Electrophysiology

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Electrophysiology
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Neurons are Electrical

• Remember that Neurons have electrically charged
  membranes
• they also rapidly discharge and recharge those
  membranes (graded potentials and action
  potentials)

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Neurons are Electrical

• Importantly, we think the electrical signals are
  fundamental to brain function, so it makes sense
  that we should try to directly measure these
  signals
• but how?

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Intracranial and single Unit

• Single or multiple electrodes are inserted into
  the brain
• may be left in place for long periods

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Intracranial and single Unit

• Single electrodes may pick up action potentials
  from a single cell
• An electrode may pick up the signals from several
  nearby cells
• spike-sorting attempts to isolate individual
  cells

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Intracranial and single Unit

• Simultaneous recording from several electrodes
  allows recording of multiple cells

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Intracranial and single Unit

• Output of unit recordings is often depicted as a
  spike train and measured in spikes/second

Stimulus on
Spikes
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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
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Intracranial and single Unit

• By carefully associating changes in spike rate
  with sensory stimuli or cognitive task, one can
  map the functional circuitry of one or more brain
  regions

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Intracranial and single Unit

• Some complications
• Suppose we observe an increase in spike rate in
  two discrete regions of the brain in response to
  a sensory stimulus What are the possible
  interpretations?

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Intracranial and single Unit

• Some complications
• Suppose we observe an increase in spike rate in
  two discrete regions of the brain in response to
  a sensory stimulus What are the possible
  interpretations?
• Area A drives area B
• Area B drives area A
• Area A and B are controlled by a third area
  independently

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Intracranial and single Unit

• Some complications
• Suppose we observe an increase in spike rate in
  two discrete regions of the brain in response to
  a sensory stimulus What are the possible
  interpretations?
• Area A drives area B
• Area B drives area A
• Area A and B are controlled by a third area
  independently and their activity is unrelated

How might you differentiate these possibilities
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Intracranial and single Unit

• How might you differentiate these possibilities
• Timing of spikes might help
• if A and B are synchronized they are probably
  functionally related
• if A leads B then it is likely to be the first in
  the signal chain

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Subdural Grid

• Intracranial electrodes cannot be used in human
  studies

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Subdural Grid

• Intracranial electrodes cannot be used in human
  studies
• It is possible to record from the cortical surface

Subdural grid on surface of Human cortex
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Electroencephalography

• It is also possible to record from outside the
  skull altogether!

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Electroencephalography

• pyramidal cells span layers of cortex and have
  parallel cell bodies
• their combined extracellular field is small but
  measurable at the scalp!

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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)

Duracell
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Electroencephalography

• Electrical potential is usually measured at many
  sites on the head surface

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Electroencephalography

• Electrical potential is usually measured at many
  sites on the head surface
• More is sometimes better

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Electroencephalography

• EEG changes with various states and in response
  to stimuli

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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.

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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

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The Event-Related Potential (ERP)

• We have an ERP waveform for every electrode

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The Event-Related Potential (ERP)

• We have an ERP waveform for every electrode

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The Event-Related Potential (ERP)

• We have an ERP waveform for every electrode
• Sometimes that isnt very useful

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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 surface
• isopotential map

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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 surface
• isopotential map

Sometimes that isnt very useful - we want to
know the generator source in 3D
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Brain Electrical Source Analysis

• Given this pattern on the scalp, can you guess
  where the current generator was?

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Brain Electrical Source Analysis

• Given this pattern on the scalp, can you guess
  where the current generator was?

Duracell
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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

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Brain Electrical Source Analysis
Initiate the model
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Brain Electrical Source Analysis
Project Forward Solution
Initiate the model
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Brain Electrical Source Analysis
Project Forward Solution
Initiate the model
Compare to actual data
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Brain Electrical Source Analysis
Project Forward Solution
Adjust the model
Compare to actual data
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Brain Electrical Source Analysis
Project Forward Solution
This is most likely location of dipole
Compare to actual data
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Brain Electrical Source Analysis

• EEG data can now be coregistered with
  high-resolution MRI image

Anatomical MRI
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Brain Electrical Source Analysis

• EEG data can now be coregistered with
  high-resolution MRI image

3D volume is rendered and electrode locations are
superimposed
Anatomical MRI
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Brain Electrical Source Analysis

• EEG data can now be coregistered with
  high-resolution MRI image

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Magnetoencephalography

• For any electric current, there is an associated
  magnetic field

Electric Current
Magnetic Field
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Magnetoencephalography

• For any electric current, there is an associated
  magnetic field
• magnetic sensors called SQuIDs can measure very
  small fields associated with current flowing
  through extracellular space

Electric Current
Magnetic Field
SquID
Amplifier
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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