Life Scisncas Vol . 17, pp .
Printed is the U .S .11 .
Psrgamon Press
683-692
T~ VISUAi. EVO~D Tiao~ip J. Tglu Dtpartraat of Pgcholop a~ Social RsLtiou Ha:Vard IIaiVartity, Cebrid=a, 1!A 02138 E. Esil Cuffia aad David Cohea >'raa~ia Eitter Batioaal Ma=aat Laboratom Maaeacbatetts Instttad of Tschaoloay Canbrid=e, 1IG 02139
(Received
in Eiaal
form
Julp 24, 1975)
Tbs Visual enokad electroeacephalosrar (VER) aad taiaetoaocepbalo=rar (IOOG), rhich ~eatores tba asEoatie lield aosral to tlr acalp, rere tirultsasooaly obtaiaad ia raa. Ths arplitnds and latency of rupoata o! r~ ras cosgiarsd u a laactioa o! atirtlw intaatity aad tha aaEnlar dLtribation of tbs 1~G atouod ths btad ; a aearab rw alto arde !br hüh lregwacy coa~oaeata oi ths LIEli (> 100 8s) . 1~G ard VER boW aborad aa iacseased arplitade of rapoese u a laoction o! ttirnlw intaosity . Ia contrat to tha VER, tba Ltaacy of ths lOG did aot appur to decrsass u a lnaetion of atirnlna intuaity . At rith tha VER, tha lOG contaiaad no hüh lrpwncy t. :ruter than tha e:periraatal aoias. Ths rssnlte ars discuaasd ia rsLti.oa to tha ~Oar:tors o! the lODG sad YER. The tare elsctroch~+~~~ activity of the brain rhich produce potential an the scalp also prodncsy a rsEOatic field aranad the head ; this ream Bald hu been datsetsd rith seaaitive rasastarstsre (6,7) and is tesrad tM ra=natosn cephalograr (l01G) . It is claiasd that tbs 1~G ha several adVaata=q aces the slsctrowcephalo=rar (EEG) (6,7) . !or asarple, at for frsgwnela (DC to 0.1 Hs) the 1DLG is free o! interlsraaca due to chaaEing skin-elactrods potsatials ; alo !or aqy lregaaacy the 1laG can raaura sane aanral source diatribetiona Which era supprseasd oa the EEG (7) . I! the IOZG is abls to record the hüh fragwacy svsnts associated rith action potential pnaration, it say help clarify the contranrq surramding the aanrowi ~assatosa o! the EEG sad evoted raspooae (8,10,16,17,21) . The 1®C is able to reveal inforrtion about brain function in a rannar diflsrsat Eras the ERG since ragnetic raasurersats raflsct charge aovusnts (carreats) rather than potential diffsreaeq (chariu) . The tw ruauru ass reltsd because the oars ltie aekaorledae the tsehaical and editorial aaistaaee of ltra . Chris Poor sad Ma . äaacy Detroad. This ruaarch ra supported is part by ARP", by a subcontract no . 110014-70-C-0330 . 683
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Magnetoencephalography
Vol . 17, No . 5
currents producing potentia]. differences within and on the surface of the tlseus aLo produce a srgaetic field . Thw the futures of the SßG and MEG are sisilar is saw canes. However, for several unique generator distributions the 88C and MEG rill be disaisilas . !or a:asple, a current dipole located on and oriented along the a>a.s of a voluse conductor having axial syartry rill produce potsntial diffgrencu on the surface of the conductor but sgro sagnetic fields outside the conductor. This situation is not unlike the aaatasical projactiona of Conversely, the optic radiations Eros the lateral geniculatg to striate torte:. a üaifoa surfue potential can bs associated with nonsero current in the voluse conductor and thus a generated sagngtic field (7) . Ths latter condition prob ably does not occur in the brain, however. In abort, the MEG and SEG in conjuaetion should M able to localise the source of a neural event better rhea either aloha. before the possible applications of the MEG can be properly unused, it is necessary to density the saia features of the MEG under a variety of coaditioos . Aa part of this effort, w haw detersined the nature of the MBG ruponu to a etaadard savory stisnlna (visual flash) cad casipared the fgaturu of the visually evoked MEG with the concurrently recorded vianal evoked EEG raponsg (~) . -8 gansa. The sagngtic liald produced by the brain is usually lean than 10 This is wry wok rhea cospargd with 1) the earth's steady field of abort 0.5 gaws asd 2) fluctuations is an urban gavirors~yt of about 10'3 genes, or even gawa (8,9) . The brain's ssg3) the heart's peak aagaetie field of abort 10 ngtic field wan rscos~dgd in the presence of urban fluctuations, hence it is s In ardes to factor of abort 10 weaker than the baekgrorad sa~agtic noise. erred this reek signal Eros the aoiag, the sgasurrsnts sere ride is a ~tically-shielded chasber to seclude the background acing, cad signal averaging was used to gstract the signal out of the rssaiaiag iatsinsic detector noise. The MEG and VER rerg evoked by a brief 11Sht flash and recorded sian].taaeowly for easparisan . A great deal of work has bean done on the VER (2,3,12,11, 15,18,20) . In general the VER evoked by a free-field, brie! flash is a cosplez distribution of scalp poteatiala having its greatest asplitude over the oaeipital lobo . AhiL the rchaniws rgspossiblg for geagratiag the VER are iaeor pLtgly ~mdgrstood (10,21), it is generally agreed that the occipital arw of the ngocorts: are the sr~or recipients of the prisary afferent ngrronal volley . Sosg of the positions of the sagagtarter abort the head rera elwagn so that occipital lobe generators sight be preferentially recorded . Methods Thru volunteer subjgeu nerved is the e:perisaat ; the dau presented here is repraeautivs of the dau collected and it lone a eels college stndgat. The subject rssoved strut clothes, which often contain sagnetic rtgrial, rapLcgd rhea with roe-~sgnetic clothing and sat inside the heavily-shielded rocsi (lie . 