Contains report on one research project.National Institutes of Health (Grant 2 PO1 MH-04737-06)Joint Services Electronics Programs (U.S. Army, U.S. Navy, and U.S. Air Force) under Contract DA 36-039-AMC-03200(E)National Science Foundation (Grant GK-835)National Aeronautics and Space Administration (Grant NsG-496

Sachs, M. B.

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XIV. COMMUNICATIONS BIOPHYSICSAcademic and Research StaffProf. D. B. Geselowitzt Prof. T. F. Weiss$ Dr. M. Nomoto***Prof. P. R. Gray Dr. J. S. Barlow t Dr. R. R. Pfeiffer$Prof. H. B. Lee Dr. A. W. B. Cunningham Dr. R. Rojas-CoronaProf. W. T. Peake$ N. I. Durlach Dr. G. F. SongsterProf. W. A. Rosenblith Dr. H. FischlerIt R. M. Brown$Prof. W. M. Siebert Dr. R. D. Hall A. H. Crist$Prof. R. Suzuki** Dr. N. Y. S. KiangJ D. P. LangbeintGraduate StudentsJ. A. Anderson R. E. Greenwood E. C. MoxonG. von Bismarck J. J. Guinan, Jr. M. NahviL. D. Braida D. K. Hartline P. H. O'LagueS. K. Burns L. K. Krakauer D. J-M. PoussartH. S. Colburn R. G. Mark M. B. SachsJ. E. Evans E. G. Merrill I. H. ThomaeJ. A. Freeman P. J. Metz III M. L. WiederholdD. C. MilneA. AUDITORY NERVE FIBER RESPONSES TO TWO-TONE STIMULI[This report gives a brief summary of a Ph.D. thesis of the same title, submitted tothe Department of Electrical Engineering, M.I.T., August 1966. Further detailsand results may be found in the thesis itself.]Electrophysiological investigations of auditory nerve fiber responses thus far havebeen limited largely to what have been called "simple" acoustic stimuli: clicks, tones,and tone bursts. Some recent findingsl' 2 have revealed rather complicated interactionsin the responses to more complex stimuli and have indicated that extension of resultsfrom simple to complex stimuli may not be straightforward. There have been fewrThis work was supported by the National Institutes of Health (Grant 2 PO1MH-04737-06), the Joint Services Electronics Programs (U.S. Army, U.S. Navy, andU.S. Air Force) under Contract DA 36-039-AMC-03200(E), the National Science Founda-tion (Grant GK-835), and the National Aeronautics and Space Administration (GrantNsG-496).TVisiting Associate Professor from the Moore School, University of Pennsylvania.Also at Eaton-Peabody Laboratory, Massachusetts Eye and Ear Infirmary, Boston,Massachusetts.Visiting Assistant Professor from the Research Institute of Dental Materials,Tokyo Medical and Dental University, Tokyo, Japan.ttResearch Affiliate in Communication Sciences from the Neurophysiological Labo-ratory of the Neurology Service of the Massachusetts General Hospital, Boston, Massa-chusetts.UFrom the Department of Electronics, Weizmann Institute of Science, Rehovoth,Israel.***Public Health Service International Postdoctoral Research Fellow from theDepartment of Physiology, Tokyo Medical and Dental University, Tokyo, Japan.QPR No. 83 159(XIV. COMMUNICATIONS BIOPHYSICS)quantitative data relating to these interactions, however. In this study we have attemptedto describe quantitatively the responses of auditory nerve fibers to somewhat more com-plex stimuli, sums of two tones. We have chosen to describe the fiber responses by asingle variable, rate of spike discharge.We have used a sweep-frequency technique 3 to construct iso-rate contours forresponses to one- and two-tone stimuli. An iso-rate contour is the locus of points inthe frequency-level plane corresponding to stimuli that evoke a specified rate of responsefrom a fiber. Such contours provide a convenient means for illustrating general charac-teristics of the dependence of rate on stimulus frequency and level. When measuringrate as a function of the frequency of one of two tones, we presented that tone as a sweep-frequency signal. If a second tone was present, it was presented as a continuous