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A review of eeg recording techniques

A review of eeg recording techniques






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    A review of eeg recording techniques A review of eeg recording techniques Document Transcript

    • 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 COMMUNICATION ENGINEERING & TECHNOLOGY (IJECET)ISSN 0976 – 6464(Print)ISSN 0976 – 6472(Online)Volume 3, Issue 3, October- December (2012), pp. 177-186 IJECET© IAEME: www.iaeme.com/ijecet.aspJournal Impact Factor (2012): 3.5930 (Calculated by GISI) ©IAEMEwww.jifactor.com 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 177
    • International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 –6464(Print), ISSN 0976 – 6472(Online) Volume 3, Issue 3, October- December (2012), © IAEMEconstantly shifting current dipole, and variations in orientation and strength of the dipole producewave like fluctuations in a volume conductor. When the sum of electrical activity is negativerelative to cell, the cell is depolarized and quite excitable. When it is positive, the cell ishyperpolarized and less excitable. EEG potentials on the scalp are usually no more than 150uVpeak to peak. The brain frequencies depended on the degree of activity of the cerebral cortex. Forexample, the waves change between states of wakefulness and sleep. Much of the time, the brainwaves are irregular and no general pattern can be observed. Yet at other times, distinct patternsdo 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 andwaveforms 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 aperson is in a conscious, relaxed state with eyes closed; the activity is suppressed when the eyesare open. The amplitude of the alpha rhythm is largest and intensely occurs in the occipitalregion and can be best recorded at parietal and frontal regions of the scalp. Beta waves normallyoccur in the frequency range of 14-30Hz and sometimes even as high as 50Hz for intenseactivity. 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 isabnormal in alert adults but seen during sleep, and small children. Theta waves occur mainly inthe parietal and temporal region. Delta waves have the largest amplitudes and the lowestfrequency in less than 3.5Hz. It is normal rhythm for infants less than one year old and in adultsin deep sleep. This wave can thus occur solely within the cortex, independent of the activities inlower regions of the brain.Electroencephalography (EEG) is the measurement of electrical activity produced by the brain asrecorded from electrodes placed on the scalp. Just as the activity in a computer can beunderstood on multiple levels from the activity of individual transistors to the function ofapplications, so can the electrical activity of the brain be described on the relatively small torelatively large scales. At one end are action potentials in a single axon or currents with a singledendrite of a single neurons at the other end is the activity measured by the EEG whichaggregates the electric voltage field from million of neurons .so called scalp EEG is collectedfrom tens to hundreds of electrodes positioned on different location at the surface of the headEEG signal ( in the range of milli volts) are amplified and digitalized for later processing. Thedata measured by the scalp EEG are used for clinical and research purposes. In neurology themain diagnostic application of EEG is for epilepsy by the this technique is also used toinvestigate many other pathologies such as sleep related disordered ,sensory deficits, braintumors, etc. In cognitive neuroscience, EEG is used to investigate the neural correlates of mental 178
    • International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 –6464(Print), ISSN 0976 – 6472(Online) Volume 3, Issue 3, October- December (2012), © IAEMEactivity from low level perceptual and motor processes to higher order cognition (attention,memory, reading etc.) EEG electrodes transfer ionic currents from cerebral tissue into electricalcurrents used in EEG preamplifiers. The electrical characteristic are determined primarily by thetype 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 muscletissue 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 thesheaf tip and used to electrically stimulate the brain.BASIC RECORDING SYSTEMReferential: The potential difference is measured between an active electrode and an inactivereference electrode Figure 2 (a) Referential methodBipolar: The potential difference is measured between two active electrode Figure 2(b) Bipolar methodEEG is a representation of the electrical activity of the brain. .The technique involve following:1.Biopotential peak up- Cranial or cerebral surface transducer electrodes2.EEG signal conditioning-Transducer output amplification and filtering (.1 to 100Hz)3.EEG signal recording-Signal displayed on graphic recorder or CRT4. EEG Signal analysis- visual or computer interpretation of resting EEGEEG is a collaborative tool is diagnosis brain function and disease. Many Physician andneurologist view EEG signal as interest artifacts but confess that then are not certain of the signal 179
    • 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 October-origins. In fact ,until recently EEG waveform were originally thought to be a summation ofaction potential of neurons as they made their was to cranial surface later idea reflectsstimulation associated by diverse neuron . Modern interpretation of EEG origin rest with knowledge of basic neuronal electrochemicalprocess. The Action potential (AP) from neuron has been recorded with microelectrodes at thecellular level. Essentially the synaptic fibers, terminal boutons ,neuronal membrane and axoncontribute the distinguishable response characteristic .Electrical reaction of neurons includes thefollowing 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 20The amplitude, phase, and frequency of EEG signals depend on electrode placement. This , signalsplacement is based on the frontal, parietal, temporal, and occipital cranial area. One of the mostpopular schemes is the 10-20 EEG electrode placement system established by the international 20Federation 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 arelocated on the surface of scalp. The position are determined as follows: Reference points are . poinasion, which is the delve at the top of the nose, level with the eyes, and inions, which is bonylump at the base of the skull on the midline at the bback of the head. From these points, the skullperimeters are measured in the transverse and media planes. Electrode are placed by measuring medianthe nasion-inions distance are marking points on the head 10%, 20%, 20%, 20%, 20%, 10% ofthis length. The vertex, c2 electrode is the mid mid-point as shown in fig below.Figure 3:10-20 EEG Electrode Placement system 20 180