1) . The VER electrode ru located at Os (on the sidling between Ol and 02) and w rgtergnced to the right earlobe. The VER vu asplifigd at a baadpasa of 0.5 Hs - 3 b8s, recorded oa lM tape for later analysis, and soaitorgd on as averaging casputer . The ssgngtic detector, a SQDID (aupercondnctiag quantum iaterfereace device) sagnatarter, van horsed is a Devar container filled with liquid helius (7) . The sagnetosetgr wan pwitionad 1-2 sa frog the scalp at several locations abort the head cad ssasurgd the cuponent of the sagwtic field ~rsal to the head . The oatpnt of the sagnataneter vas recorded onto !M ape cad dispLyed on the averaging cosputgr (nee big. 2) . Ths sagnetosrter baadpus was alto O .S Hs - 3 mss. The rgagtarter could bs located et one of five aaguLr positions
Vol . 17, No . 5
lyagnetoencephalography
68 5
The oeta~ooslly-shaped shielded roua at hZT used !or retordu rpstesscephaloirar (IDDG's) . Three of the lites Lure of aasnitic shialdia~ are shown. The sabjset's head, shown at the 0' position is this llEars, xaa placed is close praziaigr to the aaSsstaastsr . A strobe Baht vas located in front o! the sab~act, who wears nan asSnatlc clothinE . about the head, with 0' corrapondiaE to the occipital costs:. TM aaSnstcastsr locations ors shown is hig. 6. Des to the usnasd ijaatr ical nature of the brain's evoked MEG, rscordinaa at +45', +90', etc. sera not ride is this study. Visual stiauLtion vas psavidsd b7 a Grus PS2 photo stiaalator which prodaca a 10 vest !lash. The flash tabs vas inside the shielded rooa shoat 1.S esters lroa the sabjsct and level with his qsa . Doe to the aagnstic artifut usociatsd with the strobe liSht discharSe, the stzobs ru placed is an opsa ao1T-pssaalloy baz to effect a as~stic shield . The snbisct vas inatractsd to look at the aolyrpsraallo7 baz bat ~t directly at the strobe üsht, than the source o! ilhination vas a baz subtsndin; a visas! aa~i.a of 9.6' b7 19 .2' . MEG and VER rscordinss rare obtained siaultaosonsly is all aspects of the sspsriasat . Each vspssiasastal ras consisted of 100 pasha at . abont 1/sse . Cars ras taken that the stianLtions wre not titre-locked to the 60-Ss lias . Most of the szpsriasntal rims wre repeated at last once . Thra sais of szpsriasnts wrs casdoetsd to i~oatl.iate the charutaristics of the visual svotsd MiG. In the first eerie of sspsriasnts, the effect of iacssuin; stiaslaa intsnait~ vas iavsstigatsd . The iatsasiq ssttinaa on the photo stiaalator are dalanatsd 2, " , 8 and 16, slurs 16 oorrsspoods to 1.S z 106 caeWlspowr . Than asasursrnu sers all ride at sti=nstoastsr position -fi' . Ia the sat saris of sapsriasnts the angular distribution of the evoked
68 6
Magnetoencephaloqraphy
Vol . 17, No . 5
DIG. 1 A diagraratic representation of the recording and stiaulating apparatus . The electronics are located at a station ezteraal to the roua . The strobe light is activated at about 1/sec with a muai pnLar rhich delivers a prrflash sync pnlae to the tape recorder and triggers a ccaputsr of average traasisata (C .A .T .) . The sianltaneous !~G and Pb7t signala are aaplified, averaged, and recorded r tape for subsaqueat analysis .
.
T
1
r
PIG. 3 A single, non-averaged M6G respowe (0 .7 Hs - 50 Hs bandwidth) to flash intensity 16 ; ~agaetoaster position -45' . The epoch length for averaging is 22S ree (horisontal bar below response) . Ths light flash occurred at the arrow; there is a sagnetic artifact Eros the strobe light dus to ieperfect shielding of the strobe . Most of the signal is this record consists of the intrinsic noise of eha aagnetasrter at 1.6 z 10-10 gauss per root Hs . Calibrations : horisoatal ~ 50 rec, vertical ~ 5 z 10 -9 gauss .
Vol . 17, No . 5
Magnetceacaphalography
68 7
EsG va saasured abort the head . A eaostant flash intensity of 8 w wad fas these emits. Measurernta were obuiaed fron the five aagwtortar pLcweau abort the head and sera corpared dth the VEB potential recorded at Os. A seaseh for high fragneacy (up to 3000 8s) corpooeau is the 10;6 rat psrforsrd wing the recorded data . Biz frpwncy bands vers iavatlgated wing a toned as~plifier set at carious crater fragwacia and Q's . Thus center frr gwacies sad their correspoodiag Q's acs: iS2 Hs, 2 ; 351 8s, 2; gll Hs, 2; 1470 Hz, S ; sad 3000 Hs, 5. The signal is each bead vas rectified, posed throagh a low-pans filter (0-50 8s), sad then tine averaged oa as awraging corputer . Tha process of rectification and lac-paw filtering is the rar ose owed to obtain the audio portion of m AM radio signal . Lf high frequencj cas4oasais ratted to the light flash are present in the ~G, thq wild appear wing thin rthod of dstection a as iacrwa is the output lroa the lar-pare filter at a cesuia ties foilariag each light flash . IIsiag the averaging