Contains reports on two research projects

Fahringer, G. S.

Kiang, Nelson Y-S.

Raab, D. H.

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XI. COMMUNICATIONS BIOPHYSICSProf. W. A. Rosenblith Dr. D. H. Raab L. S. FrishkopfProf. R. C. Booton, Jr. R. M. Brown M. H. Goldstein, Jr.Dr. J. S. Barlow* G. S. Fahringer N. Y. S. KiangDr. M. A. B. Brazier* R. KoehlerA. CHANGES IN CORTICAL CLICK RESPONSES AS A FUNCTION OF CLICKINTENSITY AND DEPTH OF ANESTHESIAIn the Quarterly Progress Report, April 15, 1955, pp. 76-77, we noted a relationbetween a measure of summed cortical on-going activity immediately preceding theclick-evoked responses and the size of the individual click responses.In the present study, we have examined the changes in amplitude and variability ofamplitude of the cortical click response in anesthetized cats as a function of click inten-sity and depth of anesthesia.Figure XI- la indicates the location of our "active" electrode in the middle ecto-slyvian gyrus. Figure XI-lb illustrates a typical click response from this corticalpoint together with the amplitudes we have measured. The data presented below arefor the peak-to-peak amplitude a 2 .Responses to fifty clicks presented at a rate of one every four seconds wererecorded for each of five click intensities at each of four levels of anesthesia. Theintensity levels were separated by 20-db steps from 10 db to 90 db re the cat's clickthreshold. During each set of five intensities, the anesthesia level was kept as constantas possible by continually checking the electrocorticogram (ECG) and injecting Dialwhenever necessary (see Fig. XI-2). For each intensity-anesthesia condition the meanamplitude and the standard deviation were computed for fifty responses. The results4 ,SFr(b)Fig. XI-1. a. The cat brain. The active electrode was placed in the middleectosylvian gyrus in the location indicated by the dot; b. A typicalelectrical response evoked by click (upward deflection indicatesnegative polarity at the active electrode). Of the amplitudes al,a 2 , and a 3 , only the a 2 measurement was used in this study.From the Neurophysiological Laboratory of the Neurology Service of the Mass-achusetts General Hospital.(blOApV4mSECTOTAL DIAL TIMEIIT213 mg.279 mg200,VI sec1 339 mg418 mgFig. XI-2. The four (I, II, III, IV) ECG records were taken during actual presentation of clicksat each of the anesthetic levels in order of increasing depth. The markers under eachECG indicate the presentation of a click. Dial was initially given intra-peritoneally(IP) although subsequent doses were given intravenously (IV). The times show wheneach record was taken.4:50 P.M.6:45 P.M.10:50 P.M.1:45 A.M.-~--zh c- slx~vu'x~crlPIV------------------------- -- h- __--------~SSi- '* i" -- 'J.n JJln.. n._L---------- JI*~y-?r -l~m~~d,~JI.4~80 60 40 20 0DB ATTENUATIONThe mean amplitudes of groups of 50 consecutive responsesare plotted against click intensity for each level of anesthesia(I, II, III, IV) corresponding to those of Fig. XI-2. Theresponse amplitudes are in arbitrary units.r/ \ i/ 2K\/ /II I II80 60 40DB ATTENUATION20 0Fig. XI-4. The standard deviation (a 2 ) of the same groups of fiftyresponses for which the mean amplitudes were plottedin Fig. XI-3 are here plotted against click intensity forthe same parametric values of anesthesia.Fig. XI-3.(XI. COMMUNICATIONS BIOPHYSICS)are summarized in Figs. XI-3 and XI-4.In general, both the mean response amplitude and the standard deviation increasewith click intensity and decrease as the level of anesthesia is deepened.D. H. Raab, N. Y. S. KiangB. AN ELECTRONIC DEVICE FOR THE DETECTION OF EVOKED RESPONSESIN POTENTIALS RECORDED FROM THE SKULLThe amplitude of on-going electrical activity recorded from the brain of awake sub-jects is large compared to the activity evoked by sensory stimuli. It is then difficultto detect the evoked activity in the EEG; frequency characteristics of present inkwritersincrease these difficulties. The difficulty of visually observing evoked responses in anEEG is illustrated in Fig. XI-5.Identical stimuli do not cause identical responses; thus, it is impossible to deter-mine the exact waveform of a single response. However, the average waveform of theresponse to a repetitive identical stimulus may be determined by performing