A review of eeg recording techniques

International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 –

6464(Print), ISSN 0976 – 6472(Online) Volume 3, Issue 3, October- December (2012), © IAEME

177

A REVIEW OF EEG RECORDING TECHNIQUES

Imteyaz Ahmad1, F Ansari

2, U.K. Dey

3

1Dept of ECE ,

2Dept of Electrical Engg. ,

3Dept of Mining Engg.

BIT Sindri, Dhanbad, Jharkhand 1 Corresponding Author Tel.-+91-0326-2245671 Fax no.-+91-0326-2350729

Email address for correspondence [email protected]

ABSTRACT

EEG is a collaborative tool to diagnosis brain function and disease. Modern interpretation of

EEG origin rest with knowledge of basic neuronal electrochemical process. The EEG is

composed of electrical rhythms and transient discharges which are distinguished by location,

frequency, amplitude, form, periodicity and functional properties. The low-level EEG Signal

from the brain as recorded is amplified and converted to a digital signal for further processing.

EEG machines have a notch filter sharply tuned at 50Hz so as to eliminate main frequency

interference. Reading has been taken in international 10-20 Electrode system for a normal

person. EEG signal voltage amplitudes ranges 1 to 100 µv peak to peak at low frequencies(1

to 50 Hz) at the surface of scalp. In biofeedback training the ratio of amplitude of alpha to

theta waves need to be increased.

Keyword: Electroencephalogram(EEG), biofeedback, notch filter.

INTRODUCTION

The human brain contains approximately 100 billion nerve cells called neurons. Neurons have

the amazing ability to gather and transmit electrochemical signals. The Neurons have 3 basic

parts, a cell body which has the necessary cells components, Axon which is like a long cable to

carry nerve impulse and finally the Dendrites which is the nerve ending branches that connects to

other cells to allow electrical transfers between cells. The generation of EEG potentials requires

a neural source close to the inside surface of the skull that is coherent, which means all the

neurons must be aligned similarly and act together electrically. Pyramidal cells in the center of

the cerebral cortex are the major source of EEG potentials. The dendrites will receive excitatory

or inhibitory inputs from surrounding neurons and axons. When the dendrites receive an impulse

or input by an ion such as Na+ enters them (become active), current flows into and out of these

dendritic processes and the cell body. The cell to dendrite relationship is therefore one of a

INTERNATIONAL JOURNAL OF ELECTRONICS AND

COMMUNICATION ENGINEERING & TECHNOLOGY (IJECET)

ISSN 0976 – 6464(Print)

ISSN 0976 – 6472(Online)

Volume 3, Issue 3, October- December (2012), pp. 177-186

© IAEME: www.iaeme.com/ijecet.asp

Journal Impact Factor (2012): 3.5930 (Calculated by GISI)

www.jifactor.com

IJECET

© I A E M E



178

constantly shifting current dipole, and variations in orientation and strength of the dipole produce

wave like fluctuations in a volume conductor. When the sum of electrical activity is negative

relative to cell, the cell is depolarized and quite excitable. When it is positive, the cell is

hyperpolarized and less excitable. EEG potentials on the scalp are usually no more than 150uV

peak to peak. The brain frequencies depended on the degree of activity of the cerebral cortex. For

example, the waves change between states of wakefulness and sleep. Much of the time, the brain

waves are irregular and no general pattern can be observed. Yet at other times, distinct patterns

do occur. Some of these are characterized to be abnormalities of the brain such as epilepsy.

Generally there are four wave groups (alpha, beta, theta, and delta). The EEG rhythm and

waveforms are varied by the position of electrode placements on certain parts of the brain (fig.1).

Alpha wave occurs at a frequency between 7.5 and 13Hz. The alpha waves are produced when a

person is in a conscious, relaxed state with eyes closed; the activity is suppressed when the eyes

are open. The amplitude of the alpha rhythm is largest and intensely occurs in the occipital

region and can be best recorded at parietal and frontal regions of the scalp. Beta waves normally

occur in the frequency range of 14-30Hz and sometimes even as high as 50Hz for intense

activity. Beta waves activities are present when people are alert or anxious, with their eyes open.

