Contains reports on four research projects

Barlow, J. S.

Brazier, Mary A. B.

Brown, Robert M.

Frishkopf, Lawrence S.

Hooks, A. K.

Hughes, John R.

Macy, J., Jr.

Raab, D. H.

Rosenblith, Walter A.

Putter, K.

DSpace@MIT

XIX. COMMUNICATIONS BIOPHYSICSProf. W. A. Rosenblith R. M. Brown A. K. HooksDr. D. H. Raab L. S. Frishkopf K. PutterA. INTRODUCTIONThe Communications Biophysics group is concerned chiefly with the electricalactivity of the nervous system and with certain psychophysical experiments on temporalcharacteristics of hearing. The electrophysiological data with which we deal includeevoked responses (for example, specific responses that are recorded when acousticstimuli are presented) and "spontaneous" activity of the brain-wave type. We are alsoconcerned with the study of the interaction of these two types of electrical activities.Since we record the electrical activity of populations of neural elements, the mathe-matical models are, appropriately, statistical in character. We emphasize, however,close relations between mathematical models and experimental data. We require of ourprobabilistic models that they not only describe familiar phenomena, but that theysuggest and predict new experimental findings.Up to this point, we have mainly investigated a zero-interaction model, that is, amodel in which the behavior of single units is not affected by what happens to othermembers of the population. Such a model seems a useful approximation for the mostperipheral station of the auditory nervous system. It does not seem to be satisfactoryfor dealing with cortical activity.We are developing electrical instrumentation designed to speed up and simplify theanalysis of our data. We feel that at the present time small-scale computational deviceswill not only analyze our data most efficiently but also assist us in developing intuitivethinking about behavior of the nervous system.W. A. RosenblithB. EFFECT OF NOISE ON INTENSITY FUNCTIONS OF N 1Intensity functions (amplitude of neural response N I vs intensity of stimulus) havebeen obtained at the round window of cats for (a) clicks and (b) clicks against a back-ground of random noise. The results of these experiments, for several different levelsof the noise, are shown in Fig. XIX-1.Certain aspects of these data on N1 tend to confirm in the neural system underinvestigation properties that were previously suggested on the basis of other evidence.The elementary component of this system will be called a neural element, which wedefine as the complex of auditory receptor cell, first-order neuron, and receptor-neural junction.1. Experimental evidence from several sources suggests that neural elements havethresholds that fluctuate with time. This property has been made the basis of a-67--NO NOISE BACKGROUND--- NOISE BACKGROUND - 92 DB UNFILTERED- NO NOISE BACKGROUND--- NOISE BACKGROUND 82 DB UNFILTERED(DB BELOW 1.29 VOLTS INTO PHONE) /CLICK INTENSITY (DB BELOW 1.29 VOLTS INTO PHONE)(a)0IoCrzoo_mcrLCLICK INTENSITY (DB BELOW 1.29 VOLTS INTO PHONE)(b)TERED (DB BELOWTO PHONE)CLICK INTENSITY (DB BELOW 1.29 VO(c)Fig. XIX-1LTS INTO PHONE)Intensity functions for clicks with and without noise backgrounds, for several noise levels.10I Iz)a.U)t:zCO 51rUj(XIX. COMMUNICATIONS BIOPHYSICS)mathematical model of the peripheral auditory nervous system (Quarterly ProgressReport, October 15, 1953, and W. J. McGill: A Statistical Description of NeuralResponses to Clicks Recorded at the Round Window of the Cat, Doctoral Thesis, Depart-ment of Psychology, Harvard University, 1952). Additional evidence is now adducedfrom the data of Fig. XIX-1. Observe that two weak noise levels are extremelyeffective in preventing the system from responding to clicks of much greater intensity.A hypothesis of fixed thresholds would predict no response to click intensities less thanor equal to the noise intensities, and increasing response thereafter. This is clearlynot the case. The assumption of fluctuating thresholds can, on the other hand, accountfor the data. Neural elements which in the absence of noise would respond to clicks,are now continually dropping in threshold below the noise level and firing, thus beingrendered incapable of fully responding to subsequent clicks of intensity well above thenoise level. The data of Fig XIX-1 can be accounted for quantitatively if the rate ofthreshold variation is suitably chosen (see part C below).2. The two-stage growth of the intensity function without noise, seen in the uppercurves of Fig. XIX-1, is characteristic of all preparations for which intensity functionshave been determined over a sufficiently broad stimulus