    • International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 –6464(Print), ISSN 0976 – 6472(Online) Volume 3, Issue 3, October- December (2012), © IAEMESignal AcquisitionElectrode arrangements may be either unipolar or bipolar. A unipolar arrangements is composedof a number of scalp leads connected to a common indifference point such as earlobe. Henceone electrode is common to all channels. A bipolar arrangements is achieved by theinterconnection of scalp electrodes.ACTUAL CIRCUIT DIAGRAMSThe differential amplifier (as shown in Figure 4) is used where a difference in potential has to beamplified in the presence of an interfering common-mode voltage. Such interference frequentlycomes from 50 Hz power line fields, man-made electrical noise, or radiation from other electricalequipment. 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 currentflow in A will increase while the current in B will decrease. If both amplifiers and associatedparts are identical, the amplifier currents i1 and i2 will be equal and opposite, the net current willbe zero and no voltage drop will occur across resistor RC. Under these conditions the outputsignal 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 thecase of such signals from the pickup power lines, both i1 and i2 will increase or decreasesimultaneously, causing a voltage drop on RC. This common-mode voltage results indegeneration (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 byequation (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 signalsappearing simultaneously with differential signals. CMRR is a ratio of two gains, as shows inequation (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 theinput 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). 181
    • 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 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 thecommon 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 amplifiercould 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 eachamplifier. 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 inputimpedance and on the resistance of the electrodes placed on the Patient’s body. The input voltagedepends 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 goodconditions, a loss of less than one percent is possible. Since the TL074 has an input of over 1012ohms, 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, orEEG signal voltage. Two amplifiers are used for the differential input, and one for a single-endedoutput amplifier. A typical circuit arrangement is shown in Figure 5.The gain of the invertingamplifier A4 is determined by the ratio of the R1/R8 (100KΩ/8.2KΩ) resistors (A = 12). Thegain of the differential amplifier is determined from equation (2-5). Adiff = 2 R6/R5 (2-5) 182
    • 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 Fig 6 Differential input, Single-ended output amplifierThe gain (shown in the circuit diagram of Figure 5 is : Adiff = 2 x 100 KΩ/ 6.8KΩ Adiff = 29The gain of the input stage can be controlled by varying the 10 KΩ resistor (R5) between pins 6and 13. As the resistor is made smaller, the gain is increased. This resistor should not be zerosince 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 R10should be adjusted to obtain the best balance of the two signals input to A4. This balanceachieves the highest possible CMRR.PATIENT LEAD SAFETYIn medical instrumentation, patient safety is a major consideration. If patient is totally isolatedfrom ground, the potential shock hazard is greatly reduced. Figure 7 shows how, by using anoptocoupler, 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. Thesensor can be a photodiode, phototransistor, or photodarlington. The LED-to-sensor insulationbreakdown voltage ranges from 1500 volts to over 20,000 volts, depending on the device used.Fig 7: Floating differential amplifier 183
    • International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 –6464(Print), ISSN 0976 – 6472(Online) Volume 3, Issue 3, October- December (2012), © IAEMEObservationEach electrode site was rubbed with alcohol, electrode paste and electrodes firmly attached. Foreach 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 leavingswitches A1 and A2 open or closed, and the low end was rolled off at approximately 3 Hz byopening A4. For a one-volt amplifier output, a system gains of 5,000 to 50,000 (75-95 dB) wasrequired. Digital oscilloscope was used to record data from EEG recorder. The output was seenfrom TP15 on the oscilloscope and also seen the waveforms through DSO on the PC byconnecting the output (TP-15) to Ch1/Ch2 through parallel port. The output (TP15) of theexperimental panel can be seen on the Digital storage oscilloscope (DSO). Keep the DIPswitches SWA and SWB in kit as follows:The various Practical waveform at different points are as shown below:1. PointsF3-C3 4. PointsP3-O12.Points O1-O2 5.Points A2-O23.Points F4-C4 6.Points C4- P4 184
    • International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 –6464(Print), ISSN 0976 – 6472(Online) Volume 3, Issue 3, October- December (2012), © IAEMEThe various ideal waveforms at different points are shown below:CONCLUSIONThe EEG gives a coarse view of neural activity and can be used to non-invasively study cognitiveprocesses and the physiology of the brain. A typical EEG signal ranges from 1 to 100 µv peak to peak atlow frequencies(1 to 50 Hz) at the surface of scalp. Modern EEG machines are PC Based. EEG signal isusually recorded by reusable scalp disc or cup electrodes . The EEG is composed of electrical rhythmsand transient discharges which are distinguished by location, frequency, amplitude, form, periodicity andfunctional properties. The low-level EEG Signal from the brain as recorded is amplified and converted toa digital signal for further processing. EEG machines have a notch filter sharply tuned at 50Hz so as toeliminate main frequency interference Reading has been taken in international 10-20 Electrode systemfor 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. 185
    • 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[4] F. H. Lopes da Silva, J. P. Pijn, and D. N. Veli, “Signal processing of EEG: evidence for chaos ornoise. An application to seizure activity inepilepsy,” in Advances in Processing and Pattern Analysis ofBiological Signals. New York: Plenum, 1996. pp. 21–32.[5] J. R. G. Carrie, “A hybrid computer system for detecting and quantifying spike and wave EEGpatterns,” 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 DiegoBiomed. 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 AnalyticStudies 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, “EEGnonstationaritiesdetected by inverse filtering in scalp and cortical recordings of epileptics: Statistical analysis and spatialdisplay,” 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,” inEpileptology: 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 forthe 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 humanEEG,” 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, MedicalPhysics 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 channelAmplifier 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 ILlc. pp. 92-128[24]Neuman, Michael R. (1998). Medical Instrumentation Application and Design. John Wiley & Sons, Inc pp.183-233 186