Electrophysiology. Neurons are Electrical Remember that Neurons have electrically charged membranes they also rapidly discharge and recharge those membranes

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


• 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


• Simultaneous recording from several electrodes allows recording of multiple cells


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

QuickTime™ and a decompressorare needed to see this picture.

Stimulus on

Spikes


• 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


• 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


• 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




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


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!


• 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


QuickTime™ and a decompressor

are needed to see this picture.




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

• More is sometimes better


• 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.


• 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 Analysis models neural activity as one or more equivalent current dipoles inside a head-shaped volume with some set of electrical characteristics


Initiate the model


Initiate the model

Project “Forward Solution”


Initiate the model


Compare to actual data


Adjust the model




This is most likely location of dipole




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

Anatomical MRI



Anatomical MRI

3D volume is rendered and electrode locations are superimposed



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

SquIDAmplifier


• 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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Electrophysiology. Neurons are Electrical Remember that Neurons have electrically charged membranes they also rapidly discharge and recharge those membranes