1. What does EEG actually measure?
The EEG
What does EEG actually measure?

2. Neurons are Electrical
Remember that Neurons have electrically charged membranes
they also rapidly discharge and recharge those membranes (graded potentials and action potentials)

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

4. Whenever you build up charge in one place it tries to go somewhere else
Voltage is a measure of the force with which charge tries to move

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

6. Electroencephalography
Cell bodies and apical dendrites tend to be pointed in the same direction
Perpendicular to the surface of the cortex

7. 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)
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8. The “forward solution”
If we knew which parts of the brain were electrically active at a given time…
And if we knew the orientation of the cortex at those points…
And if we knew a few other parameters about the skull and scalp…
We could figure out what the voltage would be at each point on the scalp!
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9. The “inverse solution”
EEG research at its essence is the process of running those steps in reverse
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10. The “inverse solution”
The first couple weeks of class will be spent running EEG “backwards” using the BESA dipole simulator
We’ll make a hypothetical data set
Then we’ll analyze it as if it were real
Then we’ll collect real data and see how well they match
Don’t get your hopes up…they won’t match all that well

11. Electricity made inaccurately simple:
No such thing as voltage “at” a point
Battery analogy – you can’t record voltage from a single end
You always need a second point which we usually call a “ground”
Why not use “ground” as ground?
Potentially really dangerous!
Totally unnecessary

12. Why not use “ground” as ground?
Potentially really dangerous!
Voltage would fluctuate wildly due to many other factors

13. A simple virtual ground circuit
Use some other part of the head
Basically that makes a really good receive antennae – picks up EVERYTHING!
Noisy!
Picks up radio frequency noise from many sources (outside the head

14. A differential circuit
Also called “common-mode rejection”
Also called “balanced line”
Active electrode =
V – 0 = V
Notice what happens when external RF noise comes in – it gets “rejected” by the circuit
(V + noise) – (0 + noise)
= V – 0
= V
Active – ground = V
Reference – ground = “zero”

15. Common mode rejection works as long as the electrodes have the same electrical characteristics
The most important of these is impedance
We must ensure that the impedance of each electrode is low and consistent with the others

16. Choosing reference site
Ideally it would be independent of brain electrical activity but that doesn’t really exist
Instead we’ll compute a “virtual” reference that is the average of each of the electrodes