cwpater sad a larp ao bsz of etiaulw presentations, it should then be poaible to detect high fregwac~ responses to the light flash is co~pariroa to the noise present in the l~G. Besults . A siagL aon~saged ~G sapante is depicted is Fig. 3 for pnrpwa of rafereaea. The signal here rinl~ consisu of intriric dsteetor noire sad is st a bandwidth of 0.7 Hs - SO Hs . This bandwidth L eaplayed fns all subsr gwnt !OG sad VER enrages to iaprwve the signal-to-noise ratio sad to faciliute cas~pasisoas with the VER litwesra . The rgrtarta vas located is eha -4S' position (sea Fig. 6) ; the flash iateasit~ van 16 . The epoch length fns acesagiug w 2TS arse and u indicated b7 the horisonul bar below the respaae. The light flash stianlw occurred at the arrow sad a wall rgrtic signal prodndd b7 the strobe light can bs sera ; this it as artifact . The resnlu of the iateaaity series era shorn is Fig. 4. Average VER pr teatiaL for the four iateasitia of visual stinulw (snowing inteorigr fr~r 2-16) are shams in Fig. 4A . This average, as call a all others, is 225 rev in duration and negatlvit~ at the VER elecerode i. plotted a as uprard deflection . Stirulw onset i. indicated b7 an array. The arplitnde of the VER iaeswes u a fraction of stiaulw iatwity, sad the Lteacy of the aejor cars parents decrwa with snowing sti>tnlw inteasiq . Fig. 48 presents the average evoked !~G response to the variow fLsh iatwitia recorded coacnrrentl~ at >tagnstortar positiaa -4S' (sa Fig. 6) . Tha nagoatic polarity is arbitrary is thL conte~ but is coasirtsont throughout this papa . The ~agaetic artifact asociated with the strobe discharge hen bean blocked ant is all e~spt the batten ?~G. Although the artifact is actsally of very chart duration, it appears Lngtheaed is tic dw to the upper bandpass of 50 8s . The hottest average of Fig. 4B, labsled "head away," yes obtained to a flash intensity of 8. Thin trace indicate the level of mire present after averaging. äots the large difference between td "head aray" acerags and the "head in" average ; these reprereat the visual evoked ~G signals fr~r the brain. The ?~G results, as well a the VER responses, sere farad to ba highly reprodncibls as reported roar within a subject. Tha aaplitude of the IIIOä rapowe i. even to increase a a fwction of atieulna intensity. The latency of the rjor corpoaents of the IODG respease does not appear to change s. a fnnctiaa of flab intercity, hos~sver thin rasnlt ry ba a consequence of the noise still present, v+hich can shift peab ices their trw location. To cawpare the amplitudes of the VER sad IO~G a a function of atYanlw
Msgnetoencaphalography
68 8
Vol . 17, No . 5
FIG . 4 The averaged VER sad !!6G ruposse (100 epochs ; 0 .7 to 50 Hs) to increuing !ateasities of !lash stianlw (2 to 16) . A . The VER response vas recorded at O z over a 223 epoch . The flash occurred at the array, Calibrations : horisantal ~ 30 vertical ~ 5 pV . E . The averaged IBC respasse at sagastorter position -65' mended concarreatly with the V88 . The etiaulw occurred at the array . Tha artifact is folly shown only is the had-away record, underlined . The head-away record vas obtained with the snbj at's head 30 m Eras{ the srgar to~ater . Calibrations horisontal ~ 50 cosec, vertical ~ 5 z 10'9 gaws (consistent bat arbitrary poLriq) . The vertical laws indicate that the Ltency of the t~G does not appear to change u a fsmction of stiau].w intensity (sae tezt) .
na na,
intensity, the asiplitude of each vas plotted as a function of stimulus intensity . For each, the a~litude vas defined as the pre-artifact baseline to peat of the rjor upYard deflation, within the time viadov of SO-120 we poststimnlw . These data are plotted in Fig . S . It is seen that both the lDIC and VER geaerally {sows as a function of stimulus intensity, althoagh at different rates . The non-!{sanity in the VER plot is probably dw to a saturation aflat at high intensity (6,23) . The angular diatributioa of the woked IO:G was ezsmined at the five anger tosietar positions as depicted !a Fig . 6 . The flash wu coostaat at an intensity of 8 . The lfl;G response vas nurly zero at 0' and vents: sad largest at -45' . This latter position vas close to the assumed gewrator site is the occipital lobe (13,25) . The responses at -135' have a markedly different phase relationship to both the VER and the -65' I+~G, The diasiailarities in the phase rala-
Magnetoancephaloqraphy
Vol . 17, Dto . 5
VER t ~ ~ . msq t o-b V0~ ~8 ~ ' Y 6
FIG . 5
MEG
The effect of stisnlw intensity on caeponents of the VBB ana lO~G . Plotted ara the bosuns to first upward dsflectioa in both VEB sad !~G averages u a fimetion of stiaulus intensity . both functions iacreasa as a ßmetion of stislulw intensity, bat at diflerant slopes .
~~~.d 3
a
689
2
a z
0
2
RELATIVE
a
8
16 LIQNT INTENSITY
0
VER
FIG . 6 The angular distribution of the !~G about the hand as coeipared with the VEB recorded at 0 The ss~gaetae.ater positions are shorn on the drawing of the head . Epoch laagt~ " ia 225 ree . Calibrations : horisoata~ ~ 50 tact, VEB vertical 5 un, 10IG vertical ~ 5 : 10' gauss .