Theta potentials are large amplitude, low frequency between 3.5 and 7.5Hz waves. Theta is

abnormal in alert adults but seen during sleep, and small children. Theta waves occur mainly in

the parietal and temporal region. Delta waves have the largest amplitudes and the lowest

frequency in less than 3.5Hz. It is normal rhythm for infants less than one year old and in adults

in deep sleep. This wave can thus occur solely within the cortex, independent of the activities in

lower regions of the brain.

Electroencephalography (EEG) is the measurement of electrical activity produced by the brain as

recorded from electrodes placed on the scalp. Just as the activity in a computer can be

understood on multiple levels from the activity of individual transistors to the function of

applications, so can the electrical activity of the brain be described on the relatively small to

relatively large scales. At one end are action potentials in a single axon or currents with a single

dendrite of a single neurons at the other end is the activity measured by the EEG which

aggregates the electric voltage field from million of neurons .so called scalp EEG is collected

from tens to hundreds of electrodes positioned on different location at the surface of the head

EEG signal ( in the range of milli volts) are amplified and digitalized for later processing. The

data measured by the scalp EEG are used for clinical and research purposes. In neurology the

main diagnostic application of EEG is for epilepsy by the this technique is also used to

investigate many other pathologies such as sleep related disordered ,sensory deficits, brain

tumors, etc. In cognitive neuroscience, EEG is used to investigate the neural correlates of mental



179

activity from low level perceptual and motor processes to higher order cognition (attention,

memory, reading etc.) EEG electrodes transfer ionic currents from cerebral tissue into electrical

currents used in EEG preamplifiers. The electrical characteristic are determined primarily by the

type of metal used. silver-silver chloride (Ag-Ag cl) is commonly found in electrode discs.

Five types of electrode are used:

1.Sclap- silver pads, discs, or cup; stainless steel rods; and chlorides silver wires.

2. Sphenoidal- alternating insulated silver and bare wire and chloride tip inserted through muscle

tissue by a needle.

3. Nasopharyngeal- silver rod with ball at the tip inserted through the nostrils.

4.Eltrocorticographic- cotton wicks soaked in saline solution that rest on the brain surface.

5. Intercerebral- sheaves of Teflon-coated gold or platinum wire cut a various distance from the

sheaf tip and used to electrically stimulate the brain.

BASIC RECORDING SYSTEM

Referential: The potential difference is measured between an active electrode and an inactive

reference electrode

Figure 2 (a) Referential method

Bipolar: The potential difference is measured between two active electrode

Figure 2(b) Bipolar method

EEG is a representation of the electrical activity of the brain. .The technique involve following:

1.Biopotential peak up- Cranial or cerebral surface transducer electrodes

2.EEG signal conditioning-Transducer output amplification and filtering (.1 to 100Hz)

3.EEG signal recording-Signal displayed on graphic recorder or CRT

4. EEG Signal analysis- visual or computer interpretation of resting EEG

EEG is a collaborative tool is diagnosis brain function and disease. Many Physician and

neurologist view EEG signal as interest artifacts but confess that then are not certain of the signal

International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976

6464(Print), ISSN 0976 – 6472(Online) Volume 3, Issue 3, October

origins. In fact ,until recently EEG waveform were originally thought to be a summation of

action potential of neurons as they made their was

stimulation associated by diverse neuron

Modern interpretation of EEG origin rest with knowledge of basic neuronal electrochemical

process. The Action potential (AP) from

cellular level. Essentially the synaptic fibers,

contribute the distinguishable response characteristic .Electrical reaction of neurons includes the

following potential.

