PRINCIPLES OF ANALOGUE AND DIGITAL EEG

1. PRICNCIPLES OF ANALOGUE AND DIGITAL EEG
Dr Danish Ahammed P K

2. Introduction
• EEG Records electrical activities from cortex predominatly from cerebral
convexities.
• They record Extracellular Field Potentials of EPSP and IPSP generated at
apical dendrites of pyramidal cells in cerebral cortex.

3. Conduction of nerve impulse

4. Membrane Potential Changes in Impulse Conduction

5. Summation Of Potential

6. Why synaptic potentials are used an not membrane potential?
• Action potentials within neurons are brief and small hence cannot be used in
EEG.
• Synaptic potentials by Neurotransmitters and their subsequent summation
over large areas are slow and of sufficient strength to be recorded through
EEG.
• Electrial activity from atleast 6 cm2 of brain area is requird to be detected on
scalp EEG.

7. Pyramidal cells are arranged in vertical coloumns at surface of cortex.
EPSPs are present at the surface of cortex whereas IPSPs are present more proximally towards the cell
body.
• EPSP at distal part of dendrite -
Positive ions inflow to neuron --
> negative extracellular charge
• Passive flow of positive ions out
into extracellular field at
proximal part
• Resulatant current causes a
positive deflection on EEG.
• This process is vice versa in
IPSP causing negative
deflection

9. • Electrical activity from brain spreads out into
the scalp not in a focused manner , hence a
Prominent wave form in one lectrode in eeg is
an artifact unless proven otherwise

10. • Rythmicity of EEG activity is regulated by thalamus.
• Thalamus acts as a pacemaker for cortical rythms like alpha activity & sleep
spindles.
• Thalamic and cortical signals via feedback inhibition produce Rythmic and
synchronous post synaptic potentials during wakefullness and sleep.
• Input from Reticular activating system produce desynchronisation of these
synaptic potential and attenuation of EEG activity.
• High freequency activities like beta and gamma discharges are generated
within the cortex itself.
• Short lived interactions between interneurons and pyramidal cells produce
ultra fast oscillations such as ripples.

12. • For artifact free recording impedance between skin and electrode should be
below 5 Kohm.
• This is achieved by cleansing the skin with an abrasive paste to remove oil
and skin debris.
• An adhesive paste of sodium chloride solution is used to attach electrodes to
skin

13. EEG Amplifier.
• Cortical neuronal electrical potentials are
very small in the range of microvolts and
are further attenuated by intervening scalp
tissue hence requires amplification.
• Common mode rejection - Amplifier
removes commmon signals like external
signals and power line potentials.

14. • Gain = Output signal / Input signal. [ measured in Decibels] (100- 100000)
• Common Mode Rejection ratio = Gain of differential signal / Gain of common
signal. ( 100 db atleast)

15. • Analogue Filters - To reduce interference from unwanted signals.
• Low frequency filter (LFF) - Removes low frequency biological signals suc h
as those arising from respiration.
• High freequency filter - Removes high freequency filters such as myogenic
artifacts.
• Notch filter - Reduces narrow band of 50 hz

16. Analogue To Digital Converter (ADC)
• Converts physical analogue signals to digital signals.
• Continous analog signals is sampled at a fixed rate , data stored as discrete
mathematical values subsequently converted to analog signals to be
displayed on screen.
• Sampling rate should be twice as that of highest freequency to be recorded.
• As useful EEG signal is usually < 10 hz a sampling rate of 240 hz is used in
mostly availiable EEG machines.

17. • Aliasing - If the sampling rate is lower , freequencies higher than half the
sampling rate maybe erroneously displayed aslower freequencies.

18. • Resolution of ADC - Determines the smallest amplitude that can be sampled.
• Usually 12 - 16 bit that can sampl upto 0.5 microvolt.
• sampling skew - Time lags in the same waveforms in different channels when
multiple channels are sampled sequentially instead of simultaneously
• Burst Mode Sampling - Time between sampling of ifferent channels is
significiantly reduced to avoid sampling skew
• Recorded waveforms are further displayed on computer screen resolution of
which woul also affect accuracy of EEG waveforms.