690
Magnetoancephaloqraphy
Vol . 17, No . 5
tions betreen the casponants of the VER sad 1~G are striking in all sagnetorter locations. Tha ts~poral riadw esociatad with the prisary negative Vet reponsa is devoid of 1~0 utivity is all but the -133' rgnetosatsr posision, a location far from the preurd generators . lfo high frequency (100 Hs - 3 RHs) !~G ca~ponsnts reLtad to the light fLsh and greater than the rgnetic datactor noise ware found; the high frequency search ras sods at the various locations sad light inteasitie . In addition, snowing the nusber of average to 300 also failed to produce any detectable high frequency cosponsate at the -43' location . Discussion The reults of the precut e:p~riaeat indicate that sagnetic rasureaeats of sensory svokad awral utivity bear a definite relationship to the VER. It is noted that the aaplitude of both the VER and lIISG vvoted response snows (although at different slope) e a function of stiaulna intensity. This pheaosesa he been repeatedly observed is the VER (1,5,11,24,25) . Its preeacs is the 1+DDG repaase say suggest that different aspects of the sans neuronal geaerstors are being veered by the tro rthads . The Ltency of the identified co~poaents of the VER wrs seen to decrease with snowing annuls iateaaity. This pheaasenon has been docurated eLsrhere for eha VER (22,24,26) . Horwer, so~ wrters note cosponents that are stationary in ti~ as a function of stiaulw intensity (19) . The preeat suggetioa that the !gG cas4onenes are stationary as a inaction of sti~nlna intensity is thw is contrast with the rjority of the V~ literature and rich sisrv.taneauly recorded VER data reported is this paper . It is possible that the 1~G L responding to those stationary casponeats referred to above (19) . It rat be espheized that the !~C stationarity say be only as apparent result due to the effects of averaged noise. The angular distribution of the lO~C about the head has indicated that rgaetorter positions which are symmetrical to the heaisphere (positions 0' and vrte:) do not detect magnetic fields ; this probably results frame the symmstry of the brain. do~ ample, at 0' the rgneeartes "sw" the ear current fsaa both hs~isphare ; this can be shorn to giv a aorrl rgnatic cosQoneat of ssro . Asyrerical sagnetasrter locations -43' and -90' giv rise to sisiLr evoked KG's, except the asplitnds at -90' is edict. Since the presnsed geaeraeor(s) of the visual evoked response are prsdaaiaantly in the occipital lobe, it is nadarstaadable that the larger lO;G response L Etna the asymmetrical sagaetoreer location neret the occipital lobe (-45') . The -90' !~G esentially receive the ear signai, bat is further gray, hence reakar . The asgnetic fields produced by an on-going brain activity is the alpha frequencies is :Lo priaarily occipital and displays a siailar I~G aagnLr distribution about the head (7) . The rgaetic response esociatad with the first negativ casganeat of the VSft is sissing froc all è~C recorda, with the possible esception of the -135' record . The absence of an I~G signal rhea a VER event is preeat can exist for the case of radial generators in a sphere, and thin situation is apprasiaated by the thaluocortical projections to the occipital cotte:. It L thus possible that the "sd.asing" evoked lOZG cosponsor represents the condition of an approsirtaly radial dipole . The -135' utivity ry reflect non-striate voted neural utivity. The second negativ casponent of the VSR is tesporally appraziasted by the l~G cosß+oneats sera in the -45' and -90' records. It is possible that thL lODG response reflects not direct activation of the striate torte:, but rather the
Vol . 17, No . 5
Magnetoencaphaloqraphy
69 1
in uion of the aztsaatriata fialda of tkr upper aad lovas heailield retinal projactiana (14,15) . Snch cortical activation weld Banarata aan-radial cnrranta which would bs veil rspraaantad in the MEG . Tha abaaace of high frequency conponeats in the M~ is thin iavutißatian appaara to naßata 1b naa in the detection of nwronal action potantiala . Her swr, the aeoaitivity of these snasnrwanta is prsaaatly liritad by intaraal aaunetanatar noiaa. it aiay be that a ~~tactor with an internal noiu sißaificantly Uaa than the praaant 1.6 : 10Bauaa (ro) par root cycle way be able to detect these aoapvnanta of asnral activity . The state-of-tha~rt of SQUID datactora L ci~aaßiaß rapidly, aad detectors with lua noise sre a:pscted to hater readily availabls is the near fntura . Bafaraacu 1. 2. 3. 4. 5. 6. 7. 8. 9. 10 . 11 . 12 . 13 .
14 . IS . 16 . I7 . I8 . 19 . 20 . 21. 22 . 23 . 24 . 25 . 26.
J. C. ARKZRf.R01i, Docw . Ophthal. 18, 194-206 (1964) . J . S. BAItLOW, Qnartarh Protrna ltevort 75 , p. 149, Blaureh Lab. of Electronica, MIf, Caabridßa (1964) . L. EEßGAlSIIR and B. BEItGAMASCO, Cortical Evoked PotaatiaL 1n Man, Chaslu C Tbora, Sprinßfiald (1967) . M. CLTRES and M. EAH1I, tsra aad Electroaie Davicea in P chie , p. 206, Grma and Stratton, 11avZor 6 ä. CLTREB, lt. LO®1 aad L. LITSHITZ, ~. R .T . Acad . Sci. 112, 468-509 (1964) . D. COBEK, 8 iaaca 161, 784-786 (1968 . D . OOdER, Science 175, 664-666 (1972) . D. C01®, I~Eâ Tran% w ëasaatica , Maß.-11, no . 2, 694-700 (1975) . D . COBER and D. McCADGHIIR, Atas . J . Cardiolo:, 29, 678-685 E1972) . 0 . D. CBEOTLlEt.DT, of tha , p. 397, LittL, Brown and Co ., Boaton (1 8. P. DfAI~, Aaa. R.i . Acad . Sci. 112, 160-171 (1964) . J. A. HOQiRS, Interna Baviw Worobio , p" 99 . "eadaa~ic Praaa, lIw Tosk (1964 . . D. ". JEPâEElS, Ratnsa (Loed.) 229, 502-504 (1971) . D. ". JQlAEiS, and J. G. "aORD, isp. Brain Baa . 16, 1-21 (1972) . D. ". JEtlsEiS, and J. G. AaOBD, Esa. Brain La . ~6, 22-40 (1972) . D. M. MacLI, Raaroeciaaca Bu . Bull . 7, no . 3 (1969) . :. Pi.OrSET, Bioaleetric Pheno~eoa, p . 368, McOraw-5111, Rw Tork (1969) . ü. J. BEIi9fir.u, Acw Paieioi. roarrcol . Rur1 . 12, 373-407 (1963) . ü. J. 1tEITV1Q~ and ü. 8. M. TOBDOIA, Aeta Phreiol. Phar~acol. Raar1. 13, 160170 (1965) . M. SCHiIAATZ and C. BHAC"S8, Aaa. R.T . Aced . Sci. 112, SIO-525 (1964) . C. SHAG488, Ewoked Brain Potentials ia Parchiatr~Ple~ , Prus, Rw Tork (1972) . T. SHIPLEi, A. it. JQES and ". lAT, Science 150, 1162-1164 (1965) . H. SPE~EIJßE, Anal7aia of EEG Baepcoesa in lfan , Jnak Pnb., Tba Haßw (1966) . D. I . TEPAB, J. C. A10~`T011 and i1. J. QOPEL, Bio1o Protot w and S~nthetic Sntar , vol . 1, p " 13, Piw Praaa, Rw Tork 1962 . H. G. VAII6HA1(, "verap E~roiud PotentiaL , p. 4S, RASA, Vaahinßtoa (1969) . J. D, üIC~, E. DORCHIIf aad D. B . LDIDSLET, Scienca 146, 83-85 (1964) .
of
Printed is the U .S .11 .
