A review of eeg recording techniques

International INTERNATIONALCommunication Engineering & Technology (IJECET), ISSN 0976 –
Journal of Electronics and JOURNAL OF ELECTRONICS AND
6464(Print), ISSN 0976 – 6472(Online) Volume 3, Issue 3, October- December (2012), © IAEME
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ISSN 0976 – 6464(Print)
ISSN 0976 – 6472(Online)
Volume 3, Issue 3, October- December (2012), pp. 177-186
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A REVIEW OF EEG RECORDING TECHNIQUES
Imteyaz Ahmad1, F Ansari2, U.K. Dey3
1
Dept 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 author-imtiazahmadbitsindri@gmail.com

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

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

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

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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
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6464(Print), ISSN 0976 – 6472(Online) Volume 3, Issue 3, October December (2012), © IAEME
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 to cranial surface later idea reflects
stimulation associated by diverse neuron .
Modern interpretation of EEG origin rest with knowledge of basic neuronal electrochemical
process. The Action potential (AP) from neuron has been recorded with microelectrodes at the
cellular level. Essentially the synaptic fibers, terminal boutons ,neuronal membrane and axon
contribute the distinguishable response characteristic .Electrical reaction of neurons includes the
following potential.
g
1. Presynnaptic spike potential (rapid ms positive event resulting from presynaptic
(rapid-ms
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.
ke
4. After hyperpolarisation (prolong positive potential)
5. Inhibitory post synaptic potential(IPSP)

10-20 EEG Electrode Placement system
20
The amplitude, phase, and frequency of EEG signals depend on electrode placement. This
, signals
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
20
Federation of EEG Societies usually employed to record spontaneous EEG. The Head is mapped
EEG.
by four standard points: the nasion , the inions, and the left and right ears. Here 21 electrodes are
located on the surface of scalp. The position are determined as follows: Reference points are
. poi
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 bback of the head. From these points, the skull
perimeters are measured in the transverse and media planes. Electrode are placed by measuring
median
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
mid-point as shown in fig below.

Figure 3:10-20 EEG Electrode Placement system
20

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

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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).
EEGin =Zin of amplifier x EEG voltage source/Zin of amplifier + R of electrodes (2-4)
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).
Adiff = 2 R6/R5 (2-5)

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Fig 6 Differential input, Single-ended output amplifier
The gain (shown in the circuit diagram of Figure 5 is :
Adiff = 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

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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

184


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.
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A review of eeg recording techniques

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