Presentation on theme: "What does EEG actually measure?"— Presentation transcript:
1 What does EEG actually measure? The EEGWhat does EEG actually measure?
2 Neurons are Electrical Remember that Neurons have electrically charged membranesthey also rapidly discharge and recharge those membranes (graded potentials and action potentials)
3 Electroencephalography pyramidal cells span layers of cortex and have parallel cell bodiestheir 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 bodiestheir combined extracellular field is small but measurable at the scalp!
6 Electroencephalography Cell bodies and apical dendrites tend to be pointed in the same directionPerpendicular 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)Duracell
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!Duracell
9 The “inverse solution” EEG research at its essence is the process of running those steps in reverseDuracell
10 The “inverse solution” The first couple weeks of class will be spent running EEG “backwards” using the BESA dipole simulatorWe’ll make a hypothetical data setThen we’ll analyze it as if it were realThen we’ll collect real data and see how well they matchDon’t get your hopes up…they won’t match all that well
11 Electricity made inaccurately simple: No such thing as voltage “at” a pointBattery analogy – you can’t record voltage from a single endYou 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 headBasically 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 = VNotice what happens when external RF noise comes in – it gets “rejected” by the circuit(V + noise) – (0 + noise)= V – 0= VActive – ground = VReference – ground = “zero”
15 Common mode rejection works as long as the electrodes have the same electrical characteristics The most important of these is impedanceWe 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 existInstead we’ll compute a “virtual” reference that is the average of each of the electrodes
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