Contains reports on two research projects.United States Air Force (Contract AF19(604)-4112

Berger, Eda

Geisler, C. D.

Gutierrez, O. C.

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*XVIII. COMMUNICATIONS BIOPHYSICSProf. W. A. Rosenblith Dr. N. Y-S. Kiang A. H. CristProf. M. H. Goldstein, Jr. Dr. J. W. Kuiper**' G. E. ForsenDr. J. S. Barlowt Dr. T. T. Sandel Margaret Z. FreemanDr. M. A. B. Braziert W. A. Clark, Jr. tt C. D. GeislerDr. Eda Berger$ O. C. Gutierrezttt C. E. MolnarDr. M. Eden** Aurice V. Albert D. F. O'BrienDr. B. G. Farleytt J. Allen$$$ W. T. PeakeDr. G. L. Gersteinj$ R. M. Brown T. F. WeissJ. D. Cowan****A. EVOKED CORTICAL RESPONSES TO REPETITIVE PHOTIC STIMULATIONIn the first of a contemplated series of systematic studies dealing with the electro-physiology of the visual system, evoked potentials in response to photic stimulationwere recorded from the cortex of the anesthetized cat.A photic stimulator designed to provide precise control of such variables as stimu-lus intensity, duration, repetition rate, and wavelength served as the stimulus source.Stimulation was monocular; the evoked potentials were recorded from the contralateralcortex by means of monopolar surface electrodes made of nichrome wire. Activity wasrecorded from each of two cortical sites: from the striate area and from the visualassociation area on the dorsal surface of the marginal gyrus (1). Six cats served asexperimental subjects. Surgery was performed while the animals were under Dial andurethane anesthesia; no additional anesthetic was administered during the several hoursrequired for the completion of a given experiment.The stimulus variables investigated in this study were flash intensity and flash dura-tion. Intensity was varied in fixed steps over a range of at least four log units; theflash durations employed encompassed the range from 12. 5 msec to 1000 msec. Whitelight was used throughout, and flash repetition rate was kept constant at 5 sec. The-, This work was supported in part by the U. S. Air Force under Contract AF19(604)-4112.tFrom the Neurophysiological Laboratory of the Neurology Service of the Massachusetts General Hospital.TPostdoctoral Fellow of the National Institute of Mental Health.**Research Associate on leave from the National Heart Institute.tStaff Member, Lincoln Laboratory, M. I. T."Postdoctoral Fellow of the National Institute of Neurological Diseases and Blindness.Sloan Postdoctoral Fellow of the School for Advanced Study, M. I. T.ttDoherty Foundation Fellow.tlRamo-Wooldridge Fellow in Systems Engineering.****Graduate Fellow of British Tabulating Machine Company, Ltd.179(XVIII. COMMUNICATIONS BIOPHYSICS)animal was kept in a dark anechoic chamber, and adequate precautions were taken toensure a constant state of dark adaptation prior to each experimental series. For eachintensity-time combination of the stimulus, evoked potentials were recorded continu-ously for a six-minute period. These responses were subsequently processed by meansof the ARC-1 computer (2) to yield a composite averaged response.The evoked cortical potentials recorded in this fashion exhibit the following features:There is an "early" response that appears to be related to the onset of the photic stimu-lus, as well as a "late" response that seems to be related to the cessation of stimulation.Accordingly, over the range of values investigated, the early response, in its simplestform, seems to be independent of flash duration, whereas the time of occurrence of thelate response is governed by flash duration. (See Fig. XVIII-1.)Both the early and the late responses undergo characteristic changes as a functionof stimulus intensity. Their latencies (i. e., the elapsed time between the onset of thestimulus and the first measurable deflection of the evoked potential) vary inversely withthe intensity of the flash. The time of occurrence of characteristic maxima and/orminima in the waveforms of both of these responses shows a similar dependence onstimulus intensity. (See Fig. XVIII-2.)Variations in flash intensity result in another type of modification of the earlyresponse, namely, the emergence of a secondary component at higher intensities. Com-pared with the initial event of the early response, this secondary component seems morecomplex and variable, in that its elicitation is more critically dependent upon appropri-ate intensity-time relations of the stimulus. Thus, although there is no evidence for asecondary component of the early response at low levels of stimulation, it does manifestitself at high flash intensities. At intermediate intensity levels, where the secondarycomponent first emerges, it seems to interact with the late response; the nature of thisinteraction is unpredictable and typically distorts the time sequence among the variouscomponents of the total response pattern. Furthermore, even at high intensities whereit can be identified, the latency of this secondary component (approximately 100 to200 msec) is such that it may effectively block the appearance of the late response.Consequently, with stimuli whose duration is less than approximately 200 msec,the complete response pattern in all its details may not emerge. However, whenflash duration is sufficiently long and intensity sufficiently high that interaction (seeFig. XVIII-3) among various response components is minimized or eliminated, thetotal response pattern can be seen. (See Fig. XVIII-4.)The rate of stimulus presentation also seems to affect the evoked cortical potentials,most notably their amplitude. (See Fig. XVIII-5.)Precise quantitative evaluation of various features of evoked cortical potentialsobtained under these conditions is complicated by our inability to specify the depth ofanesthesia of the animal at the time when a given response sample is being recorded.180LiLJ------ r--100 .