Subdural Grid Intracranial electrodes typically cannot be used in human studies It is possible to record from the cortical surface Subdural grid on surface

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?


• 1929 – first measurement of brain electrical activity from scalp electrodes (Berger, 1929)


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


• 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!


• 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)


• 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


• 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


• 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.


• 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


• 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 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?


• 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


• 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


• 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


• Simultaneous recording from many electrodes allows recording of multiple cells


• 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


• 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

Documents

Subdural Grid Intracranial electrodes typically cannot be used in human studies It is possible to record from the cortical surface Subdural grid on surface