Psrgamon Press
683-692
T~ VISUAi. EVO~D Tiao~ip J. Tglu Dtpartraat of Pgcholop a~ Social RsLtiou Ha:Vard IIaiVartity, Cebrid=a, 1!A 02138 E. Esil Cuffia aad David Cohea >'raa~ia Eitter Batioaal Ma=aat Laboratom Maaeacbatetts Instttad of Tschaoloay Canbrid=e, 1IG 02139
(Received
in Eiaal
form
Julp 24, 1975)
Tbs Visual enokad electroeacephalosrar (VER) aad taiaetoaocepbalo=rar (IOOG), rhich ~eatores tba asEoatie lield aosral to tlr acalp, rere tirultsasooaly obtaiaad ia raa. Ths arplitnds and latency of rupoata o! r~ ras cosgiarsd u a laactioa o! atirtlw intaatity aad tha aaEnlar dLtribation of tbs 1~G atouod ths btad ; a aearab rw alto arde !br hüh lregwacy coa~oaeata oi ths LIEli (> 100 8s) . 1~G ard VER boW aborad aa iacseased arplitade of rapoese u a laoction o! ttirnlw intaosity . Ia contrat to tha VER, tba Ltaacy of ths lOG did aot appur to decrsass u a lnaetion of atirnlna intuaity . At rith tha VER, tha lOG contaiaad no hüh lrpwncy t. :ruter than tha e:periraatal aoias. Ths rssnlte ars discuaasd ia rsLti.oa to tha ~Oar:tors o! the lODG sad YER. The tare elsctroch~+~~~ activity of the brain rhich produce potential an the scalp also prodncsy a rsEOatic field aranad the head ; this ream Bald hu been datsetsd rith seaaitive rasastarstsre (6,7) and is tesrad tM ra=natosn cephalograr (l01G) . It is claiasd that tbs 1~G ha several adVaata=q aces the slsctrowcephalo=rar (EEG) (6,7) . !or asarple, at for frsgwnela (DC to 0.1 Hs) the 1DLG is free o! interlsraaca due to chaaEing skin-elactrods potsatials ; alo !or aqy lregaaacy the 1laG can raaura sane aanral source diatribetiona Which era supprseasd oa the EEG (7) . I! the IOZG is abls to record the hüh fragwacy svsnts associated rith action potential pnaration, it say help clarify the contranrq surramding the aanrowi ~assatosa o! the EEG sad evoted raspooae (8,10,16,17,21) . The 1®C is able to reveal inforrtion about brain function in a rannar diflsrsat Eras the ERG since ragnetic raasurersats raflsct charge aovusnts (carreats) rather than potential diffsreaeq (chariu) . The tw ruauru ass reltsd because the oars ltie aekaorledae the tsehaical and editorial aaistaaee of ltra . Chris Poor sad Ma . äaacy Detroad. This ruaarch ra supported is part by ARP", by a subcontract no . 110014-70-C-0330 . 683
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currents producing potentia]. differences within and on the surface of the tlseus aLo produce a srgaetic field . Thw the futures of the SßG and MEG are sisilar is saw canes. However, for several unique generator distributions the 88C and MEG rill be disaisilas . !or a:asple, a current dipole located on and oriented along the a>a.s of a voluse conductor having axial syartry rill produce potsntial diffgrencu on the surface of the conductor but sgro sagnetic fields outside the conductor. This situation is not unlike the aaatasical projactiona of Conversely, the optic radiations Eros the lateral geniculatg to striate torte:. a üaifoa surfue potential can bs associated with nonsero current in the voluse conductor and thus a generated sagngtic field (7) . Ths latter condition prob ably does not occur in the brain, however. In abort, the MEG and SEG in conjuaetion should M able to localise the source of a neural event better rhea either aloha. before the possible applications of the MEG can be properly unused, it is necessary to density the saia features of the MEG under a variety of coaditioos . Aa part of this effort, w haw detersined the nature of the MBG ruponu to a etaadard savory stisnlna (visual flash) cad casipared the fgaturu of the visually evoked MEG with the concurrently recorded vianal evoked EEG raponsg (~) . -8 gansa. The sagngtic liald produced by the brain is usually lean than 10 This is wry wok rhea cospargd with 1) the earth's steady field of abort 0.5 gaws asd 2) fluctuations is an urban gavirors~yt of about 10'3 genes, or even gawa (8,9) . The brain's ssg3) the heart's peak aagaetie field of abort 10 ngtic field wan rscos~dgd in the presence of urban fluctuations, hence it is s In ardes to factor of abort 10 weaker than the baekgrorad sa~agtic noise. erred this reek signal Eros the aoiag, the sgasurrsnts sere ride is a ~tically-shielded chasber to seclude the background acing, cad signal averaging was used to gstract the signal out of the rssaiaiag iatsinsic detector noise. The MEG and VER rerg evoked by a brief 11Sht flash and recorded sian].taaeowly for easparisan . A great deal of work has bean done on the VER (2,3,12,11, 15,18,20) . In general the VER evoked by a free-field, brie! flash is a cosplez distribution of scalp poteatiala having its greatest asplitude over the oaeipital lobo . AhiL the rchaniws rgspossiblg for geagratiag the VER are iaeor pLtgly ~mdgrstood (10,21), it is generally agreed that the occipital arw of the ngocorts: are the sr~or recipients of the prisary afferent ngrronal volley . Sosg of the positions of the sagagtarter abort the head rera elwagn so that occipital lobe generators sight be preferentially recorded . Methods Thru volunteer subjgeu nerved is the e:perisaat ; the dau presented here is repraeautivs of the dau collected and it lone a eels college stndgat. The subject rssoved strut clothes, which often contain sagnetic rtgrial, rapLcgd rhea with roe-~sgnetic clothing and sat inside the heavily-shielded rocsi (lie . 