1. Presynnaptic spike potential (rapid

depolarization)

2. Excitatory postsynaptic potential (EPSP) (Prolonged 2ms graded positive potential)

3. Spike potential (High voltage ,sudden 2 ms positive discharge of 10 to 30 mv.

4. After hyperpolarisation (prolong positive potential)

5. Inhibitory post synaptic potential(IPSP)

10-20 EEG Electrode Placement system

The amplitude, phase, and frequency of EEG sig

placement is based on the frontal, parietal, temporal, and occipital cranial area. One of the most

popular schemes is the 10-20 EEG electrode placement system established by the international

Federation of EEG Societies usually employed to record spontaneous EEG

by four standard points: the nasion , the

located on the surface of scalp.

nasion, which is the delve at the top of the nose, level with the eyes, and inions, which is bony

lump at the base of the skull on the midline at the b

perimeters are measured in the transverse and media

the nasion-inions distance are marking points on the head 10%, 20%, 20%, 20%, 20%, 10% of

this length. The vertex, c2 electrode is the mid

Figure 3:10-20 EEG Electrode Placement system

International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976

6472(Online) Volume 3, Issue 3, October- December (2012), © IAEME

180


action potential of neurons as they made their was to cranial surface later idea reflects

stimulation associated by diverse neuron .


process. The Action potential (AP) from neuron has been recorded with microelectrodes at the

Essentially the synaptic fibers, terminal boutons ,neuronal membrane and axon


g potential.

Presynnaptic spike potential (rapid-ms positive event resulting from presynaptic

Excitatory postsynaptic potential (EPSP) (Prolonged 2ms graded positive potential)

ke potential (High voltage ,sudden 2 ms positive discharge of 10 to 30 mv.

After hyperpolarisation (prolong positive potential)

Inhibitory post synaptic potential(IPSP)

20 EEG Electrode Placement system

, and frequency of EEG signals depend on electrode placement. This


20 EEG electrode placement system established by the international

usually employed to record spontaneous EEG. The Head is mapped

by four standard points: the nasion , the inions, and the left and right ears. Here

. The position are determined as follows: Reference poi


lump at the base of the skull on the midline at the back of the head. From these points, the skull

perimeters are measured in the transverse and median planes. Electrode are placed by measuring

distance are marking points on the head 10%, 20%, 20%, 20%, 20%, 10% of

electrode is the mid-point as shown in fig below.

20 EEG Electrode Placement system


December (2012), © IAEME


to cranial surface later idea reflects


neuron has been recorded with microelectrodes at the

,neuronal membrane and axon


ms positive event resulting from presynaptic

Excitatory postsynaptic potential (EPSP) (Prolonged 2ms graded positive potential)

ke potential (High voltage ,sudden 2 ms positive discharge of 10 to 30 mv.

nals depend on electrode placement. This


20 EEG electrode placement system established by the international

. The Head is mapped

Here 21 electrodes are

The position are determined as follows: Reference points are


From these points, the skull

Electrode are placed by measuring

distance are marking points on the head 10%, 20%, 20%, 20%, 20%, 10% of



181

Signal Acquisition

Electrode arrangements may be either unipolar or bipolar. A unipolar arrangements is composed

of a number of scalp leads connected to a common indifference point such as earlobe. Hence

one electrode is common to all channels. A bipolar arrangements is achieved by the

interconnection of scalp electrodes.

ACTUAL CIRCUIT DIAGRAMS

The differential amplifier (as shown in Figure 4) is used where a difference in potential has to be

amplified in the presence of an interfering common-mode voltage. Such interference frequently

comes from 50 Hz power line fields, man-made electrical noise, or radiation from other electrical

equipment.

Figure 4 shows the basic circuit arrangement for a differential amplifier.

If an out-of-phase (differential) voltage is applied to the inputs of amplifiers A and B, the current

flow in A will increase while the current in B will decrease. If both amplifiers and associated

parts are identical, the amplifier currents i1 and i2 will be equal and opposite, the net current will

be zero and no voltage drop will occur across resistor RC. Under these conditions the output

signal is a function of twice the gain of one amplifier.

