Niko Busch Charité University Medicine Berlin

Introduction to the EEG technique

Part 1: neural origins of the EEG

The History of the EEG 18th cent. Physiologists discover elctrical

properties of living tissue (Galvani, Ohm, Faraday)

1870ies Caton records brain potentials from cortex

1929 Berger records electrical activity from the scalp

1930ies Studies of abnormal activity with epilepsy and tumors;

first single-trial ERPs

1940ies commercial EEG system with multi-electrode montages (up to 16 channels!)

1950ies differential amplifiers

1957 The toposcope (imaging of electrical brain activity)

1962 Computerized ERP analyses

1964/65 Discovery of CNV and P3

1980 digital EEG systems, source analysis, etc.

What you see in the EEG ─ spontaneous rhythms

Frequency Ranges:

Beta: 14 – 30 Hz

Alpha: 8 – 13 Hz

Theta: 5 – 7 Hz

Delta: 1 – 4 Hz

What you see in the EEG ─ epileptic activity

Seizure-related and inter-ictal activity

Can be used to localize epileptic focus

What you see in the EEG ─ event-related signals

Event-related potentials

Scalp topographies

Time-frequency analysis of event-related rhythms

Source analysis

What is electroencephalography (EEG)?

“It is generally accepted that the EEG reflects activity originating in the brain” (Coles & Rugg, 1995, Electrophysiology of Mind)

EEG reflects voltages generated (mostly) by excitatory postsynaptic potentials from apical dendrites of massively synchronised neocortical pyramidal cells.

A few electrical concepts Voltage

the potential of current to flow from one point to another.

think of it as “water pressure”.

this is a relative measure!

Current

number of charged particles (electrons, ions) that flow in a given time.

think of it as “water flow”.

Resistance

resistance to movement of charges

like having a skinny or blocked hose segment

Ohm’s Law: Voltage = Current * Resistance

The neuron

Signal transmission:

chemical between neurons at the synapse

electrical within neuron

The neuron’s resting potential

Ion concentrations: extracellular: Sodium (Na+) and Chloride

(Cl-)

intracellular: Potassium (K+) and organic anions (-)

Potential differences: extracellular excess of positive charges

polarisation

resting potential ~ 80 mV

Forces: Diffusion to areas of low concentration

Electrostatics: negative and positive attract

Membrane permeability

Sodium–Potassium–pump (Na+ out, K+ in)

outside

inside

Generation of the action potential

1. Resting potential

Na+ outside; K+ inside

2. Depolarization

Na+ influx

3. Action potential start

Na+ influx

4. Action potential stop

K+ outflux

The postsynaptic potential

Neurotransmitters open ion channels

Sodium (Na+) influx

Depolarisation

Local reduction of Na+ concentration

Relative negative charge

Current inflow at synapse current sink

Current outflow at soma current source

Source and sink are poles of a dipole.

Postsynaptic potentials and the scalp EEG

EPSP at apical dendrites negative EEG polarity on the scalp relative to electrically neutral reference.

EEG voltages are potential differences: there is no EEG at a single location.

scalp electrode (-)

neutral reference electrode (+)

Summation of signals

A single neural event is too small to be detected on the scalp.

Action potentials do not sum up – too short.

EPSPs/IPSPs sum up in time through synchronisation,

and in space due to cortical architecture (closed electrical fields).

Closed fields in glial cells and subcortical structures no EEG.

Interim summary

What EEG measures:

Excitatory and inhibitory PSP at apical dendrites of many synchronised cortical neurons.

What EEG does not measure:

Single neurons

Asynchronous activity

Glial cells

Subcortical structures

From dipoles to sources I

EEG generators are electrical dipoles.

Many tiny dipoles result in an equivalent current dipole.

The dipole results in a topography at the scalp.

From dipoles to sources II

Scalp topography ≠ source.

Distance, volume conduction, dipole orientation, superposition of sources.

Radial dipole: source is under topography maximum.

Two or more dipoles: source is somewhere else.

Tangential dipole: source is where topography reverses.

Dipole simulator – Download from www.besa.com

The inverse problem

Any dipole produces a certain scalp topography (forward problem).

Any topography could have been produced by an infinite number of possible sources (inverse problem).

Be very careful to infer EEG sources from EEG topographies!

Recommended literature

Collura: History and evolution of electroencephalographic instruments and techniques. J Clin Neurophysiol. 1993

Luck: Introduction to the event-related potential technique. MIT Press. 2005

Niedermeyer & Lopes da Silva: Electroencephalography: basic principles, clinical applications, and related fields. Lippincott Williams & Wilkins. 2005

Thank you very much for your attention!