Electrophysiology
Neurons are Electrical
• Remember that Neurons
have electrically charged
membranes
• they also rapidly
discharge and recharge
those membranes (graded
potentials and action
potentials)
• Review pgs 31 – 50 if this
isn’t familiar to you
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
• “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
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
Stimulus on
Spikes
Intracranial and “single” Unit
• Output of unit recordings is
often depicted as a “spike
train” and measured in
spikes/second
Stimulus on
• Spike rate is almost never
zero, even without sensory
input
– in visual cortex this gives
rise to “cortical grey”
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 typically cannot be used in
human studies
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
• 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
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