1) . The VER electrode ru located at Os (on the sidling between Ol and 02) and w rgtergnced to the right earlobe. The VER vu asplifigd at a baadpasa of 0.5 Hs - 3 b8s, recorded oa lM tape for later analysis, and soaitorgd on as averaging casputer . The ssgngtic detector, a SQDID (aupercondnctiag quantum iaterfereace device) sagnatarter, van horsed is a Devar container filled with liquid helius (7) . The sagnetosetgr wan pwitionad 1-2 sa frog the scalp at several locations abort the head cad ssasurgd the cuponent of the sagwtic field ~rsal to the head . The oatpnt of the sagnataneter vas recorded onto !M ape cad dispLyed on the averaging cosputgr (nee big. 2) . Ths sagnetosrter baadpus was alto O .S Hs - 3 mss. The rgagtarter could bs located et one of five aaguLr positions
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lyagnetoencephalography
68 5
The oeta~ooslly-shaped shielded roua at hZT used !or retordu rpstesscephaloirar (IDDG's) . Three of the lites Lure of aasnitic shialdia~ are shown. The sabjset's head, shown at the 0' position is this llEars, xaa placed is close praziaigr to the aaSsstaastsr . A strobe Baht vas located in front o! the sab~act, who wears nan asSnatlc clothinE . about the head, with 0' corrapondiaE to the occipital costs:. TM aaSnstcastsr locations ors shown is hig. 6. Des to the usnasd ijaatr ical nature of the brain's evoked MEG, rscordinaa at +45', +90', etc. sera not ride is this study. Visual stiauLtion vas psavidsd b7 a Grus PS2 photo stiaalator which prodaca a 10 vest !lash. The flash tabs vas inside the shielded rooa shoat 1.S esters lroa the sabjsct and level with his qsa . Doe to the aagnstic artifut usociatsd with the strobe liSht discharSe, the stzobs ru placed is an opsa ao1T-pssaalloy baz to effect a as~stic shield . The snbisct vas inatractsd to look at the aolyrpsraallo7 baz bat ~t directly at the strobe üsht, than the source o! ilhination vas a baz subtsndin; a visas! aa~i.a of 9.6' b7 19 .2' . MEG and VER rscordinss rare obtained siaultaosonsly is all aspects of the sspsriasat . Each vspssiasastal ras consisted of 100 pasha at . abont 1/sse . Cars ras taken that the stianLtions wre not titre-locked to the 60-Ss lias . Most of the szpsriasntal rims wre repeated at last once . Thra sais of szpsriasnts wrs casdoetsd to i~oatl.iate the charutaristics of the visual svotsd MiG. In the first eerie of sspsriasnts, the effect of iacssuin; stiaslaa intsnait~ vas iavsstigatsd . The iatsasiq ssttinaa on the photo stiaalator are dalanatsd 2, " , 8 and 16, slurs 16 oorrsspoods to 1.S z 106 caeWlspowr . Than asasursrnu sers all ride at sti=nstoastsr position -fi' . Ia the sat saris of sapsriasnts the angular distribution of the evoked
68 6
Magnetoencephaloqraphy
Vol . 17, No . 5
DIG. 1 A diagraratic representation of the recording and stiaulating apparatus . The electronics are located at a station ezteraal to the roua . The strobe light is activated at about 1/sec with a muai pnLar rhich delivers a prrflash sync pnlae to the tape recorder and triggers a ccaputsr of average traasisata (C .A .T .) . The sianltaneous !~G and Pb7t signala are aaplified, averaged, and recorded r tape for subsaqueat analysis .
.
T
1
r
PIG. 3 A single, non-averaged M6G respowe (0 .7 Hs - 50 Hs bandwidth) to flash intensity 16 ; ~agaetoaster position -45' . The epoch length for averaging is 22S ree (horisontal bar below response) . Ths light flash occurred at the arrow; there is a sagnetic artifact Eros the strobe light dus to ieperfect shielding of the strobe . Most of the signal is this record consists of the intrinsic noise of eha aagnetasrter at 1.6 z 10-10 gauss per root Hs . Calibrations : horisoatal ~ 50 rec, vertical ~ 5 z 10 -9 gauss .
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Magnetceacaphalography
68 7
EsG va saasured abort the head . A eaostant flash intensity of 8 w wad fas these emits. Measurernta were obuiaed fron the five aagwtortar pLcweau abort the head and sera corpared dth the VEB potential recorded at Os. A seaseh for high fragneacy (up to 3000 8s) corpooeau is the 10;6 rat psrforsrd wing the recorded data . Biz frpwncy bands vers iavatlgated wing a toned as~plifier set at carious crater fragwacia and Q's . Thus center frr gwacies sad their correspoodiag Q's acs: iS2 Hs, 2 ; 351 8s, 2; gll Hs, 2; 1470 Hz, S ; sad 3000 Hs, 5. The signal is each bead vas rectified, posed throagh a low-pans filter (0-50 8s), sad then tine averaged oa as awraging corputer . Tha process of rectification and lac-paw filtering is the rar ose owed to obtain the audio portion of m AM radio signal . Lf high frequencj cas4oasais ratted to the light flash are present in the ~G, thq wild appear wing thin rthod of dstection a as iacrwa is the output lroa the lar-pare filter at a cesuia ties foilariag each light flash . IIsiag the averaging cwpater