range. Two categories ofneural elements are thereby suggested (see Quarterly Progress Report and DoctoralThesis as quoted above); a neural element is either sensitive or insensitive accordingas the range of its threshold values coincides with the stimulus range of the low-intensity or high-intensity portion of the growth curve. The alternative to this view,barring fixed thresholds, is that the range of possible threshold values for each neuralelement extends over the entire range of stimuli which evoke response. In its idealizedform, this is the question of two populations vs one population. By 'population' wemean a group of neural elements, all of which have identical threshold distributions.Previous evidence has favored two populations. Additional evidence in its favor isobtained from the present data. For the intensity function for clicks in the presence oflow-level noise shows a two-phase behavior of the sort one would expect from two popu-lations whose average thresholds are clearly separated. Neural elements contributingto the insensitive portion of the intensity function are unaffected by noise of sufficientlylow intensity, while sensitive units are completely prevented from firing to the click.On the other hand, if the system consisted of a single population, one would expectan intensity function of quite a different shape in the presence of low-level noise.L. S. Frishkopf, D. H. Raab, R. M. BrownC. NOTE ON A PROBABILISTIC MODEL FOR THE BEHAVIOR OF NEURALELEMENTS (PERIPHERAL AUDITORY SYSTEM)A model has been proposed (Quarterly Progress Report and Doctoral Thesis asquoted in subsection B) to account for the observed behavior of the first neural-69-(XIX. COMMUNICATIONS BIOPHYSICS)component N 1 of the peripheral response, recorded at the round window. Neural units,mathematical idealizations of neural elements, with the following properties are postu-lated.a. The response of a unit obeys the all-or-none law.b. The threshold of a unit, in its resting state, fluctuates and is described by athreshold probability distribution.c. A unit is characterized as either sensitive or insensitive, depending upon thevalue of its mean threshold. A simplification is introduced by assuming that each ofthese populations consists of units having identical threshold distributions.d. The firing of a unit alters its threshold distribution, which then becomes afunction of the time AT since firing. For times AT that are sufficiently long the unitreturns to the resting distribution.The experiments described in subsection B give additional validity to postulates band c.An extension of the model to include some description of the rate of threshold fluctu-ation is made possible by the data of Fig. XIX-1, as follows:Associated with a neural unit is a threshold probability distribution, determinedexperimentally from the intensity function of the appropriate neural population. Let usassume that threshold fluctuations occur in the following way: a unit occupies a definitethreshold state (or value) for a time T whereupon it 'jumps' or makes a transition, in atime short compared to 7, to a new threshold state, and so forth. Let us assume inaddition that threshold is a random variable on the set of possible threshold states.The probability that the threshold of a unit will in time T (T/T transitions) occupy one ormore states in an interval I subtending a total threshold probability a is given byp = 1 -(1 - a)T/T (1)If a is now chosen as the probability that the threshold lies below the noise level(obtained from the sensitive population intensity function), then for a selected value ofT, p gives the probability that in time T a unit will fire to the noise. With T of the orderof 2 msec, p is approximately given as 1-RN/R, where R N and R are respectively themaximal responses of the sensitive population to clicks with and without noise back-ground. For the actual situation is approximately equivalent to one in which amplitudeof response is zero if a unit is excited within about 2 msec after firing, and totallyrecovered thereafter. Inserting values obtained from the data of Fig. XIX-1 (noiselevel -92 db) and T = 2 msec into Eq. 1, yields T 1/2 msec. Using this value of Tin conjunction with the data for the -82 db noise level gives p - 15/16, which agreeswith the data within the limits of error of the experiment. This is to be regarded onlyas an order-of-magnitude calculation for T.A somewhat different experiment is underway that we hope will test the consistency-70-(XIX. COMMUNICATIONS BIOPHYSICS)of the model, provide a more accurate value of T, and provide data concerning thechange and recovery