Slide 1

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.

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

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

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

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

Sometimes that isnt very usefulThe Event-Related Potential (ERP)We have an ERP waveform for every electrode

Sometimes that isnt very useful

Sometimes we want to know the overall pattern of potentials across the head surfaceisopotential map

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

Sometimes that isnt very useful

Sometimes we want to know the overall pattern of potentials across the head surfaceisopotential map

Sometimes that isnt very useful - we want to know the generator source in 3D Brain Electrical Source AnalysisGiven this pattern on the scalp, can you guess where the current generator was?

Brain Electrical Source AnalysisGiven 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

DuracellBrain Electrical Source AnalysisEEG data can be coregistered with high-resolution MRI image

Source ImagingResultStructural MRI with EEG electrodes coregisteredIntracranial and single UnitSingle or multiple electrodes are inserted into the brain

chronic implant may be left in place for long periods

Intracranial and single UnitSingle electrodes may pick up action potentials from a single cell

An electrode may pick up the combined activity from several nearby cellsspike-sorting attempts to isolate individual cells

Intracranial and single UnitSimultaneous recording from many electrodes allows recording of multiple cells

Intracranial and single UnitOutput of unit recordings is often depicted as a spike train and measured in spikes/second

Spike rate is almost never zero, even without sensory inputin visual cortex this gives rise to cortical grey

Stimulus onSpikesIntracranial and single UnitLocal Field Potential reflects summed currents from many nearby cells

Stimulus onSpikesRelationship between EEG / LFP / spike trainsAll three probably reflect related activities but probably dont share a 1-to-1 mappingFor example: there could be some LFP or EEG signal that isnt associated with a change in spike rates.

WHY?

Whittingstall & Logothetis (2009)Synthesize the Big PictureSynthesize the Big PictureLesion StudiesLogic of Lesion Studies:damaged area plays a role in accomplishing whatever task is deficient after the lesionLesion StudiesTypes of LesionsAnimalHuman

Lesion StudiesAnimal Lesion TechniquesAspiration LesionsElectrolytic Lesions

Lesion StudiesAnimal Lesion TechniquesAspiration LesionsElectrolytic Lesions

Problems:These can damage surrounding tissue - especially white matter tracts nearby (fibers of passage)

Irreversible

eventual degradation of connected areas

Lesion StudiesAnimal Lesion TechniquesVascular Lesionsendothelin-1good model of human strokesevere damagenot pinpoint accuracy

Lesion StudiesAnimal Lesion TechniquesReversible LesionscoolingLocal anesthetic, other drugshighly selectivecan cool specific layers of cortexcan be reversed!

Lesion StudiesAnimal Lesion TechniquesSelective Pharmacological lesionsdamage or destroy entire pathways that have a specific sensitivity to a particular chemical

e.g. MPTP model of Parkinsons Disease (frozen addicts)e.g. scapolomine - acetylcholine antagonist - temporary amnesia

Can be selective for specific circuits but not for specific brain areascan be reversible in some cases (e.g. scopolamine, but not MPTP)

Lesion StudiesAnimal Lesion TechniquesGene Knock-Out/Knock-In (Transgenics)can selectively block/enhance expressionViral vectors, electroporationanimal develops differently

Can have temporal/regional/molecular specificity

Lesion StudiesHuman LesionsIschemic EventsStroke and Hemorrhage:typically due to blood clot or hemorrhagesize of lesion depends on where clot gets lodgedamount of damage depends on how long clot remains lodged Lesion StudiesHuman LesionsTraumaFrontal lobes are particularly susceptibleSome famous cases (e.g. Phineas Gage) Lesion StudiesHuman LesionsSurgeryOften surgery done to treat epilepsyOccasionally corpus callosum is severed

Problem: patient wasnt normal before the surgeryLesion StudiesHuman LesionsTranscranial Magnetic StimulationElectromagnet Induces current in the brainvery transient, very focal reversible lesion

Believed to be safesites that can be studied are limited by the geometry of the head

32Figure 4.25b Transcranial magnetic stimulation. Here the TMS coil is being applied by an experimenter. Both the coil and the subject have affixed to them a tracking device to monitor the head/coil position in real time.

33Figure 4.25d Transcranial magnetic stimulation. The TMS pulse directly alters neural activity in a spherical area of approximately 1 cm3.

34Figure 4.26a Transcranial magnetic stimulation over the occipital lobe. The center of the figure-eight coil is placed over the targeted area. When a large electrical field is passed through the coil, a magnetic pulse is generated and passes through the skull. This pulse activates neurons in the underlying cortex.