Contains reports on two research projects

Goldstein, M. H., Jr.

Kiang, Nelson Y-S.

Lehmann, W. B.

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XIV. COMMUNICATIONS BIOPHYSICSProf. W. A. RosenblithProf. M. H. Goldstein, Jr.Dr. J. S. Barlow*Dr. M. A. B. Brazier*Dr. E. de BoerDr. L. S. FrishkopfDr. F. J. Julian**Dr. Melek Khairy* **Dr. N. Y. S. KiangDr. T. T. Sandel****F. AmorosoR. M. BrownMrs. M. Z. FreemanG. E. ForsenC. D. GeislerW. B. LehmannP. NathanW. T. PeakeA. RESPONSES AT THE AUDITORY CORTEX TO REPETITIVE ACOUSTIC STIMULISynchronous neural activity has been recorded from the whole auditory nerve inresponse to repeated stimuli presented at rates up to 2000 per second. However,cortical activity, recorded by gross electrodes, exhibits no visually detectable primaryresponses to individual stimuli at relatively low rates (approximately 100 per second),for several reasons:1. As the repetition rate increases, the amplitude of response decreases(Fig. XIV-1).2. The duration of the cortical response results in overlapping and interference ofresponses at the higher repetition rates.3. As the responses become smaller, it becomes more difficult to detect themvisually, since they are imbedded in ongoing activity. Although it is feasible to reducethe ongoing activity by the use of anesthesia, its possible effects upon the "following" ofthe evoked cortical responses necessitates the study of unanesthetized animals.o UNANESTHETIZED0 DIAL (075 cc per kg p )00ao60 .0r 40n05 I 2 5 10 20 50REPETITION RATE (sec )Fig. XIV- 1. Peak-to-peak amplitudes of average responses from unanesthetizedand anesthetized cats plotted against repetition rates for click stimuli.Steady-state responses are averaged for more than 2 minutes foreach point. Intensity level of clicks, -62 db re 1 volt across thephone. (Cat 427: active electrode near posterior ectosylvian sulcus;indifferent lead on headholder.)From the Neurophysiological Laboratory of the Neurology Service of the Massachusetts General Hospital.Postdoctoral Fellow in Neurophysiology of the National Foundation for Infantile Paralysis.Visiting Fellow, Electrical Engineering Department.Postdoctoral Fellow of the National Institute of Neurological Diseases and Blindness.135CLICK RATE- - / /I/SEC20/SEC320/SECFig. XIV-2. Averaged cortical responses from unanesthetized cat toclicks at different repetition rates. Stimulus intensitylevel, -62 db re 1 volt across phone. (Cat 430: activeelectrode near posterior ectosylvian sulcus; indifferentlead on headholder.)136A i I-O-"-IOOPV40msIOOPV40 msIOPV5 msUNANESTHETIZEDIOjv jbpi :L: Il. a1 .4, . - - ZOO/SECFig. XIV-3. Averaged cortical responses to clicks delivered at variousrepetition rates under unanesthetized and anesthetized con-ditions. Markers under each record indicate the time atwhich clicks were presented. Stimulus intensity level, -62db re 1 volt across phone. (Cat 427: active electrode nearposterior ectosylvian sulcus; indifferent lead on headholder.)137DIAL 0.75 cc/kg50/SEC100/SEC__ I ~__F-t- L I-A-kl- 1416h.- IDIAL 0.75 cc/kg20 pv 50/SECIO0/SEC'I,200/SECFig. XIV-4. Averaged cortical responses to repeated bursts of noise as recorded by concentricelectrodes. Stimulus intensity, -75 db re 1 volt rms across phone. (Cat 449:electrodes in middle ectosylvian gyrus.)UNANESTHETIZED- - -, I~ I _t.it AN MIN&W ------ k -AARM. I-L, .(XIV. COMMUNICATIONS BIOPHYSICS)In the present study, we have used both anesthetized and unanesthetized (enciphaleisole) cats. Responses were recorded on magnetic tape and processed by an electronicaverage-response computer (1, 2). By these means small responses are detectable inthe presence of large ongoing activity.Stimuli were delivered monaurally by a type PDR-10 earphone at intensity levels atwhich the auditory system is undoubtedly mechanically linear.Figure XIV-2 shows electronically computed responses to clicks at rates of 1, 20,and 320 per second. The responses at the highest repetition rate are apparently not ofcortical origin but result from spread from remote regions. These responses disappearwhen the recording electrodes are concentric: one electrode is 2 mm deep in the cor-tex, while the other is the sleeve resting on the surface of the cortex.Figure XIV-3 shows responses recorded by a monopolar electrode to clicks deliveredat rates of 50, 100, and 200 per second. Since the time scale has been expanded, thewaveforms of the responses are more clearly outlined. At 200 clicks per second onlyactivity of short duration remains in responses from both unanesthetized and anesthe-tized preparations. It is this type of activity, locked to the stimulus, that constitutesthe responses to the 320 per second clicks shown in Fig. XIV-2; the concentric elec-trodes do not record this activity. In unanesthetized cats, but not in cats under Dialanesthesia, the concentric electrodes record synchronous activity in response to 100clicks per second.Similar results are obtained when bursts of noise, rather than clicks, are used asstimuli (Fig. XIV-4). With the concentric electrodes, synchronous responses arerecorded when the stimuli are delivered at rates up to 200 per second in the unanesthe-tized animal, but only up to 100 per second in the anesthetized cat.It is possible that at different stimulus intensities, the limits of synchronous follow-ing would occur at different repetition