in time of the threshold distribution of a unit after firing.L. S. FrishkopfD. ANALOG CORRELATOR FOR ELECTROENCEPHALOGRAPHYMinor modifications have been made on the analog correlator for the analysis ofelectroencephalographic (EEG, or brain potentials) activity (Quarterly Progress Report,Research Laboratory of Electronics, M. I. T., January 15, 1953, pp. 47-49). This com-puter has been adapted from the correlator at the Imperial College of Science and Tech-nology, London (J. N. Holmes, J. M. C. Dukes: A Correlation Function Computer forthe Analysis of Speech Wave Forms, Proc. Inst. Elec. Engrs., Part III (in press)).Additional correlation functions (see, for example, Fig. XIX-2) have been obtained withthe modified apparatus in conjunction with the magnetic drum developed by K. Goff ofthe M. I. T. Acoustics Laboratory. A description of the magnetic drum has appeared(K. Goff: Proc. I.R.E., 41, 1578-1584, 1953) and the operating principles of thecorrelator are described in a paper to be published (J. S. Barlow, M. A. B. Brazier:A Note on a Correlator for Electroencephalographic Work, EEG. Clin. Neurophysiol.(in press)).The number of record-playback heads on the FM magnetic tape recorder (Ampex)has been increased from two to five. This modification will make it possible to repro-duce for correlation analysis any two of five channels of EEG recorded with a similarfive-channel instrument in the Neurophysiological Laboratory at the MassachusettsGeneral Hospital. To shorten the time of analysis, a speedup of 5 or 10 times takesplace between the recording and the playing back on the two instruments.Fig. XIX-2Autocorrelation of a square wave (150 cps). Integration time, 5 sec; AT = 1/16 msec.-71-(XIX. COMMUNICATIONS BIOPHYSICS)A magnetic drum similar to that developed by Goff is being constructed, with whichthe FM carrier frequency of the tape recorder will be delayed during computation of thecorrelation functions. In contrast to the earlier method of making an endless loop oftape for analysis, an automatic recycling system for the magnetic tape recorder isplanned similar to that now in operation for the M. I. T. digital correlator.Several recordings of both human and animal EEG's, with and without evokedresponses to flash, have been recorded on tape at the Massachusetts General Hospital,and two such recordings have been analyzed. The EEG of a cat has been recordedon paper in the Research Laboratory of Electronics and the recorded tracing isnow being autocorrelated by means of the manual correlator of the M. I. T. Servo-mechanisms Laboratory (N. Zabusky: A Mechanical Correlation Computer, Engi-neering Report No. 32, Servomechanisms Laboratory, M. I. T., 1951). This analysisshould yield information regarding the maximum delays that it will be necessaryfor the analog instrument to handle.J. S. Barlow and Mary A. B. Brazier(Massachusetts General Hospital Neurophysiological Laboratory),W. A. RosenblithE. THE TIME-GATED AMPLITUDE QUANTIZER (TGAQ)Since the completion of final tests on the TGAQ, the operation of the equipment hasbeen checked under actual experimental conditions. Eighty-six electrical responses tosingle high-intensity clicks from the round windows of cats have been analyzed by com-paring the results obtained by the TGAQ with those obtained by conventional visualanalysis with the aid of a microfilm reader.1. Visual analysis using quantized categories placed 78 percent of the responsesin or on the boundary of the levels registered by the TGAQ.2. Visual quantization placed the remaining responses into the two levels adjacentto the level indicated by the TGAQ.3. The unquantized data showed that the absolute value of the amount of dis-crepancy between the results of the two techniques was less than one-half level out of40 levels.Although the results of visual quantization are not sufficiently accurate to be usedas a standard of absolute accuracy for the TGAQ, the experiment shows that the TGAQadequately replaces the visual technique, at least for work with peripheral responsesN 1 to single clicks.Use of the TGAQ in this and other experiments has also shown that althoughthe assumption of constant latency of components of peripheral responses for astimulus of fixed intensity is a reasonable one, the necessity for readjusting theclamp circuit for changes in stimulus intensity limits the usefulness of the instrument.-72-(XIX. COMMUNICATIONS BIOPHYSICS)Electronic equipment designed to measure the latency of response components in orderto trigger the signal selection circuits of the TGAQ at the proper time is now beingdeveloped by A. K. Hooks and R. M. Brown.K. Putter-73-

