What does EEG actually measure? The EEG What does EEG actually measure?

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

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

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

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

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

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

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! Duracell

The “inverse solution” EEG research at its essence is the process of running those steps in reverse Duracell

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

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

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

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

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”

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

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