tone atthe fiber's characteristic frequency (CTCF). Figure XIV-1 shows iso-rate contours fora typical fiber for both one- and two-tone stimuli. In this figure we. have specified stim-ulus levels in terms of peak-to-peak stapes displacement. In doing so, we hope to iso-late effects produced by the cochlea from effects produced by the frequency dependence4of the middle-ear transmission.8/SECUNIT 400-19 / 100/SEC - 70RATE OF SPONTANEOUS10- 3  ACTIVITY = 8/ SEC 1 EC *8SEC 6RATE OF RESPONSE TO 20/ECCTCF @ 3 x 10-7 = 25/SEC 5010- 4  - CF = 22.8 kHz 20/SEC 4030 V-510 20S STIMULUS:CTCF ® 3.0 x 10 10- 6106 -0SWEEP FREQUENCY SIGNAL 0CTCF I-- ' - "1-TO10- 7  I | I ,I , , I I -200.1 0.5 10.0 5.0 10.0 20.0 40.0SWEEP FREQUENCY (kHz)10- 3  100/SEC -- 60140/SECS -4 10/SEC -50- 510 - 20STIMULUS: - 10SWEEP FREQUENCY SIGNAL1-6 - 010 0ci- -10-710-   I l -200.1 0.5 1.0 5.0 10.0 20.0 40.0SWEEP FREQUENCY ( kHz)Fig. XIV-1. Iso-rate contours.QPR No. 83 160(XIV. COMMUNICATIONS BIOPHYSICS)For one-tone stimuli (lower portion of Fig. XIV-1) we find a limited region in thestimulus plane within which a fiber's rate of discharge is increased above its spontane-ous level. For each fiber the "excitatory response area" is defined to be that area inthe stimulus plane bounded below by the iso-rate contour corresponding to a rate20 per cent greater than the spontaneous rate. From Fig. XIV-1 it is evident that allstimuli lying within a fiber's excitatory response area evoke a rate of response at least20 per cent above the spontaneous rate.In the two-tone stimulus situation (CTCF plus sweep-frequency signal - upper sectionof Fig. XIV-1) the fiber exhibits a region in the sweep-frequency stimulus plane in whichthe effect of the sweep-frequency signal is to increase the rate of discharge aboveits value for the CTCF presented alone. To the right and left of this region are regionsin which the effect of the sweep-frequency signal is to reduce the rate below its valuefor the CTCF alone. For any CTCF level, we define the "inhibitory response area" tobe the region in the sweep-frequency stimulus plane bounded below by the iso-rate con-tours corresponding to a rate equal to 80 per cent of the rate to the CTCF alone. Anytone with stimulus parameters within the inhibitory response area causes a reduction inthe rate of response to the CTCF of at least 20 per cent. We call the reduction in the- 100 -100~10-2  - 80 10-2- - 80Z Z Z Z- 70 L w 7010 60 10-3 60CTCF L - 50 -E5010 - 400 0 10 4030 0 30 C-5 < 5105  - 20 10-  - 2010- 6  I I llit I I Ill I I 1 IIl 0 10-6  I I 111111 I I I 11111 I l l lli 00.1 0.5 1.0 5.0 10.0 50.0 100.0 0.1 0.5 1.0 5.0 10.0 50.0 100.0SWEEP FREQUENCY ( kHz) SWEEP FREQUENCY ( kHz)100UNIT 359-2 _ 90 UNIT 400-18 -100i-2  80 1-2  - 90 %zU - 70 W z 800 -3 -3 - 70_450 i - 4 50U 10 CTCF 0 - 10  0<30 30U " 1-. "310- 5  2T0 10- 5  20- 0 ' -1010-6 I 11 11111 I I111111 I I 11111 0 10-6 011111 I I Illll I IIIII 0.1 0.5 1.0 5.0 10.0 50.0 100.0 0.1 0.5 1.0 5.0 10.0 50.0 100.0SWEEP FREQUENCY (kHz) SWEEP FREQUENCY (kHz)Fig. XIV-2. Excitatory and inhibitory response areas for four fibers.QPR No. 83 161(XIV. COMMUNICATIONS BIOPHYSICS)fiber's rate of discharge to a CTCF by the presentation of a second tone "two-tone inhi-bition." All second tones with stimulus parameters within the inhibitory response areaproduce two-tone inhibition. We have observed two-tone inhibition for more than 300fibers; we have never failed to find two-tone inhibition for any fiber when we were ableto look for it systematically.Figure XIV-2 shows excitatory and inhibitory response areas for four fibers coveringa broad range of characteristic frequencies. These areas are typical of more than 200fibers. The fibers all have inhibitory areas on both sides of their excitatory responseareas. We have found