If both signal inputs are in phase, both (+) or (–), this is called the common-mode signal. In the

case of such signals from the pickup power lines, both i1 and i2 will increase or decrease

simultaneously, causing a voltage drop on RC. This common-mode voltage results in

degeneration (voltage drop on RC) and a reduction in amplifier gain.

Of key importance is the ratio of the [differential signal gain (Adiff)] to the [common-mode gain

(Acm)]. This relationship is called the common-mode rejection ratio (CMRR) and is expressed by

equation (2-1)

CMRR = 20LOG10 Adiff /Acm (2-1)

Where the CMRR is expressed in decibels.

The CMRR shows the ability of a differential amplifier to attenuate common-mode signals

appearing simultaneously with differential signals. CMRR is a ratio of two gains, as shows in

equation (2-2)

(Voutdiff/Vindiff)/(VOut/Vincm)

In determining the CMRR, the two signals (common-mode and differential) are adjusted at the

input to produce the same output voltage. For equation (2-2), Voutcm–Voutdiff.

Therefore, CMRR can be computed from the input voltages as shows in equation (2-3).

CMRR = 20 Log10 (2-2)

CMRR = 20 Log10 Vindiff/Vincm (2-3).



182

The CMRR is then +65 dB. Good amplifiers have CMRR values which range from +60 dB to

+100 dB. When two or more such amplifiers are cascaded, the CMRR of each can be add.

Fig 5 One Quarter of a TL074 IC (LF347)

The transistors Q1-Q2 are the input differential pair, while transistors Q3, Q5 and Q6 make the

common emitter resistor (RE). CMRR is 80 to 86 dB, and the bandwidth (open loop) is 3 MHz.

The input impedance is high, about 1012

ohms and the output impedance is low under 100 ohms.

The high-input impedance of the amplifier is a necessary design element, since the amplifier

could easily load down the signal source.

The adverse effect of electrode contact resistance on signal input is an additional consideration.

Losses incurred at the lead contacts with the skin decrease the available input signal to each

amplifier. In the circuit shown in Figure 6, the electrode contact resistors (R3, R4) are in series.

High input impedance of the amplifier, however, minimizes the signal loss.

The input signal to an amplifier from an EEG voltage is dependent both on the amplifier’s input

impedance and on the resistance of the electrodes placed on the Patient’s body. The input voltage

depends on the losses at the lead contacts. This is shown by equation (2-4).

=Zin of amplifier x EEG voltage source/Zin of amplifier + R of electrodes

It can be shown using equation (2-4) that only 5-10% of the EEG signal is lost. Under good

conditions, a loss of less than one percent is possible. Since the TL074 has an input of over 1012

ohms, only a very small portion of the input signal voltage would be lost.

Since the TL074 is a quad, three of the amplifiers can be used for recording the ECG, EMG, or

EEG signal voltage. Two amplifiers are used for the differential input, and one for a single-ended

output amplifier. A typical circuit arrangement is shown in Figure 5.The gain of the inverting

amplifier A4 is determined by the ratio of the R1/R8 (100KΩ/8.2KΩ) resistors (A = 12). The

gain of the differential amplifier is determined from equation (2-5).

R6/R5

EEGin (2-4)

Adiff = 2 (2-5)



183

Fig 6 Differential input, Single-ended output amplifier

The gain (shown in the circuit diagram of Figure 5 is :

2 x 100 KΩ/ 6.8KΩ

Adiff = 29

The gain of the input stage can be controlled by varying the 10 KΩ resistor (R5) between pins 6

and 13. As the resistor is made smaller, the gain is increased. This resistor should not be zero

since the circuit might oscillate under high gain conditions. In the circuit shown, resistors of 1%

tolerance (or less) should be used, since both halves of the circuit must match. The value of R10

should be adjusted to obtain the best balance of the two signals input to A4. This balance

achieves the highest possible CMRR.

