Contains a report on a research project.National Science Foundation (Grant GP-2495)National Institutes of Health (Grant MH-04737-04)National Aeronautics and Space Administration (Grant NsG-496

Barlow, J. S.

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XIII. COMMUNICATIONS BIOPHYSICSProf. M. Eden Dr. R. Rojas D. P. Langbein**Prof. J. L. Hall II Dr. R. H. Wendt P. L. MarcusProf. W. T. Peake** R. M. Brown** R. G. MarkProf. R. R. Pfeiffer** S. K. Burns E. G. MerrillProf. W. A. Rosenblith R. R. Capranica M. NahviProf. W. M. Siebert R. J. Clayton P. H. O'LagueProf. T. F. Weiss** A. H. Crist** Ann M. O'RourkeDr. J. S. Barlowtt N. I. Durlach D. J. M. PoussartDr. A. W. B. Cunninghaml$ J. L. Elliot Cynthia M. PyleDr. E. Gibermant P. R. Gray M. B. SachsDr. Gisele Guilbaudt J. J. Guinan, Jr.** J. J. SingerDr. R. D. Hall F. N. Jordan Judith E. SwensenDr. N. Y-s. Kiang** K. C. Koerber** I. H. ThomaeDr. G. P. Moore*** M. L. WiederholdA. RHYTHMIC AFTERDISCHARGE TO FLASHESIn response to slowly repeated flashes of light, the parieto-occipital EEG of humansubjects with closed eyes frequently exhibits a series of rhythmic voltage fluctuations,a phenomenon that has been called the rhythmic sensory afterdischarge (references havebeen given elsewhere ). This rhythmicity is usually not evident upon visual inspectionof the EEG inked trace, but becomes detectable by the use of averaging techniques. Thefrequency of the rhythmic afterdischarge may be quite close to, if not identical with, thepredominant frequency of alpha activity. Therefore components of the EEG might appearin the averaged responses merely as sampling artefact and would not represent a trueafterdischarge. Results from a series of mock-up experiments to illustrate this pointhave been described elsewhere. Although the possibility of such an occurrence isgreater the smaller the number of responses averaged, it cannot be eliminated even foraverages of as many as 200 stimuli.Several types of controls to test the validity of such averaged responses weredescribed previously. 3' 4 These include the use of arhythmically as well as rhythmicallyrepeated stimuli, the use of several slightly different flash rates for periodically repeatedThis work was supported in part by the National Science Foundation (Grant GP-2495);in part by the National Institutes of Health (Grant MH-04737-04); and in part by theNational Aeronautics and Space Administration (Grant NsG-496).'From the Department of Physics, Weizmann Institute of Science, Rehovot, Israel.t Attach6 de, Recherches, Inst. Nat. d'Hygione and Centre d'Etudes de PhysiologieNerveuse et d'Electrophysiologie, Paris, France.Also at Eaton-Peabody Laboratory, Massachusetts Eye and Ear Infirmary, Boston,Massachusetts.tResearch Affiliate in Communication Sciences from the Neurophysiological Labora-tory of the Neurology Service of the Massachusetts General Hospital, Boston, Massachu-setts."Postdoctoral Fellow, National Institutes of HealthPostdoctoral Fellow, National Academy of Science.QPR No. 75 149(XIII. COMMUNICATIONS BIOPHYSICS)flashes, and the use of an opaque cover on the flash source so that no light reaches thesubject's eyes. It should be noted that when no stimuli are applied it is assumed that allthe statistical characteristics of the EEG are the same as they are when stimuli are pre-sented. Such an assumption is not made, however, if the "control average traces" arecarried out on the same recordings that were used to determine the averaged evokedresponses. For these controls the averaging computer can be triggered at times thatare shifted with respect to the actual instant of stimulation. When the stimuli are pre-sented at random intervals, triggering the computer after a fixed initial delay, which isseveral times the average interval between stimuli, provides one such control. 3 Thisprocedure, however, does not eliminate the possibility that there might be remnants ofstimulus-locked activity in the EEG after such large delays. On the other hand, trig-gering the computer at a fixed interval of time before the occurrence of a stimulusentirely eliminates this possibility.For periodic stimuli, the averaged response is simply repeated for integralpositive or negative multiples of the interstimulus interval; hence this proceduredoes not provide a control for a sampling artifact. For periodic stimuli, however,the computer can be triggered by repeated pulses separated by intervals that differslightly from the interflash interval. Alternatively, the computer can be triggeredwith randomly recurring pulses whose average interval is approximately the inter-stimulus interval.Illustrations of these controls for averaged evoked responses are shown inFigs. XIII-1, XIII-2, and XIII-3. The visual stimuli produced by a stroboscope (GrassModel PS-2, intensity setting 4, peak flash intensity 4, 000, 000 lumens). The stroboscopewas triggered by means of a repetitive-pulse generator (Tektronix Type 162). For ran-domly recurring stimuli, the latter was triggered by random low-frequency electricalnoise from a noise generator (Elgenco Model 312A). The minimum interstimulus inter-val was 0. 