the cross-correlation4(T) = fT f(t) u (t-) dt (1)in which f(t) is the potential recorded from the brain, u p(t) is a train of impulse func-tions which is time-locked to the stimulus, T is the relative delay of the impulses, andT is the period of observation (1, 2, 3, 4, 5). All of the operations necessary for per-forming the computation in Eq. 1 are identical with those performed in computing a reg-ular crosscorrelation function except that here a continuous function is being multipliedby a series of unit impulses. Therefore, the analog correlator for electroencephalo-graphy described earlier (6) may be used for this computation by simply substituting adevice capable of multiplying a continuous function by unit impulses for the regularmultiplier. A block diagram of the modified correlator is shown in Fig. XI-6.STROBOSCOPIC FLASHES10 uvI SECONDFig. XI-5. Potentials recorded from skull of subject receivingstroboscopic flashes at times indicated by pipsFig. XI-6. Block diagram of system for computing theaverage response to repetitive stimuliFig. XI-7. Block diagram of device for multiplyingbioelectric potentials by unit impulsesFig. XI-8. Average of 100 responses to stroboscopic flashes presented every850 msec. Each line represents a change in 7 of 10 msec, whereT is measured from the onset of the flash. Recording electrodeswere placed as indicated in Fig. XI-5.(XI. COMMUNICATIONS BIOPHYSICS)If the unit impulse multiplier performs its function accurately, its output will beimpulses with amplitudes equal to the instantaneous values of the brain potential at thetimes of occurrence of the impulses. Since the integrator is incapable of integratingpulses of zero width, the output of the multiplier has to be converted to rectangularpulses to provide adequate voltage-time area input to the integrator. A block diagramof a unit impulse multiplier with rectangular pulse output is shown in Fig. XI-7.The important elements of this system are the gate pulse generator, the signal gate,the storage stage, and the output gate. The gate pulse generator converts the pulseswhich indicate the onset of the stimulus into two sets of rectangular gate pulses whichare opposite in phase so that when the signal gate is closed the output gate is opened,and vice versa.The signal gate is normally opened so that the storage section can follow the input.However, when the signal gate is closed, the value in storage is constrained at theinstantaneous value of the signal at the time of closure. At this same instant, the out-put gate opens so that the constant value in storage is fed into the integrator until theend of the gating pulse. The width of the gating pulse is variable, but is kept shortso that the system will be capable of handling random stimulation and still give equalweighting to each sample.Figure XI-8 shows the average waveform of the activity evoked by a stroboscopicflash as recorded from the electrode positions indicated in Fig. XI-5. This averageof 100 responses was computed from an 85-second sample of the activity from whichFig. XI-5 was taken. The 10-cps periodic activity resembles activity not ordinarilyevoked by a flash but it is apparently time-locked to the periodic flashes.G. S. FahringerReferences1. Y. W. Lee, Application of statistical methods to communication problems, Techni-cal Report 181, Research Laboratory of Electronics, M.I.T. (Sept. 1950).2. Y. W. Lee, T. P. Cheatham, and J. B. Wiesner, Application of correlation analysisto the detection of periodic signals in noise, Proc. IRE 38, 1165-1171 (1950).3. M. A. B. Brazier and J. U. Casby, Crosscorrelation and autocorrelation studiesof electroencephalographic potential, EEG Clin. Neurophysiol. 4, 201-211 (1952).4. G. D. Dawson, A summation technique for the detection of small evoked potentials,EEG Clin. Neurophysiol. 6, 65-84 (1954).5. J. S. Barlow and M. A. B. Brazier, Quarterly Progress Report, ResearchLaboratory of Electronics, M.I.T., April 15, 1955, pp. 79-82.6. Quarterly Progress Report, Research Laboratory of Electronics, M.I.T., Jan. 15,1953, pp. 47-49; Jan. 15, 1954, pp. 71-72; April 15, 1955, pp. 77-79.

Communications Biophysics

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Communications Biophysics

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