PATIENT LEAD SAFETY

In medical instrumentation, patient safety is a major consideration. If patient is totally isolated

from ground, the potential shock hazard is greatly reduced. Figure 7 shows how, by using an

optocoupler, a differential instrumentation amplifier can be isolated from the circuits that follow,

as well as from the earth ground.The optocoupler contains an infrared LED and a sensor. The

sensor can be a photodiode, phototransistor, or photodarlington. The LED-to-sensor insulation

breakdown voltage ranges from 1500 volts to over 20,000 volts, depending on the device used.

Fig 7: Floating differential amplifier

Adiff =



184

Observation

Each electrode site was rubbed with alcohol, electrode paste and electrodes firmly attached. For

each pressing of amplitude variation switch, there will be waveform generated for earlobe,

central, parietal, frontal, occipital points. The high-frequency roll-off was controlled by leaving

switches A1 and A2 open or closed, and the low end was rolled off at approximately 3 Hz by

opening A4. For a one-volt amplifier output, a system gains of 5,000 to 50,000 (75-95 dB) was

required. Digital oscilloscope was used to record data from EEG recorder. The output was seen

from TP15 on the oscilloscope and also seen the waveforms through DSO on the PC by

connecting the output (TP-15) to Ch1/Ch2 through parallel port. The output (TP15) of the

experimental panel can be seen on the Digital storage oscilloscope (DSO). Keep the DIP

switches SWA and SWB in kit as follows:

The various Practical waveform at different points are as shown below:

1. PointsF3-C3 4. PointsP3-O1

2.Points O1-O2 5.Points A2-O2

3.Points F4-C4 6.Points C4- P4



185

The various ideal waveforms at different points are shown below:

CONCLUSION

The EEG gives a coarse view of neural activity and can be used to non-invasively study cognitive

processes and the physiology of the brain. A typical EEG signal ranges from 1 to 100 µv peak to peak at

low frequencies(1 to 50 Hz) at the surface of scalp. Modern EEG machines are PC Based. EEG signal is

usually recorded by reusable scalp disc or cup electrodes . The EEG is composed of electrical rhythms

and transient discharges which are distinguished by location, frequency, amplitude, form, periodicity and

functional properties. The low-level EEG Signal from the brain as recorded is amplified and converted to

a digital signal for further processing. EEG machines have a notch filter sharply tuned at 50Hz so as to

eliminate main frequency interference Reading has been taken in international 10-20 Electrode system

for a normal person. EEG signal contain a mixture of signals

1) Evoked potential(EP) and event related potential(ERP)components associated with the task

monitored.

2) 50 Hz main interference and electrophysiological signals such as eye movements, blinks and

muscle activity.

REFERENCES [1] P. Loiseau, “Epilepsies,” in Guide to Clinical Neurology. New York:Churchill Livingstone, 1995, pp.

903–914.

[2] G. Dumermuth, “Possibilities of electronic EEG processing in epileptology,”in Epileptology: Proc.

7th Int. Symp. on Epilepsy, 1976. pp.365–372.

[3] J. M. Clark, “Oxygen toxicity,” in The Physiology and Medicine of Diving, 3rd ed. San Pedro, CA:

Best. 1978, pp. 200–238.



186

[4] F. H. Lopes da Silva, J. P. Pijn, and D. N. Veli, “Signal processing of EEG: evidence for chaos or

noise. An application to seizure activity inepilepsy,” in Advances in Processing and Pattern Analysis of

Biological Signals. New York: Plenum, 1996. pp. 21–32.

[5] J. R. G. Carrie, “A hybrid computer system for detecting and quantifying spike and wave EEG

patterns,” Electroencephalogr. Clin. Neurophysiol., vol. 33. pp. 339–341, 1972.

[6] E. E. Gose, S. Werner, and R. C. Bickford, “Computerized EEG spike detection,” in Proc. San Diego

Biomed. Symp., vol. 13. pp. 193–198,1974.

[7] J. R. Smith, “Automatic analysis and detection of EEG spikes,” IEEETrans. Biomed. Eng., vol. BME-

21. pp. 1–7, 1974.