~V100 MSECFig. XVIII-1. Evoked cortical potentials from visual association area to low-intensityphotic stimuli. Each tracing represents the average of 70 consecutiveresponses. Flash durations: 12. 5, 25, 50, 100, and 200 msec. (Cat Sh.).Fig. XVIII-2. Evoked cortical potentials from visual association area to a 100-msecflash at 2 intensity levels, 4 log units apart. Each record is the aver-aged response to 70 consecutive flashes. (Cat Reu.).181HIGH INTENSITY100 P v100 MSECI. JLOW INTENSITYMEDIUM INTENSITYHIGH INTENSITY100v100 MSECFig. XVIII-3. Evoked cortical potentials from visual association area to a 200-msecflash at 3 intensity levels, separated by 2 log unit steps. Averagedresponse to 70 consecutive flashes. (Cat Reu.).0 MSEC100 MSEC1000 2000Fig. XVIII-4. Evoked cortical potentials from visual association area to a high inten-sity flash. Averaged response to 64 consecutive flashes. Flash dura-tions: 100, 200, 500, and 1000 msec. Last two traces refer to the1000-msec stimulus and cover a total time period of 2000 msec to showlate response following cessation of stimulus. A and B, primary andsecondary component of the early response; C, late response. (Cat Go.).182AAB CTIME AFTERANESTHESIA2 HR 30 MINI I3 HRREPETITION RATEI ISE5 SEC30 SECOOv100 MSECFig. XVIII-5. Evoked cortical potentials from vis-ual association area to a 100-msecflash of low intensity. Averagedresponse to 70 consecutive flashes.Elapsed time between consecutivestimulus presentations: 1, 5, and30 sec. (Cat Sh.).5 HR7 HR 35 MIN100 /v100 MSECFig. XVIII-6. Evoked cortical potentials from visual asso-ciation area to a 100-msec flash of low inten-sity at various times following administrationof Dial and urethane anesthesia. Averagedresponse to 70 consecutive flashes. (Cat Sh.).r_jr(XVIII. COMMUNICATIONS BIOPHYSICS)TIME AFTERANESTHESIA2 HR 50 MIN4 HR 16 MIN5 HR 31 MIN7 HR 5 MIN8 HR 25 MIN100 /v100 MSECFig. XVIII-7. Evoked cortical potentials from visual association area to a 100-msecflash of low intensity at various times following administration of Dialand urethane anesthesia. Averaged response to 70 consecutive flashes.(Cat Es.).In an attempt to gauge the effects produced by this variable, each of our animals wasexposed to a uniform stimulus situation at various times following the last injection ofDial. A 100-msec flash at each of two intensity values was employed for this purpose.The averaged responses to these test stimuli fail to reveal any consistent trend. Forsome animals there is a progressive reduction in response latency, as well as anincrease in response amplitude as a function of elapsed time following administrationof. Dial (Fig. XVIII-6), but others manifest no consistent changes over similar time peri-ods. Still others show first a decrease in response amplitude, followed by an increase(Fig. XVIII-7). These factors must be taken into consideration for each particularanimal in evaluating response parameters of evoked cortical potentials.O. C. Gutierrez, Eda BergerReferences1. R. W. Doty, Potentials evoked in cat cerebral cortex by diffuse and by punctiformphotic stimuli, J. Neurophysiol. 21, 437-464 (1958).2. W. A. Clark, Jr., Average response computer (ARC-I), Quarterly ProgressReport, Research Laboratory of Electronics, M. I. T., April 15, 1958, p. 114.B. ELECTRICAL RESPONSES TO ACOUSTIC CLICKS IN MONKEYSDuring a study of potentials evoked by acoustic clicks in human subjects (1), it wasobserved that the responses recorded from scalp electrodes were not localized over the184ELECTRODE22 ,L-- ----(a)/IV S5 MSE' 50 MSEC -i(b)Fig. XVIII-8. Mean responses recorded from an intact monkey. 900 clicks, approx-imately 30 db above visually detectable threshold were presented ata rate of 10 per second. Monopolar recordings; negativity is plottedupward. Solder-blob electrodes on the scalp were used to record (a);needle electrodes on the skull were used (skin intact) in (b). Dialanesthesia.100 MSECFig. XVIII-9. Mean responses recorded from exposed pia. 900 clicks 20 dbabove threshold were presented at a rate of 5 per second.Monopolar recordings; negativity is plotted upward. SF standsfor sylvian fissure; STS, superior temporal sulcus; SS, simiansulcus; C, central sulcus; IPS, inferior postcentral sulcus. Dialanesthesia.185(XVIII. COMMUNICATIONS BIOPHYSICS)temporal regions. Rather, responses were obtained from widespread electrode loca-tions. The intervening layers of dura, skull, and scalp would be expected to have someeffect upon