sad a larp ao bsz of etiaulw presentations, it should then be poaible to detect high fregwac~ responses to the light flash is co~pariroa to the noise present in the l~G. Besults . A siagL aon~saged ~G sapante is depicted is Fig. 3 for pnrpwa of rafereaea. The signal here rinl~ consisu of intriric dsteetor noire sad is st a bandwidth of 0.7 Hs - SO Hs . This bandwidth L eaplayed fns all subsr gwnt !OG sad VER enrages to iaprwve the signal-to-noise ratio sad to faciliute cas~pasisoas with the VER litwesra . The rgrtarta vas located is eha -4S' position (sea Fig. 6) ; the flash iateasit~ van 16 . The epoch length fns acesagiug w 2TS arse and u indicated b7 the horisonul bar below the respaae. The light flash stianlw occurred at the arrow sad a wall rgrtic signal prodndd b7 the strobe light can bs sera ; this it as artifact . The resnlu of the iateaaity series era shorn is Fig. 4. Average VER pr teatiaL for the four iateasitia of visual stinulw (snowing inteorigr fr~r 2-16) are shams in Fig. 4A . This average, as call a all others, is 225 rev in duration and negatlvit~ at the VER elecerode i. plotted a as uprard deflection . Stirulw onset i. indicated b7 an array. The arplitnde of the VER iaeswes u a fraction of stiaulw iatwity, sad the Lteacy of the aejor cars parents decrwa with snowing sti>tnlw inteasiq . Fig. 48 presents the average evoked !~G response to the variow fLsh iatwitia recorded coacnrrentl~ at >tagnstortar positiaa -4S' (sa Fig. 6) . Tha nagoatic polarity is arbitrary is thL conte~ but is coasirtsont throughout this papa . The ~agaetic artifact asociated with the strobe discharge hen bean blocked ant is all e~spt the batten ?~G. Although the artifact is actsally of very chart duration, it appears Lngtheaed is tic dw to the upper bandpass of 50 8s . The hottest average of Fig. 4B, labsled "head away," yes obtained to a flash intensity of 8. Thin trace indicate the level of mire present after averaging. äots the large difference between td "head aray" acerags and the "head in" average ; these reprereat the visual evoked ~G signals fr~r the brain. The ?~G results, as well a the VER responses, sere farad to ba highly reprodncibls as reported roar within a subject. Tha aaplitude of the IIIOä rapowe i. even to increase a a fwction of atieulna intensity. The latency of the rjor corpoaents of the IODG respease does not appear to change s. a fnnctiaa of flab intercity, hos~sver thin rasnlt ry ba a consequence of the noise still present, v+hich can shift peab ices their trw location. To cawpare the amplitudes of the VER sad IO~G a a function of atYanlw
Msgnetoencaphalography
68 8
Vol . 17, No . 5
FIG . 4 The averaged VER sad !!6G ruposse (100 epochs ; 0 .7 to 50 Hs) to increuing !ateasities of !lash stianlw (2 to 16) . A . The VER response vas recorded at O z over a 223 epoch . The flash occurred at the array, Calibrations : horisantal ~ 30 vertical ~ 5 pV . E . The averaged IBC respasse at sagastorter position -65' mended concarreatly with the V88 . The etiaulw occurred at the array . Tha artifact is folly shown only is the had-away record, underlined . The head-away record vas obtained with the snbj at's head 30 m Eras{ the srgar to~ater . Calibrations horisontal ~ 50 cosec, vertical ~ 5 z 10'9 gaws (consistent bat arbitrary poLriq) . The vertical laws indicate that the Ltency of the t~G does not appear to change u a fsmction of stiau].w intensity (sae tezt) .
na na,
intensity, the asiplitude of each vas plotted as a function of stimulus intensity . For each, the a~litude vas defined as the pre-artifact baseline to peat of the rjor upYard deflation, within the time viadov of SO-120 we poststimnlw . These data are plotted in Fig . S . It is seen that both the lDIC and VER geaerally {sows as a function of stimulus intensity, althoagh at different rates . The non-!{sanity in the VER plot is probably dw to a saturation aflat at high intensity (6,23) . The angular diatributioa of the woked IO:G was ezsmined at the five anger tosietar positions as depicted !a Fig . 6 . The flash wu coostaat at an intensity of 8 . The lfl;G response vas nurly zero at 0' and vents: sad largest at -45' . This latter position vas close to the assumed gewrator site is the occipital lobe (13,25) . The responses at -135' have a markedly different phase relationship to both the VER and the -65' I+~G, The diasiailarities in the phase rala-
Magnetoancephaloqraphy
Vol . 17, Dto . 5
VER t ~ ~ . msq t o-b V0~ ~8 ~ ' Y 6
FIG . 5
MEG
The effect of stisnlw intensity on caeponents of the VBB ana lO~G . Plotted ara the bosuns to first upward dsflectioa in both VEB sad !~G averages u a fimetion of stiaulus intensity . both functions iacreasa as a ßmetion of stislulw intensity, bat at diflerant slopes .
~~~.d 3
a
689
2
a z
0
2
RELATIVE
a
8
16 LIQNT INTENSITY
0
VER
FIG . 6 The angular distribution of the !~G about the hand as coeipared with the VEB recorded at 0 The ss~gaetae.ater positions are shorn on the drawing of the head . Epoch laagt~ " ia 225 ree . Calibrations : horisoata~ ~ 50 tact, VEB vertical 5 un, 10IG vertical ~ 5 : 10' gauss .