Communications Biophysics

XVI. COMMUNICATIONS BIOPHYSICSProf. W. A. Rosenblith Dr. D. H. Raab L. S. FrishkopfDr. J. S. Barlow* R. M. Brown A. K. HooksDr. M. A. B. Brazier* J. Macy, Jr.A. ELECTRICAL RESPONSES TO CLICKS AND TONE PIPS AS RECORDED FROMTHE AUDITORY CORTEXWe are studying the responses to acoustic stimuli which can be recorded at the sur-face of the auditory cortex of the anesthetized cat. Presented below are results recordedsimultaneously from two points, 1 cm apart, one in Area I and the other in Area II, onthe middle ectosylvian gyrus.1. Responses to ClicksFigure XVI-1 illustrates the response to the same click stimulus, 40 db abovethreshold, recorded from Area I (lower trace) and Area II (upper trace). Both tracesbegin with the delivery of the stimulus, show some activity (attenuated at these elec-trode locations) of subcortical auditory centers, followed by the classic triphasic (- + -)cortical response. Both cortical points swing surface-negative together, whereas thesurface-positive response (peak) leads by approximately 0.5 msec in Area I. This con-firms a recent finding by Mickle and Ades.As the click stimulus is attenuated towards the threshold of Cat-322 this 0.5-mseclatency difference between the two points is increased to 1 msec. In addition, 'theresponse amplitudes Al and A 2 are reduced. Figure XVI-2 illustrates the unequalchanges (with click intensity) of A l and A2, especially for the response from Area II.2. Responses to Tone PipsSample responses from the same two cortical points to brief (10-msec) tone pipsare presented in Fig. XVI-3.The usual analysis of the cortical response has aimed at identifying the positive andnegative components of the response with events or structures within the cortex. Manyinvestigators, for example, have manipulated these response components by applyingprocaine, strychnine, thermocoagulation, and the like, to the cortex itself.Since it is possible to alter the response potentials differentially merely by changingthe intensity or frequency of the acoustic input to the ear, it would seem that analyseswhich omit consideration of the stimulus parameters are incomplete.D. H. Raab, R. M. Brown*From the Neurophysiological Laboratory of the Neurology Service of the Massachu-setts General Hospital.-75-S25- 20-- 154n- 10Zw-500 10 20 30 40CLICK INTENSITY IN DB ABOVE "THRESHOLD"Fig. XVI- 1Responses to the same click stimulus from audi-tory Area I (lower trace) and Area II (upper trace).(Negative up in this and in Fig. XVI-3.)4000 CPSFig. XVI-2Amplitude growth of A I and Awith click intensity.1000 CPS 500 CPSC 322Fig. XVI-3Responses from the same cortical points as those in Fig. XVI-1to three frequencies of tone pips.C-322I , I , I I ,- '-----'(XVI. COMMUNICATIONS BIOPHYSICS)B. ANALOG CORRELATOR FOR ELECTROENCEPHALOGRAPHYConstruction of the mechanical parts of the magnetic delay drum is nearing comple-tion, and its associated electronic circuits are under construction. The automatic con-trol system for recycling the magnetic tape between computation of successive pointson the correlation curve is also under construction.This device is modeled after a similar system that has been used in conjunction withthe M.I.T. digital correlator, with modifications to accommodate the present system.The drum-advance mechanism is designed to operate independently of the taperecorder if desired, for computation of correlograms from signal generators, noisesources, and the like, retaining the multiple-step feature for these alternate sources.Automatic operation of the correlator is not only of great convenience; it is alsoinstrumental in making the computational process as short as possible (one or twohours) so that major errors are not introduced by drift of the various dc stages