8 sec; the average interval was 1. 2 sec.The averaged evoked responses were computed with the aid of the Analog Computerfor Neurophysiology, ' 6 modified for the simultaneous averaging of two channels on iden-tical or on different time bases.7 The autocorrelograms, which were computed with theAnalog Correlator for Brain Potentials, 8 and the averaged responses were evaluatedsimultaneously. The two results were written out on the same time scale by a two-channel ink-writing galvanometer (Texas Instruments Dual Rectiriter). From the dis-play of such a write-out (Fig. XIII-3), the time structure of the averaged response andthe period of the rhythmic afterdischarge can be compared directly with the time struc-ture of the EEG from which the average was obtained (for example, its average periodand degree of rhythmicity). Such a direct comparison of the averaged evoked responsewith the EEG from which it is derived would not be possible, of course, if the summaryanalysis of the EEG were displayed as a spectrum.QPR No. 75 150R)0-.J(3' (a)T0M(b)o=-I.5(c)ro= -0.8(d)1o-+0.8DELAY (SEC)0 0.2 0.4 0.6 0I'I_ M-----.81-- -I0 0.2o=(6)+ .60.4 0.6 0.8I .... IIo-,i .. .C,-L--~~~~~~~~ r--- --- -- I----- --- -i --- I '... ...i t- .. .. . .---t  .. .. r -~ + ... ; .... ... t......  . ...- - (g) r * tWIBM(f)I/SEC 91Tg"+3.... ~ i- ~ -- ,------- ...... d _ . . .:+ (g) F ........ .- ,+o+.---" (h)- -----;--* I----- ! ,=,. : L -- - 1 --- ---- -I ---F /- -SEG ! --  ......../Fig. XIII-1. Averaged responses with different initial delays for a 4-minute parieto-occipitalEEG recording. Aperiodic flashing, eyes closed. Subject: W. A. R.I 5 vEYES CLOSED(a)APERIODIC(b)PERIODICDELAY (SEC)0 0.2 0.4 0.6 0.8-~I.... ... --- I -- .... O T _ _ ....---_ .. ..i ....... A ...... I ,,......EYES OPEN-- t-t- --- 2-?----- V-1----.(c)APERIODICT=0(d)T =-I SECFig. XIII-2. Averaged responses for aperiodic and periodic flashing witheyes closed and open. Subject: W. A. R.QPR No. 75I5 v~-i ~~~-:-I-~ ~~~- ~ ---------'----I---:---...a.r.-~Jr-1BBr--~--- - ;;;;- i--------t,-----~3ifir- --- ~P~i-l--152EYES CLOSEDAPERIODIC STROBEDELAY (SEC)0.2 0.4 0.6EYES OPENAPERIODIC STROBE0.8 0 0.2I I IDELAY (SEC)0.4 0.6 0.8I I I-1 c ------ -- I ------ i -----~ -------: :~--i --; -----"- -'- -;~-:-i~ -~~.i,..--L--.;.:j-- 1--- -6:- --iI:11(b)(C-i-t 7... . .... t-" -.... ... . . ..EYES CLOSEDPERIODIC STROBEwa ---- -~- -" 1~;~-Y. OEYES CLOSEDRESTING EEG- # '-- -4. 1(g)t(d) -Fig. XIII-3. Averaged responses and autocorrelograms for the midline parieto-occipital EEG under various conditions. Subject: W. A. R.QPR No. 750(a) jI5 vif153i ,,i; Air;r::(h)(XIII. COMMUNICATIONS BIOPHYSICS)In Fig. XIII-la, the averaged evoked response from a 4-minute midline parieto-occipital EEG recording (eyes closed) is shown for randomly recurring flashes. Twohundred responses were averaged. There is a very prominent rhythmic afterdischargewith a frequency of "10/sec. The peak-to-peak amplitude of individual waves rangesfrom 5 to 10 pv. The averaged responses shown in Fig. XIII-lb through h were com-puted as controls for this result. The results of b through g in Fig. XIII-1 may be con-sidered as an ensemble of averages whose initial delays (T ) range from -1. 5 sec to+6 sec. The departures from a flat baseline indicate the extent of artifact in the aver-aging process due to sampling error. The maximum peak-to-peak of these randomexcursions in any one average is approximately 2. 5 pv, or approximately 1/4 the max-imum peak-to-peak amplitude of the "real" response shown in Fig. XIII-la. For theresult shown in Fig. XIII-lh, the computer was triggered completely independently ofthe original EEG recording by a periodically recurring pulse which had an effective rep-etition rate of 1/sec. Because of this complete independence of computer triggering andoriginal EEG recording, the successive pen excursions in Fig. XIII-lh are also independ-ent, in contrast to those in Figs. XIII-lb through g. The peak-to-peak amplitude of thesampling-artifact error in Fig. XIII-lh is approximately 2. 