690
Magnetoancephaloqraphy
Vol . 17, No . 5
tions betreen the casponants of the VER sad 1~G are striking in all sagnetorter locations. Tha ts~poral riadw esociatad with the prisary negative Vet reponsa is devoid of 1~0 utivity is all but the -133' rgnetosatsr posision, a location far from the preurd generators . lfo high frequency (100 Hs - 3 RHs) !~G ca~ponsnts reLtad to the light fLsh and greater than the rgnetic datactor noise ware found; the high frequency search ras sods at the various locations sad light inteasitie . In addition, snowing the nusber of average to 300 also failed to produce any detectable high frequency cosponsate at the -43' location . Discussion The reults of the precut e:p~riaeat indicate that sagnetic rasureaeats of sensory svokad awral utivity bear a definite relationship to the VER. It is noted that the aaplitude of both the VER and lIISG vvoted response snows (although at different slope) e a function of stiaulna intensity. This pheaosesa he been repeatedly observed is the VER (1,5,11,24,25) . Its preeacs is the 1+DDG repaase say suggest that different aspects of the sans neuronal geaerstors are being veered by the tro rthads . The Ltency of the identified co~poaents of the VER wrs seen to decrease with snowing annuls iateaaity. This pheaasenon has been docurated eLsrhere for eha VER (22,24,26) . Horwer, so~ wrters note cosponents that are stationary in ti~ as a function of stiaulw intensity (19) . The preeat suggetioa that the !gG cas4onenes are stationary as a inaction of sti~nlna intensity is thw is contrast with the rjority of the V~ literature and rich sisrv.taneauly recorded VER data reported is this paper . It is possible that the 1~G L responding to those stationary casponeats referred to above (19) . It rat be espheized that the !~C stationarity say be only as apparent result due to the effects of averaged noise. The angular distribution of the lO~C about the head has indicated that rgaetorter positions which are symmetrical to the heaisphere (positions 0' and vrte:) do not detect magnetic fields ; this probably results frame the symmstry of the brain. do~ ample, at 0' the rgneeartes "sw" the ear current fsaa both hs~isphare ; this can be shorn to giv a aorrl rgnatic cosQoneat of ssro . Asyrerical sagnetasrter locations -43' and -90' giv rise to sisiLr evoked KG's, except the asplitnds at -90' is edict. Since the presnsed geaeraeor(s) of the visual evoked response are prsdaaiaantly in the occipital lobe, it is nadarstaadable that the larger lO;G response L Etna the asymmetrical sagaetoreer location neret the occipital lobe (-45') . The -90' !~G esentially receive the ear signai, bat is further gray, hence reakar . The asgnetic fields produced by an on-going brain activity is the alpha frequencies is :Lo priaarily occipital and displays a siailar I~G aagnLr distribution about the head (7) . The rgaetic response esociatad with the first negativ casganeat of the VSft is sissing froc all è~C recorda, with the possible esception of the -135' record . The absence of an I~G signal rhea a VER event is preeat can exist for the case of radial generators in a sphere, and thin situation is apprasiaated by the thaluocortical projections to the occipital cotte:. It L thus possible that the "sd.asing" evoked lOZG cosponsor represents the condition of an approsirtaly radial dipole . The -135' utivity ry reflect non-striate voted neural utivity. The second negativ casponent of the VSR is tesporally appraziasted by the l~G cosß+oneats sera in the -45' and -90' records. It is possible that thL lODG response reflects not direct activation of the striate torte:, but rather the
Vol . 17, No . 5
Magnetoencaphaloqraphy
69 1
in uion of the aztsaatriata fialda of tkr upper aad lovas heailield retinal projactiana (14,15) . Snch cortical activation weld Banarata aan-radial cnrranta which would bs veil rspraaantad in the MEG . Tha abaaace of high frequency conponeats in the M~ is thin iavutißatian appaara to naßata 1b naa in the detection of nwronal action potantiala . Her swr, the aeoaitivity of these snasnrwanta is prsaaatly liritad by intaraal aaunetanatar noiaa. it aiay be that a ~~tactor with an internal noiu sißaificantly Uaa than the praaant 1.6 : 10Bauaa (ro) par root cycle way be able to detect these aoapvnanta of asnral activity . The state-of-tha~rt of SQUID datactora L ci~aaßiaß rapidly, aad detectors with lua noise sre a:pscted to hater readily availabls is the near fntura . Bafaraacu 1. 2. 3. 4. 5. 6. 7. 8. 9. 10 . 11 . 12 . 13 .
14 . IS . 16 . I7 . I8 . 19 . 20 . 21. 22 . 23 . 24 . 25 . 26.
J. C. ARKZRf.R01i, Docw . Ophthal. 18, 194-206 (1964) . J . S. BAItLOW, Qnartarh Protrna ltevort 75 , p. 149, Blaureh Lab. of Electronica, MIf, Caabridßa (1964) . L. EEßGAlSIIR and B. BEItGAMASCO, Cortical Evoked PotaatiaL 1n Man, Chaslu C Tbora, Sprinßfiald (1967) . M. CLTRES and M. EAH1I, tsra aad Electroaie Davicea in P chie , p. 206, Grma and Stratton, 11avZor 6 ä. CLTREB, lt. LO®1 aad L. LITSHITZ, ~. R .T . Acad . Sci. 112, 468-509 (1964) . D. COBEK, 8 iaaca 161, 784-786 (1968 . D . OOdER, Science 175, 664-666 (1972) . D. C01®, I~Eâ Tran% w ëasaatica , Maß.-11, no . 2, 694-700 (1975) . D . COBER and D. McCADGHIIR, Atas . J . Cardiolo:, 29, 678-685 E1972) . 0 . D. CBEOTLlEt.DT, of tha , p. 397, LittL, Brown and Co ., Boaton (1 8. P. DfAI~, Aaa. R.i . Acad . Sci. 112, 160-171 (1964) . J. A. HOQiRS, Interna Baviw Worobio , p" 99 . "eadaa~ic Praaa, lIw Tosk (1964 . . D. ". JEPâEElS, Ratnsa (Loed.) 229, 502-504 (1971) . D. ". JQlAEiS, and J. G. "aORD, isp. Brain Baa . 16, 1-21 (1972) . D. ". JEtlsEiS, and J. G. AaOBD, Esa. Brain La . ~6, 22-40 (1972) . D. M. MacLI, Raaroeciaaca Bu . Bull . 7, no . 3 (1969) . :. Pi.OrSET, Bioaleetric Pheno~eoa, p . 368, McOraw-5111, Rw Tork (1969) . ü. J. BEIi9fir.u, Acw Paieioi. roarrcol . Rur1 . 12, 373-407 (1963) . ü. J. 1tEITV1Q~ and ü. 8. M. TOBDOIA, Aeta Phreiol. Phar~acol. Raar1. 13, 160170 (1965) . M. SCHiIAATZ and C. BHAC"S8, Aaa. R.T . Aced . Sci. 112, SIO-525 (1964) . C. SHAG488, Ewoked Brain Potentials ia Parchiatr~Ple~ , Prus, Rw Tork (1972) . T. SHIPLEi, A. it. JQES and ". lAT, Science 150, 1162-1164 (1965) . H. SPE~EIJßE, Anal7aia of EEG Baepcoesa in lfan , Jnak Pnb., Tba Haßw (1966) . D. I . TEPAB, J. C. A10~`T011 and i1. J. QOPEL, Bio1o Protot w and S~nthetic Sntar , vol . 1, p " 13, Piw Praaa, Rw Tork 1962 . H. G. VAII6HA1(, "verap E~roiud PotentiaL , p. 4S, RASA, Vaahinßtoa (1969) . J. D, üIC~, E. DORCHIIf aad D. B . LDIDSLET, Scienca 146, 83-85 (1964) .
of
