Analog analysis of EEG activity

ELECTROENCEPHALOGRAPHY AND CLINICAL NEUROPHYSlOLOGY

TECHNICAL

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NOTES

ANALOG ANALYSIS OF EEG ACTIVITY JEAN-LOuIS RIEHL,

M.D., M.S.

Divisionof Neurology, Department of Medicine, U.C.L.A. Medical Center,Los Angeles,Calif.(U.S.A.) (Reeeived for publication: December 26, 1962) then low U,t values will be present during sleep or drowsiness and in contrast, the fast activity of an alerting response will yield high Us values. This technique was used to study the variations of the frequency-voltage complexes of the EEG during activation responses in animals (Riehl et aL 1960). A numerical

In the analysis of any complex process it is useful to search for "natural" relationships between dimensions, whenever they occur. There exists a reciprocal relationship between the frequency and the voltage of the EEG: sleep or drowsiness (low level cerebral activity) are characterized by low fre-

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Fig. 1 Theory of the analog EEG analysis. For explanation see text. quency, high voltage wave forms, whereas alertness, wakefulness, responses to sensory stimulation are accompanied by relatively high frequency, low voltage tracings. If one defines a "unit of activity" Us --- f (F, V)and more specifically Us = F/V (where F is the frequency of the EEG wave form and V the corresponding voltage), s The experiments described in this study were in part carried out at the School of Aerospace Medicine, USAF, Brooks Air Force Base.

example will demonstrate the order of magnitude of the change in "Uat value". Let a resting EEG show an average frequency of I0 c/see with a corresponding averap voltage of 200 p~V. Us = F/F or 10/200 -~- 0.05. Similarly with an EEG of fast activity where the average frequency is 30 c/see and average voltage 50/sV, Us = F / V = 30/50 --- 0.6. Thus a twelve-fold difference in Us exists between the 2 EEG states. The function Us can be displayed continuously on the EEG record thus giving an instantaneous "level of

Electroenceph.din. NeurophysioL, 1963, I$: 1039-1042

J.L. RIEHL

1040

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activity" and/or it can be inteffrated with ~'espect to time in suitable epochs, The purpme of this report is to presmtt the automatic processinll of such an analog analysis.

Tkeoretleal considerations The computation steps are summarized in Fi$. I. in ,4, a synthetic resting "EEG" (high voltage, low fre. quency) gives a low value for the quotient F/V (or U,¢). The inte~ation of U.¢ over the 10 see epoch f~o (C'~) dt yields the small triangle a, b, ¢ (shaded). In B, a fast activity "EEG" gives a high UA value and correspondingly the inte|grated f~o (u,O dt yields the large triangle a', b', e' (shaded). It can be observed that the primary "EEGs" being sinusoidal traces, the values F and V are constant and thus the quotient U,t is also a constant.

InJIrumentatlon The instrumentation is shown in a block diaMam

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The primary EEG signals am obtained from the Jones plug normally going into the Grass instrument's penmotor. These signals enter a function separator unit (vide i~'ra). From the latter a rectifier ¢i.~dt provides a voltap £a proportional to th~ avexa~ EEG voltage (V); a second voltage £z is obtained through a frequency meter (Hewlett-Packard Model $00&) and is proportioned to the averap EEO fr~luency (F). Both voltages £1 (~ F) and £: (~ V) am impressed on the Donner Analog Computer (Model 3000) which performs the Analog Division EllEn (or FIg), The output, termed U,, (t) is boosted through a d.c. amplifier and finally displayed on the EEG record via a separate p:n-motor. At the same time, the output voltqle U~ (t) is placed in the intqgratinr circuit of the computer. A cyclic reset generator (Donner Model 3720) provides a I0 sec epoch during which U.4 (t) is intelp~ted with respect to time.

f~" (U.4")d, Electroe~cepl,. din. Neuropkysiol,, 1~3, 15:1039-1042

AIqALOG ANALYSISOF EEG

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Fig. 4 Analog analysis of EEG activity. Upper tracing: moderately active record. The function (/A (t) oscillates around 10. The integration of U,t gives a largt triangle f (UA) dt. Middle tracing: same subject, asleep. A predominance of hitlh ac,plitude slow waves gives low U,, values - around 6. The integration of U,~ yields a small triangle. Lower tracing: a short lived alerting response gives a transient rise in L/a. The increa~ in activity is reflected in the integration. leads: LP-LO: left parietal - left occipital. (Note that the second channel has been inverted and should read LO-LP). Another d.c. amplifier is necessary to actuate a second pen.motor and inscribe the resulting integrated signals on the EEG record. The function separator unit is of critical importance. The schematic of the circuitry appears in Fig. 3. The primary EEG signals are led to the frequency meter and the bridge rectifier through a transformer coupling. Further inductance-capacitance filtering is needed to smooth out the signals Ex (FM output) and Ee (AM outpu0. The specific component parts are detailed in the schematic and all are commercially available.

