Introduction to the EEG methodology - Timely COST

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What you see in the EEG ─ event-related signals. ▻ Event-related potentials. ▻ Scalp topographies. ▻ Time-frequency analysis of event-related rhythms.
Introduction to the EEG technique Part 1: neural origins of the EEG Niko Busch Charité University Medicine Berlin

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 multielectrode 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:  



Potential differences:  



extracellular: Sodium (Na+) and Chloride (Cl-) intracellular: Potassium (K+) and organic anions (-)

outside

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)

inside

Generation of the action potential Resting potential

1. 

Na+ outside; K+ inside

Depolarization

2. 

Na+ influx

Action potential start

3. 

Na+ influx

Action potential stop

4. 

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!