that the extent of the inhibitory areas depends on the level of CTCFemployed. An increase in CTCF level causes an upward shift in the boundaries of theinhibitory areas in the sweep-frequency stimulus plane. As is illustrated by Unit 359-2(Fig. XIV-2), if the level of the CTCF is low enough, the inhibitory area above the CFextends down to levels very close to the tip of the excitatory response area. Higherlevels are needed, however, to produce two-tone inhibition at frequencies below fiberCF. The minimum levels necessary to reduce rate by a given amount at frequenciesbelow the CF are at least 20 dB greater than the minimum levels of stapes displacementneeded at frequencies above the CF.0 0o00oo00I I I I I I I0.1 0.2 0.5 1.0 2.0 5.0CHARACTERISTIC FREQUENCY (kHz)10.0 20.0 50.0Fig. XIV-3. "Q INH" as a function of CF.INHQPR No. 83I I I I I I I , I162(XIV. COMMUNICATIONS BIOPHYSICS)An appropriately defined "Q" can be taken as a measure of the extent of the inhib-itory areas in the sweep-frequency stimulus frequency dimension. We have adoptedthe following definition of the "Q" of an inhibitory area: QINH = CF/bandwidth of theinhibitory area 10 dB above the CTCF level. Figure XIV-3 shows a scatter plot of QINHversus fiber CF for 35 fibers. The dependence of these inhibitory Q's on CF is similarto the dependence of the tuning curve Q's given by Kiang,1 in that both the tuning curveQ's and inhibitory Q's are roughly constant for CF below 2 kHz, and for higher frequen-cies both Q's increase with increasing CF.In order to collect data with which we could test mathematical descriptions of the two-responses, we used tone-burst stimuli and measured the discharge rate after the tran-sient at the tone-burst onset. Using this method, we developed the following expressionto describe the relationship between discharge rate and the stimulus parameters of thetwo-tones. (Our data are restricted to the case in which one tone is at fiber CF.)r(PCF C' P 2'f 2 ) = RSP + R(PCF fC)  f) + R(P'f2) g CF f 'wherer(PCF' C' P 2 f 2 ) = spike discharge rate as a function of the two-tone stimulusparametersPCF = sound-pressure level of the tone at the characteristic frequencyf = characteristic frequencyP 2 = sound-pressure level of the second tonef2 = frequency of the second toneRSP = r(O, fC 0, f ) = spontaneous discharge rateR(PCF' fC ) = r(PCF' fC' 0, f 2 ) - RSP = "driven part" of the rate to one tone aloneg C fC f) = "inhibitory multiplier. "The function R(P, f) is a monotone nondecreasing function of P. The inhibitory multi-plier g ( fC 2 is 1 for a small value of P2/PCF and is a monotone decreasingfunction of P 2 /PCF. This mathematical form has been adequate to "fit" all data that wehave obtained, although we lack sufficient data for certain conditions. We have sug-gested how this result might be extended to more general two-tone stimuli (neither toneat the CF) and to the sum of an arbitrary number of tones.M. B. SachsQPR No. 83 163(XIV. COMMUNICATIONS BIOPHYSICS)References1. N. Y. S. Kiang, Discharge Patterns of Single Fibers in the Cat's Auditory Nerve,M.I.T. Research Monograph No. 35 (the M.I.T. Press, Cambridge, Mass., 1965).2. M. Nomoto, N. Suga, and Y. Katsuki, "Discharge Patterns and Inhibition of PrimaryAuditory Nerve Fibers in the Monkey," J. Neurophysiol. 27, 768-787 (1964).3. R. R. Pfeiffer and A. H. Crist, "A Method for Studying Response and Inhibition Areasof Some Single Units in the Auditory System," Quarterly Progress Report No. 80,Research Laboratory of Electronics, M.I.T., January 15, 1966, pp. 232-238.4. J. J. Guinan and W. T. Peake, "Motion of Middle-Ear Bones," Quarterly ProgressReport No. 74, Research Laboratory of Electronics, M.I.T., July 15, 1964, pp. 219-221.QPR No. 83 164

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