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Electroencephalography and the Event-Related Potential
Time
Volta
ge
-Place an electrode on the scalp and another one somewhere else on the body
-Amplify the signal to record the voltage difference across these electrodes
-Keep a running measurement of how that voltage changes over time
-This is the human EEG
Electroencephalography
• pyramidal cells span layers of cortex and have parallel cell bodies
• their combined extracellular field is small but measurable at the scalp!
Electroencephalography
• The field generated by a patch of cortex can be modeled as a single equivalent dipolar current source with some orientation (assumed to be perpendicular to cortical surface)
Electroencephalography
• Electrical potential is usually measured at many sites on the head surface
• More is sometimes better
Magnetoencephalography
• For any electric current, there is an associated magnetic field
Magnetic Field
Electric Current
Magnetoencephalography
• For any electric current, there is an associated magnetic field
• magnetic sensors called “SQuID”s can measure very small fields associated with current flowing through extracellular space
Magnetic Field
Electric Current
SQuID
Amplifier
Magnetoencephalography
• MEG systems use many sensors to accomplish source analysis
• MEG and EEG are complementary because they are sensitive to orthogonal current flows
• MEG is very expensive
EEG/MEG• Any complex waveform can be decomposed into
component frequencies– E.g.
• White light decomposes into the visible spectrum• Musical chords decompose into individual notes
EEG/MEG
• EEG is characterized by various patterns of oscillations
• These oscillations superpose in the raw data
4 Hz
8 Hz
15 Hz
21 Hz
4 Hz + 8 Hz + 15 Hz + 21 Hz =
How can we visualize these oscillations?
• The amount of energy at any frequency is expressed as % power change relative to pre-stimulus baseline
• Power can change over time
Freq
uenc
y
Time0
(onset)+200 +400
4 Hz
8 Hz
16 Hz
24 Hz
48 Hz
% changeFromPre-stimulus
+600
Where in the brain are these oscillations coming from?
• We can select and collapse any time/frequency window and plot relative power across all sensors
Win Lose
Where in the brain are these oscillations coming from?
• Can we do better than 2D plots on a flattened head?
• As in ERP analysis we (often) want to know what cortical structures might have generated the signal of interest
• One approach to finding those signal sources is Beamformer
Beamforming• Beamforming is a signal processing technique used in a variety of
applications:– Sonar– Radar– Radio telescopes– Cellular transmision
Beamforming in EEG/MEG
• It then adjusts the signal recorded at each sensor to tune the sensor array to each voxel in turn
Q = % signal change over baseline
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
• 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?
Brain Electrical Source Analysis
• Source Analysis models neural activity as one or more equivalent current dipoles inside a head-shaped volume with some set of electrical characteristics
Brain Electrical Source Analysis
This is most likely location of dipole
Project “Forward Solution”
Compare to actual data