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 isn’t very useful
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
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
Brain Electrical Source Analysis
• Given this pattern on the scalp,
can you guess where the
current generator was?
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
Brain Electrical Source Analysis
• EEG data can be coregistered with highresolution MRI image
Source
Imaging
Result
Structural
MRI with EEG
electrodes
coregistered
Intracranial and “single” Unit
• Single or multiple electrodes
are inserted into the brain
• “chronic” implant may be left in
place for long periods
Intracranial and “single” Unit
• Single electrodes may pick up
action potentials from a single
cell
• An electrode may pick up the
combined activity from several
nearby cells
– spike-sorting attempts to
isolate individual cells
Intracranial and “single” Unit
• Simultaneous recording from
many electrodes allows
recording of multiple cells
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
Intracranial and “single” Unit
• Local Field Potential reflects
summed currents from many
nearby cells
Stimulus on
Spikes
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)
Synthesize the Big Picture
Metabolic
Imaging
• fMRI/PET
Extracranial
electrophysiology
Intracranial
• LFP/single-unit
• EEG/MEG
Understanding
Brain-wide
neural circuits
Synthesize the Big Picture
Intracranial
• LFP/single-unit
Metabolic
Imaging
Lesion Studies
• fMRI/PET
Extracranial
electrophysiology
• EEG/MEG
Understanding
Brain-wide
neural circuits
Computational
Models
Lesion Studies
• Logic of Lesion Studies:
– damaged area plays a role in accomplishing
whatever task is deficient after the lesion
Lesion Studies
• Types of Lesions
– Animal
– Human
Lesion Studies
• Animal Lesion Techniques
– Aspiration Lesions
– Electrolytic Lesions
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
Lesion Studies
• Animal Lesion Techniques
– Vascular Lesions
•
•
•
•
endothelin-1
good model of human stroke
severe damage
not pinpoint accuracy
Lesion Studies
• Animal Lesion Techniques
– Reversible Lesions
•
•
•
•
•
cooling
Local anesthetic, other drugs
highly selective
can cool specific layers of cortex
can be reversed!
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)
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
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
Lesion Studies
• Human Lesions
– Trauma
• Frontal lobes are particularly susceptible
• Some famous cases (e.g. Phineas Gage)
Lesion Studies
• Human Lesions
– Surgery
• Often surgery done to treat epilepsy
• Occasionally corpus callosum is severed
• Problem: patient wasn’t “normal” before the surgery
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