5 [1v, which is in agreementwith that shown in Figs. XIII-lb through g. (A similar computation for the resting EEGis shown in Fig. XIII-3g. )After consideration of the results described above, there is little question about thesignificance of the rhythmic afterdischarge shown in Fig. XIII-la; it is clearly not aresult of a sampling artifact.In Figs. XIII-2a and b, the responses evoked by arhythmic and by rhythmic stimula-tion (flash-rate 1/sec) are compared, and it is evident that the two are quite similar. InFig. XIII-2c the evoked response for arhythmic stimulation for open eyes is shown. Theinitial delay of the control for this computation was set at -1 sec (Fig. XIII-2d). It isevident that an afterdischarge is definitely present when the eyes are open, although itis neither as great in amplitude nor as long in duration as that for eyes closed (seeFig. XIII-2a). The frequency of the afterdischarge in the two instances is very muchthe same. Comparison of the corresponding peaks of the waves for averages for eyesopen and for eyes closed indicates that the latency of the afterdischarge is decreased byapproximately 30 msec if the eyes are open. Alternatively, this apparent difference inlatency could be ascribed to an initiation of the afterdischarge in an earlier phase if theeyes are open. Whether this effect is due wholly, or only in part, to the removal of thefiltering effect of the eyelids cannot be stated from these results alone.In Figs. XIII-3a and b, c and d, e and f, the averaged evoked responses for aperi-odic stimulation with eyes closed, for periodic stimulation with eyes closed, and foraperiodic stimulation with eyes open are compared with the autocorrelograms ofthe corresponding EEG recordings from which the averaged evoked responses wereQPR No. 75 154(XIII. COMMUNICATIONS BIOPHYSICS)computed. For the averaged response shown in Fig. XIII-3g, and computed from theresting EEG recording, the computer was triggered entirely independently of the EEGrecording by a pulse whose effective repetition rate was I/sec.From the autocorrelograms in Fig. XIII-3 it is evident that the average or dominantfrequency is ~10/sec in all cases. The degree of rhythmicity, as evidenced by the rateof decrement of the amplitude of the envelope of the correlogram, is highest for theresting EEG recording, and lowest for photic stimulation with open eyes. It is of par-ticular interest that the shorter persistence of rhythmicity in the autocorrelogram foreyes open is paralleled by a shorter duration of the sensory afterdischarge for the samerecording (Figs. XIII-3e and f).These results confirm and extend earlier reports 4 of the extent to which the rhythmicsensory afterdischarge and the intrinsic alpha activity exhibit parallel behavior in theparieto-occipital region of the cortex in man. Additional statistical tests have been car-ried out as a part of the computation in order to establish the validity of the phenomenonof the rhythmic afterdischarge. It is hoped that further study of the rhythmic afterdis-charge may shed some light on the still unsolved problem of the exact origin and natureof intrinsic alpha activity.This work was supported at the Massachusetts General Hospital by a U. S. PublicHealth Service Career Program Award (Number 5-K3-NB-9201), and by a Public HealthService Research Grant (NB-03752), from the National Institute of NeurophysiologicalDiseases and Blindness.J. S. BarlowReferences1. J. S. Barlow, Evoked responses in relation to visual perception and oculomotorreaction times in man, Ann. N.Y. Acad. Sci. 112 (Art. 1), 432-467 (1964).2. J. S. Barlow, Discussion of papers, Ann. N.Y. Acad. Sci. 112 (Art. 1), 219-223 (1964).3. J. S. Barlow, Relation of intrinsic alpha activity to rhythmic activity induced byphotic stimulation in man, Quarterly Progress Report No. 48, Research Laboratory ofElectronics, M.I.T., January 15, 1958, pp. 128-133.4. J. S. Barlow, Rhythmic activity induced by photic stimulation in relation tointrinsic alpha activity of the brain in man, EEG Clin. Neurophysiol. 12, 317-326 (1960).5. J. S. Barlow, A versatile analog computer for neurophysiological research,Quarterly Progress Report No. 61, Research Laboratory of Electronics, M. I. T., April1961, pp. 184-185.6. J. S. Barlow, A versatile analog computer for neurophysiological research,Digest of the 1961 International Conference on Medical Electronics, p. 27.7. J. S. Barlow, Some statistical characteristics of electrocortical activity in rela-tion to visual-oculomotor tracking in man, Bol. Inst. Estud. M6d. Biol. (Mexico) 21,497-518 (1963).8. J. S. Barlow and R. M. Brown, An analog correlator system for brain poten-tials, Technical Report 300, Research Laboratory of Electronics, M.I.T., July 14, 1955.QPR No. 75 155

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