[8] A. S. Gevins, C. L. Yeager, S. L. Diamond, G. M. Zeitlin, J. P. Spire, and A. H. Gevins, “Sharp-

transient analysis and threshold linear coherence spectra of paroxysmal EEG’s,” in Quantitative Analytic

Studies in Epilepsy. New York: Raven, 1976, pp. 463–481.

[9] F. H. Lopes da Silva, W. ten Broeke, K.van Hulten, and J. G. Lommen, “EEGnonstationarities

detected by inverse filtering in scalp and cortical recordings of epileptics: Statistical analysis and spatial

display,” in Quantitative Analytic Studies in Epilepsy. New York: Raven, 1976,pp. 375–387.

[10] J. Gotman and P. Gloor, “Automatic recognition and quantification of interictal epileptic activity,”

Electroenceph. Clin. Neurophysiol., vol. 4. pp. 513–529, 1976.

[11] K. M. Ma, G. G. Celesia, and W. P. Birkemeier, “Cluster analysis and spike detection in EEG,” in

Epileptology: Proc. 7th Int. Symp. On Epilepsy, 1976, pp. 386–396.

[12] W. P. Birkemeier, A. B. Fontaine, G. G. Celesia, and K. M. Ma, “Pattern recognition techniques for

the detection of epileptic transients of EEG,” IEEE Trans. Biomed. Eng., vol. BME-25, pp. 213–217,

1978.

[13] J. D. Frost, Jr., “Automatic recognition and characterization of epileptiform discharges in the human

EEG,” J. Clin. Neurophysiol., vol. 2. pp. 231–249, 1985.

[14] A. Babloyantz and A. Destexhe. “Low dimensional chaos in an instance of epilepsy,” Proc. Nat.

Acad. Sci. USA, vol. 83, pp. 3513–3517, 1986.

[15] P. E. Rapp, “Oscillations and chaos in cellular metabolism and physiological systems,” in Chaos-

Nonlinear Science: Theory and Applications.Manchester, U.K.: Manchester Univ. Press, 1986. pp. 179–

208.

[16] J. Gotman and L. Y. Wang, “State-dependent spike detection: Concepts and preliminary results,”

Electroencephalogr. Clin. Neurophysiol., vol. 79, pp. 11–19, 1991.

[17] A. P. Pjin, J. Van Neerven, N. Noestt, and F. H. Lopes Da Silva, “Chaos or noise in EEG signals:

Dependence on state and brain site,” Electroencephalorgr. Clin. Neurophysiol., vol. 79. pp. 371–381,

1991.

[18] A.C. Metting VanRijn, A. Peper, C.A. Grimbergen. Instrumentation Amplifier for bioelectric events:

a design with minimal number of parts. August, 21, 2008 Academic Medical Center, Medical

Physics Department

[19]“The Isolation Mode Rejection Ratio in Bioelectric Amplifiers.” A. C. MettingVanRijn, A. Peper,C.

A. Grimbergen. Academic Medical Center, Medical Physics Department

[20]“High Quality Recording of Bioelectric Events. II : A Low-Noise, Low-Power Multi channel

Amplifier Design.A. C. MettingVanRijn, A. Peper, C. A. Grimbergen. Academic Medical Center,

Medical Physics Department.

[21] Ranggayan, Rangaraj M. (2002). Biomedical Signal Analysis. IEEE Press. John Wiley & Sons, Inc.

pp. 28-31 [22] Reddy DC. (2005). Biomedical Signal Processing: Principles and Techniques. Mc Graw Hill pp. 128-135

[23]Northrop, Robert B. (2001). Noninvasive Instrumentation and Measurement in Medical Diagnosis. CRC Pr I

Llc. pp. 92-128

[24]Neuman, Michael R. (1998). Medical Instrumentation Application and Design. John Wiley & Sons, Inc pp.183-

233

A review of eeg recording techniques

Documents

Transcript of A review of eeg recording techniques