of thecorrelator.An autocorrelation function of a 30-second sample of electrical potentials recordedfrom a cat's auditory cortex (ectosylvian gyrus) is shown in Fig. XVI-4. The cat wasunder light nembutal anesthesia. The pen-and-ink record made by an Offner electro-encephalograph was analyzed by means of the manual correlator of the M.I.T. Servo-mechanisms Laboratory. Though a Fourier transform of this correlogram is not yetavailable, the graph exhibits at least two kinds of periodic activity: one at approxi-mately 15 cps and another of lesser amplitude of approximately 4-5 cps. The 15-cpsactivity does not persist as long as the slower activity. At the longest delays used,machine noise becomes sufficiently important to obscure the analysis.These findings may be interpreted as reflecting the well-known barbiturate burstssuperimposed, at this level of anesthesia, on the cat's intrinsic 5-cps rhythm.40Z< 20Ir:0r 200 0.4 0.8 L2 1.6 2.0 2.4 2.8DELAY T(SECONDS)Fig. XVI-4Autocorrelation of electrical activity from cat's (308) auditory cortex(ectosylvian gyrus). Nembutal anesthesia, 30-second sample.-77-(XVI. COMMUNICATIONS BIOPHYSICS)Evidence from the correlogram suggests that maximum values of T of the order of2 sec will be adequate for the analog electronic correlator. With a record-playbackratio of 10 to 1 (60 inches/6 inches per sec) and maximum delay on the drum of about190 msec, the maximum delay now planned is 1.9 sec. Longer delays will necessitateeven slower recording speeds.J. S. Barlow,-M. A. B. BrazierR. M. Brown, W. A. RosenblithC. AUDITORY SENSITIZATION (ELECTROPHYSIOLOGICAL RESULTS)Psychophysical experiments at the Harvard Psycho - Acoustic Laboratory haverevealed a sensitization in the absolute threshold of pure tones after stimulation withrelatively intense low-frequency tones. These studies used the Beksy audiometer inorder to record continuously the absolute threshold of human subjects. Experimentson cats were conducted at M.I.T. in an attempt to obtain electrophysiological evidencefor auditory sensitization.In these experiments the test stimuli were short tone pips of 300 cps (6 cycles witha rise-fall time of 5 msec, presented every 2.4 sec). Tone pips were used rather thancontinuous tones, since responses to the latter exhibit a steadily increasing adaptation(or "equilibration"). Though adaptation is also present in the responses to tone pips,9 -z--Z04-a.ZW_a,;-L00ELECTROPHYSIOLOGICAL DATA (CAT - 315)AMPLITUDE CHANGES OF N,AFTER EXPOSURE (300GPS,IMINUTE,IOBDB)TIME AFTER CESSATION OF CONTINUOUS EXPOSURE TONE (SECONDS)Fig. XVI-5Auditory sensitization from electrophysiological and psychophysical data.-78-V(XVI. COMMUNICATIONS BIOPHYSICS)the stimuli are sufficiently separated in time to yield a fair measure of the responsive-ness of the peripheral auditory system to supra-threshold stimuli. Twenty pips (at118 db, above 0.0002 dyne/cm ) were presented as test stimuli before the cat was stim-ulated with a continuous tone of the same frequency (300 cps) for 1 minute at 108 dbSPL. Following this prolonged stimulation, the same test tone pips at 118 db were pre-sented every 2.4 sec for as long as 6 minutes.Platinum wire electrodes, placed within a few millimeters of the cat's round windowand posterior-medial to it, record the first neural response, N 1 , to the pure tones. Atappropriate locations one can detect N 1-responses to each cycle of the tone pip. Theamplitude of the neural responses before and after the 1-minute stimulation was meas-ured in arbitrary units. The average (mean) amplitude of the 20 prestimulationresponses serves as a base line from which to measure sensitization and fatigue fol-lowing continuous stimulation.Figure XVI-5 shows the variations in amplitude of the N 1 -response to