Subdural Grid Intracranial electrodes typically cannot be used in human studies It is possible to...
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Transcript of Subdural Grid Intracranial electrodes typically cannot be used in human studies It is possible to...
Subdural Grid
• Intracranial electrodes typically cannot be used in human studies
• It is possible to record from the cortical surface
Subdural grid on surface of Human cortex
Electroencephalography and the Event-Related Potential
• Could you measure these electric fields without inserting electrodes through the skull?
Electroencephalography and the Event-Related Potential
• 1929 – first measurement of brain electrical activity from scalp electrodes (Berger, 1929)
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 and the Event-Related Potential
• 1929 – first measurement of brain electrical activity from scalp electrodes (Berger, 1929)
– Initially believed to be artifactual and/or of no significance
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
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
• EEG/MEG changes with various states and in response to stimuli
Electroencephalogram
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
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 high-resolution MRI image
Source ImagingResult
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