the spread of these cortical potentials, yet potentials evoked from cortexlying directly beneath the skull (sensorimotor cortex) have been obtained from well-localized regions on the human scalp (2). The human auditory cortex, however, isknown to lie within the sylvian fissure and its greater distance from the skull mayaccount for the widespread responses to auditory stimuli.A study of the problem of localization of electric potentials evoked by acoustic clickswas undertaken. Cynomolgus monkeys were used because their auditory cortex lieswithin the sylvian fissure. Observations were limited to two monkeys. Auditory clicks,generated by 0. l-msec electric pulses delivered to a PDR-10 earphone, were presentedto the anesthetized monkeys, and responses were recorded at various levels between thescalp and the cortex. Both monopolar and bipolar recordings were made; in the firstinstance, electrodes imbedded in the cheek bones served as indifferent electrodes. Sincethe evoked potentials were too small to be visually detectable in the background activity,the average response computer (3) was used. The curves shown represent the averageof responses (or mean response) to 900 consecutive click presentations.Figure XVIII-8 shows responses recorded monopolarly from the intact head. Themean responses shown in Fig. XVIII-8a were recorded from the shaven surface of amonkey's scalp with small solder-blob electrodes similar to those used on human sub-jects. Notice that although responses are seen, they were obtained from such widelyseparated locations as those situated over the temporal and occipital regions. However,when needle electrodes (insulated, except at the tip) are driven into the skull at thesesame spots, the responses recorded from the temporal electrode increase considerablyin amplitude (Fig. XVIII-8b), while responses from other electrodes show little change.Figure XVIII-9 shows monopolarly recorded responses from the pial surface of theexposed cortex. As previously reported by Pribram, Rosner, and Rosenblith (4) forrhesus and mangabey monkeys, responses were obtained from widespread locations.Mean responses were obtained from all points tested, although waveshape and magnitudeare quite variable. Notice the large well-localized response at point 2, just rostral tothe sylvian fissure. In this particular experiment, recordings were also made at thevarious levels encountered as the cortex was being exposed. Responses obtained fromdural electrodes directly above the numbered points on the pia closely resemble, inwaveform and magnitude, the responses obtained from the pia, except at point 2, wherethe response amplitude dropped from 5 pv to 1 Lv. Responses were also obtained fromthe skull (scalp intact) directly above these same five points, and showed little changein waveshape, although they were only about half as great in magnitude as thoserecorded from the dura. When the scalp was removed, responses obtained from skullelectrodes closely matched those recorded from the corresponding dural electrodes.186(XVIII. COMMUNICATIONS BIOPHYSICS)Finally, responses were recorded from the exposed auditory cortex, after subpialresection of the superior temporal gyrus. Responses of the same shape as thoserecorded from point 2 in Fig. XVIII-9 were obtained, but the amplitudes ranged from50 4v to 70 .Lv.These data show that although 70-Lv responses were recorded from the monkey'sexposed auditory cortex, responses recorded from the surface of the intact cortex were,at most, 5 4v in amplitude. The attenuating effect of dura and scalp is such thatresponses obtained from solder electrodes on the intact scalp record widespreadresponses of approximately 1 4v. The shunting effects of the dura and scalp, bothpresumably good conductors, are striking. At both of these layers, when well-localizedresponses were recorded from points beneath them, greatly reduced responses wererecorded from the corresponding points above them. Poorly localized responses, onthe other hand, are almost unaffected. Clearly, some sort of spatial averaging istaking place.Although these conclusions are not directly applicable to human subjects, certainlyhuman dura and scalp will produce similar attenuation of cortical responses. Andsince human auditory cortex is also within the sylvian fissure, responses recordedfrom the surface of the cortex and from the scalp may be expected to be small andwidespread.C. D. GeislerReferences1. C. D. Geisler, L. S. Frishkopf, and R. M. Brown, The "early" response toclicks in awake subjects, Quarterly Progress Report, Research Laboratory ofElectronics, M.I.T., July 15, 1957, pp. 144-148.2. G. D. Dawson, Cerebral responses to electrical stimulation of peripheral nervein man, J. Neurol. Neurosurg. Psychiat. 10, 134-140 (1947).3. W. A. Clark, Jr., Average response computer (ARC-1), Quarterly ProgressReport, Research Laboratory of Electronics, M. I. T., April 15, 1958, pp. 114-117.4. K. H. Pribram, B. S. Rosner, and W. A. Rosenblith, Electrical response toacoustic clicks in monkey: extent of neocortex activated, J. Neurophysiol. 17,336-344 (1954).187

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