Transcript Lecture 7 Powerpoint file

Electrophysiology
Neurons are Electrical
• Remember that Neurons have electrically charged
membranes
• they also rapidly discharge and recharge those
membranes (graded potentials and action potentials)
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?
Intracranial and “single” Unit
• Single or multiple electrodes
are inserted into the brain
• 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
signals from several nearby
cells
– spike-sorting attempts to
isolate individual cells
Intracranial and “single” Unit
• Simultaneous recording from
several 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
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Stimulus on
Spikes
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”
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Stimulus on
Spikes
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
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?
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?
1. Area A “drives” area B
2. Area B “drives” area A
3. Area A and B are controlled by a third area independently
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?
1. Area A “drives” area B
2. Area B “drives” area A
3. Area A and B are controlled by a third area independently
and their activity is unrelated
How might you differentiate these possibilities
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
Subdural Grid
• Intracranial electrodes cannot be used in human
studies
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
Electroencephalography
• It is also possible to record from outside the skull
altogether!
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
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QuickTime™ and a
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QuickTime™ and a
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Electroencephalography
• Electrical potential is usually measured at many sites
on the head surface
• More is sometimes better
Electroencephalography
• EEG changes with various
states and in response to
stimuli
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?
Brain Electrical Source Analysis
• Source Analysis models
neural activity as one or
more equivalent current
dipoles inside a headshaped volume with some
set of electrical
characteristics
Brain Electrical Source Analysis
Initiate the model
Brain Electrical Source Analysis
Initiate the model
Project “Forward
Solution”
Brain Electrical Source Analysis
Initiate the model
Project “Forward
Solution”
Compare to actual data
Brain Electrical Source Analysis
Adjust the model
Project “Forward
Solution”
Compare to actual data
Brain Electrical Source Analysis
This is most likely
location of dipole
Project “Forward
Solution”
Compare to actual data
Brain Electrical Source Analysis
• EEG data can now be coregistered with highresolution MRI image
Anatomical MRI
Brain Electrical Source Analysis
• EEG data can now be coregistered with highresolution MRI image
Anatomical MRI
3D volume is rendered and
electrode locations are
superimposed
Brain Electrical Source Analysis
• EEG data can now be coregistered with highresolution MRI image
Magnetoencephalography
• For any electric current,
there is an associated
magnetic field
Electric
Current
Magnetic
Field
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
Electric
Current
Magnetic
Field
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
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