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Model 3731) is placed in the feedback loop of an operational amplifier. The multiplication (or division) is based on the time.division principle. An input X modulates the height of a high frequency rectangular pulse, whereas the input )'determines the duty cycle of the pulse (i.e. its duration). The resultant is proportional to X¥. The integration of XY gives the analog ouL~ut Z. When the multiplier is inserted in the ft~lba~k l~o~ o~' a d.c. amplifier the output of the latter becomes X/Y ner

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Electroenceph. din. Neurophysiol., 1963, 15:1039-1042

J . L . RIEHL

1042

Calibration of" the analyzer Although specific frequency-voltage complexes can be identified on the EEG record, they seldom occur in pure form for any length of time. It is thus not necessary to use the extreme values of the frequency-voltage spectrum for the purpose of calibration as this would result in a sharp decrease in resolution. For this reason, the upper and lower limits of the scale of EEG activity were chosen from average values computed in actual records (Riehl et al. 1960). Drowsiness and sleep in the lower range were equated to a 200/AV-IO c[sec sine wave. At the other end of the scale, fast activity was represented by a sinusoidal signal of 50/~V amplitude at a frequency of 30 c/see. An oscillator with suitable attenuating network can be used as a signal source for the calibration. RESULTS A sample of the analog analysis is shown in Fig. 4. The upper tracing is that of a moderately active human EEG. The function U~ oscillates around 10 (arbitrary scale); the integration yields a large triangle; in the middle

tracing the same subject is asleep. The U,4 values fluctuate, slightly above 6. The triangles of integration are small. The alerting response shown in the lower tracing results in a sharp rise in U~ (from 4 to 10), followed by a return to base line in 12 sec. The integration of U,4 (t) during the alerting response gives a large triangle. In all three records the primary EEG signals are presented with the real-time analysis of U~t (t) and the integration of U,¢ (t) in 10 sec epochs. SUMMARY An analog analysisof E E G activityhas been described. The quotient of two E E G parameters, frequency (F) and voltage (V), is used as a unit of activity (UA)such that UA -- F/V, The function l./Acan be displayed in real-time or integrated in suitable epochs, REFERENCES

RIEHL, J.-L., PAUL, J. C. and UNNA, K. R. Quantification of the EEG activation response: effects of atropine. Psychopharmacologia (Berl.), 1960, 1: 200-209.

Reference: RtEttL, J.-L. Analog analysis of EEG activity. Electroenceph. clin. NeurophysioL, 1963, 15: 1039-1042.

A DESIGN FOR A RELIABLE A N D INEXPENSIVE BIPHASIC STIMULATOR t HARRY O. PETROHILOS, ROBERT SIMPSON, NORMAN T. WELFORD AND ELLIOT S. VALENSTEIN

Yellow Sprlng,v Instrument Company and Fel~ Research Instltut¢,Yellow Sprlns#, Ohlo ( U,$,A,) (Received for publication: February 25, 1963) lnvestipto~ interested in minimizing tissue damaJe with electrical ~timulating ~.rrcnts have generally con. eluded that biphasio pulses produce less destruction than that which results from unidirectional pulses (Lilly et al. 1955a and b; Rowl.nd et cal. 1960), In spite of this con. dusion there are few stimulators available tl--" can conveniently supply biphasie pulses. Thesecould be produced by combining two pulse generators, but there are several disadvantages to this approach, Besidesthe higher expense involved, the n__,~ess_ ity of matching amplitudes of positive and neptive pulses with separate adjustment knobs of the two units is both time consuming and subject to error, In addition, the combining of the output from separate pulse generators generally requires some additional circuitry which may reduce the effective output below require. merits for some purposes. During the past year and one half, we have had in almost daily use, six biphasic stimulators of our design, The completely transistorized circuit has pn)ved to be trouble free and can easily be assembled in 1 day. The ,mmm,mmmmm

l This work was supported by USPHS Oraqt M-4529.

total cost of components including all hardware is less than 65 dollars. With a 20,000 CI load the stimulator is capable of delivering 100 V (peak to peak) Lnd the positive and negative pulses can be adjusted by a single knob. The circuit is so designed that a perfect matching of amplitude and duration of the rectangular pulses is assured. Other features of this stimulator include an output that is floating with respect to chassis ground and convenient triggering and monitoring a ~ t s . While the frequency, pulse duration and pulse delay are fixed, these may be varied by changing the values of capacitors as indicated in the following circuit description. In our experiments we have used both a pulse duration and delay of 0.2/msec and a repetition frequency of 100c/see,and in repeated brain stimulation sessions over many months with current values as high as 1.5 mA (peak to peak) we have found no evidence of histological d a m ~ attributable to the electric c~,a'ent. The basic circuit is presented in Fig. 1. Q I and Q2 constitute a free running multivibrator which oscillates when the NC relay contacts (RYI) open. The frequency of oscillation is determined by CI and C2 and in our applica-

Electroenceph. clin. Newophystol., 1963, 15:1042-1044