INTERNATIONAL JOURNAL OF ELECTRONICS AND COMMUNICATION ENGINEERING & TECHNOLOGY (IJECET)

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1 INTERNATIONAL JOURNAL OF ELECTRONICS AND COMMUNICATION ENGINEERING & TECHNOLOGY (IJECET) International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 ISSN (Print) ISSN (Online) Volume 3, Issue 3, October- December (2012), pp IAEME: Journal Impact Factor (2012): (Calculated by GISI) IJECET I A E M E A REVIEW OF EEG RECORDING TECHNIQUES Imteyaz Ahmad 1, F Ansari 2, U.K. Dey 3 1 Dept of ECE, 2 Dept of Electrical Engg., 3 Dept of Mining Engg. BIT Sindri, Dhanbad, Jharkhand 1 Corresponding Author Tel Fax no 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 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 177

2 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 178

3 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 179

4 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 diversee 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. 1. Presynnaptic spike potential (rapid-ms positive event resulting from presynaptic 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) EEG Electrode Placement system The amplitude, phase, and frequency of EEG signals depend on electrode placement. This placement is based on the frontal, parietal, temporal, and occipital cranial area. One of the most popular schemes is the EEG electrode placement system established by the international Federation of EEG Societies 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 21 electrodes are located on the surface of scalp. The position are determined as follows: Reference points are 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 back of the head. From thesee points, the skull perimeters are measured in the transverse and median planes. Electrode are placed by measuring the nasion-inions distance are marking points on the head 10%, 20%, 20%, 20%, 20%, 10% of this length. The vertex, c 2 electrode is the mid-point as shown in fig below. Figure 3:10-20 EEG Electrode Placement system 180

5 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 i 1 and i 2 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 i 1 and i 2 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 (A diff )] to the [common-mode gain (A cm )]. This relationship is called the common-mode rejection ratio (CMRR) and is expressed by equation (2-1) CMRR = 20LOG 10 A diff /A cm (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 Log 10 (Vout diff /Vin diff )/(VOut / Vin cm ) (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), Vout cm Vout diff. Therefore, CMRR can be computed from the input voltages as shows in equation (2-3). CMRR = 20 Log 10 V indiff /V incm (2-3). 181

6 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 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). EEG in =Z in of amplifier x EEG voltage source/z in 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 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). A diff = 2 R6/R5 (2-5) 182

7 Fig 6 Differential input, Single-ended output amplifier The gain (shown in the circuit diagram of Figure 5 is : A diff = 2 x 100 KΩ/ 6.8KΩ A diff = 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 183

8 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

9 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 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 [2] G. Dumermuth, Possibilities of electronic EEG processing in epileptology, in Epileptology: Proc. 7th Int. Symp. on Epilepsy, pp [3] J. M. Clark, Oxygen toxicity, in The Physiology and Medicine of Diving, 3rd ed. San Pedro, CA: Best. 1978, pp

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