aspiration Lesions • Electrolytic Lesions

23. Lesion Studies • Animal Lesion Techniques • Aspiration Lesions • Electrolytic Lesions • Problems: • These can damage surrounding tissue - especially white matter tracts nearby (“fibers of passage”) • Irreversible • eventual degradation of connected areas

24. Lesion Studies • Animal Lesion Techniques • Vascular Lesions • endothelin-1 • good model of human stroke • severe damage • not pinpoint accuracy

25. Lesion Studies • Animal Lesion Techniques • Reversible Lesions • cooling • Local anesthetic, other drugs • highly selective • can cool specific layers of cortex • can be reversed!

26. Lesion Studies • Animal Lesion Techniques • Selective Pharmacological lesions • damage or destroy entire pathways that have a specific sensitivity to a particular chemical • e.g. MPTP model of Parkinson’s Disease (frozen addicts) • e.g. scapolomine - acetylcholine antagonist - temporary amnesia • Can be selective for specific circuits but not for specific brain areas • can be reversible in some cases (e.g. scopolamine, but not MPTP)

27. Lesion Studies • Animal Lesion Techniques • Gene Knock-Out/Knock-In (Transgenics) • can selectively block/enhance expression • Viral vectors, electroporation • animal develops differently • Can have temporal/regional/molecular specificity

28. Lesion Studies • Human Lesions • Ischemic Events • Stroke and Hemorrhage: • typically due to blood clot or hemorrhage • size of lesion depends on where clot gets lodged • amount of damage depends on how long clot remains lodged

29. Lesion Studies • Human Lesions • Trauma • Frontal lobes are particularly susceptible • Some famous cases (e.g. Phineas Gage)

30. Lesion Studies • Human Lesions • Surgery • Often surgery done to treat epilepsy • Occasionally corpus callosum is severed • Problem: patient wasn’t “normal” before the surgery

31. Lesion Studies • Human Lesions • Transcranial Magnetic Stimulation • Electromagnet Induces current in the brain • very transient, very focal reversible “lesion” • Believed to be safe • sites that can be studied are limited by the geometry of the head