the first cycleof the post-stimulation tone pips as a function of time. The heavy horizontal line rep-resents the mean amplitude of N 1 in the prestimulation test, that is, the base line. Eachpoint on the curve represents the mean of five successive post-stimulation amplitudereadings. The curve shows an initial decrease in the N1 response, then a sensitization(the supernormality is significant beyond the 1-percent level by Fisher's Exact Test),followed by a secondary fatigue. Five similar sensitization curves have been obtainedon three different cats. Figure XVI-5 also shows psychophysical data on the absolutethreshold changes in humans after exposure to 500 cps, 1 minute, 122 db (test tone wasa continuous 500 cps). There appears to be a marked similarity between the time courseof the sensitization curve obtained electrophysiologically on cats and psychophysicallyon humans.John R. Hughes(Psycho-Acoustic Laboratory, Harvard University)D. AN ELECTRONIC DEVICE FOR THE MEASUREMENT OF THE LATENCY OF"NEURALS"The present technique for the measurement of the latency of neural componentsrecorded near the round window involves photographing the response and determiningthe latencies by means of the microfilm reader. This procedure is tedious and has thedisadvantage that there is a considerable interval between the actual experiment and thetime at which the data become available. The device under construction will provide aninstantaneous measurement of the latencies, allowing practically immediate evaluationof the latency data. This device will be called a neural latency monitor (NLM).The block diagram is shown in Fig. XVI-6. A highly schematized response to an-79-(XVI. COMMUNICATIONS BIOPHYSICS)NEURAL SIGNALWITHOUT "NOISE"CLSI,Fig. XVI-6Block diagram of the neural latency monitor.acoustic click emphasizing the first neural component, N 1 , is shown. The gateis opened at approximately 0.5 msec before the first neural. This delay time and theduration of the gating pulse are both adjustable. A clamp holds the amplitude of thelatest, most negative excursion of the signal during the gating interval. This clampsignal is subsequently differentiated, clipped, and again differentiated. This procedureresults in two pips of opposite polarity. Note that the second pip will occur at a timejust before the neural potential exhibits a zero slope. Since the shape of the neural ismore triangular than sinusoidal and since the amount of amplification following theclamp differentiator is limited only by noise generated after the clamp, this time dif-ference between the most negative excursion of the last output pip can be made as smallas necessary. This pip is used to trigger the memory circuit that follows the counter.The counter began its count at the time of the click stimulus, so that the count held bythe memory circuit can now be read from the indicator.The need for some memory scheme is brought about by the presence of noise andmicrophonic potentials in the response signal. The clamp holds to the most negativeswings of the total signal and the following circuits generate negative pips correspondingin time to the zero slope points of these negative "pulses." Thus, the last pip gener-ated while the signal gate is open corresponds to the neural, provided that the neuralis a greater magnitude "pulse" than the other negative "pulse" in the signal. The neuralis almost always greater than other parts of the response signal. An oscilloscope isused to monitor the response in any case, so that this condition can be checked. Sincethe memory circuit holds the count of the counter at the instant of each pip, the finalcount indicated corresponds to the latency of N 1 . A.K. HooksA. K. Hooks-80-

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