rates; however, the present experiments are con-fined to approximately 30 db above the threshold of the cat's electric responses. Itshould be noted that following, as recorded by gross electrodes, deals with averagedresponses from many neurons. Such records do not necessarily imply that single unitswould follow at these rates.In summary, at low intensity levels the auditory cortex of unanesthetized cats appearsto follow stimulation by clicks and bursts of noise up to rates of 200 per second.M. H. Goldstein, Jr., N. Y. S. KiangReferences1. J. S. Barlow and M. A. B. Brazier, Quarterly Progress Report, Research Labora-tory of Electronics, M.I.T., April 15, 1955, p. 79.2. J. S. Barlow and R. M. Brown, Technical Report 300, Research Laboratory ofElectronics, M.I.T., July 14, 1955.139(XIV. COMMUNICATIONS BIOPHYSICS)B. A VERSATILE STIMULUS PROGRAMMING DEVICEA wide-range time-delay generator for electrophysiological and psychoacousticexperimentation was designed and built. The device enables the experimenter to controlthe temporal spacing and the duration of stimuli. In each of these ranges the delay canbe varied over a decade by means of a continuous control. In addition, the stimulus pro-gramming device will - after the occurrence of a prescribed delay - either initiate orterminate a stimulus the first time a reference waveform passes through a given ampli-tude in a given direction.The stimulus programming device is composed of four "building blocks" that can beinterconnected in whatever combinations the experimenter desires. One combination isshown in Fig. XIV-5. With the switch in the other position, the device must be exter-nally triggered at the beginning of each cycle. When the equipment is first turned on,or after the cycle has been interrupted for any reason, points F, G, and H (and there-fore K) will be at a dc potential of less than 150 volts, with the result that the initial-pulse generator initiates a rundown in delay generator I. Thereafter, unless operationREFERENCE SINUSOID PHASEDISCRIMINATORTIME-DELAY "AND" TIME- DELAYINPUTGENERATOR I CIRCUIT -GENERATOR EINITIAL-PULSE0 @ 0GENERATORWAVEFORMS® TI I l I I@ I I i+ 300VFig. XIV-5. Block diagram of stimulus programming device.140(XIV. COMMUNICATIONS BIOPHYSICS)is interrupted, either F, G, or H will be at a potential of greater than 150 volts, andthe initial-pulse generator will be effectively disconnected from the circuit. The "AND"circuit stores the information that the delay generated by the first time-delay generatorhas occurred until the reference sinusoid reaches the desired phase, and then triggersdelay generator II. The output of the second delay generator re-initiates the cycle.(The outputs of the time-delay generator and the "AND" circuit are identical, so thateither could have been used to trigger the second delay unit.) As many delay genera-tors and phase discriminators as desired may be included in the loop, and both pulseand gate outputs are available.The time-delay generator has 11 coarse ranges, with each range providing a decadeof variation of output duration. Fine control is accomplished by means of a variable decontrol voltage, which may be located many feet from the equipment. The time-delaygenerator is linear to higher than ±0. 2 per cent over the entire range, and exhibits lessthan 0.05 per cent jitter. A 10 per cent change in heater voltage produces a 0.1 percent change in output duration. A 10 per cent change in plate-supply voltage also pro-duces a 0. 1 per cent change in output duration, but in the opposite direction, therebymaking the performance of the delay generator extremely insensitive to line-voltagevariations. The input of the delay unit must be greater than 10 volts in magnitude andhave a rise time of less than 2 psec. A cathode-coupled phantastron is used to generatethe time delays. Since the phantastron rise time is a function of the delay length, theoutput of the phantastron is used to trigger a Schmitt circuit whose output is differenti-ated, clipped, and shaped, and then fed to a cathode follower to provide uniform, low-impedance output pulses.The phase discriminator has a flat response from 0. 25 cps to 20 kc. The referencesignal is first amplified 30 times in an ac amplifier, with 40 db of negative feedback andnegligible phase shift over the operating range. By means of an attenuator, the outputof the amplifier is set to 100 volts peak-to-peak, and this signal is impressed on oneinput of a difference amplifier. A control voltage on the other grid of the differenceamplifier makes it possible to select any 5-volt "slice" of the 100-volt signal andamplify it at the plate of the difference amplifier. This amplified horizontal "slice" ofthe reference voltage is used to trigger a Schmitt circuit whose output is differentiatedand used as an input of the "AND" circuit. The linearity of the circuit is betterthan ±1 per cent over the entire range, and jitter is negligible. Short-term de drift isminimized by putting both halves of the difference amplifier in the same envelope.Long-term drift is not troublesome, since neon indicators allow quick calibration with-out the use of meters or additional equipment.W. B. Lehmann141

Communications Biophysics

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Communications Biophysics

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