CHARACTERISTICS OF THE NORMAL ELECTROENCEPHALOGRAM. I. A STUDY OF THE OCCIPITAL CORTICAL POTENTIALS IN 500 NORMAL ADULTS 1 By MARY A. B. BRAZIER AND JACOB E. FINESINGER (From the Deprtment of Neuropsychitry, Hrvrd Medicl School, nd the Psychitric Deprtment nd Electroencephlogrphic Lbortory, Msschusetts Generl Hospitl, Boston) This study represents n ttempt to codify the min chrcteristics of the electroencephlogrm in the norml dult. This ttempt ws inspired by the recent interest in the use of the electroencephlogrm s possible method of grding normls in the selection of ir pilots. The chief chrcteristics of the norml electroencephlogrm my be studied by considering the following components: (1) Dominnt frequency... which is here defined s the frequency in cycles per second of the mjority of the wves present. The degree of fluctution in the dominnt frequency of n individul in repeted recordings hs lso been studied. (2) Percentge time lph... the percentge of the record occupied by wves of 8.0 to 13.0 cycles per second, whether occurring singly or in chins. (3) Percentge time intermedite frequencies...the percentge of the record occupied by wves in the intermedite bnd (13.5 to 17.5 per second). (4) Percentge time bet... the percentge of the record occupied by corticl potentils of frequencies bove 17.5 per second, nd of voltges so low s to mke them individully uncountble. (5) Percentge time slow ctivity... i.e., wves slower thn 8.0 cycles per second. (6) Voltge... In this study, the voltge chrcteristic studied ws the mximum voltge of the potentils from the bipolr occipitl leds. The chrcteristics listed bove will be found to vry in the sme person, ccording to the prt of the hed exmined. Throughout the present study, ll nlyses were mde from bipolr recordings from the occiput. 1 This study ws ided by grnt from the Hrrington Fund. (Received for publiction August 2, 1943) METHOD OF ANALYZING THE RECORDS After mny ttempts t esier nd more rpid methods of nlysis, the method finlly chosen for this reserch, becuse it gve more informtion thn ny other, ws the frequency distribution of the wves, compiled by counting the percentge time covered by wves of ech different number of cycles per second. In order to compile frequency distribution curve, 2-minute record, tken when the subject ws lying quietly nd brething normlly, is first inspected for the presence of rtifcts. Any portion showing rtifcts due to eyeblinks, muscle movements, etc., is omitted from the smple for nlysis. The reminder is mesured for totl length of time, nd this figure becomes the totl on which ll percentges re clculted. A trnsprent grting (designed by Dvis), mrked off in intervls equivlent to ech of the frequencies, is then lid on the record, nd the frequency of ny chins of wves is thus esily determined. The time covered by wves of ech frequency is then totlled, the results being expressed s percentges of the whole period mesured. This process cn be shortened by mesuring only chins in which t lest 3 wves of the sme frequency occur together; in the mjority of norml records, this rbitrry rule gives n dequtely representtive picture of the record, lthough its only specific merit is s time sver. When these figures hve been compiled, they cn be presented either in tbulr form, or in digrmmtic form (Figure 1). On the whole, the digrm is to be preferred, becuse it gives n immedite representtion of the most chrcteristic fetures of the electroencephlogrm. The chrcteristics listed in the introduction were studied in detil in series of 500 young norml dults nd the records of these subjects form the bsis for the mjor portion of this pper. Most of these subjects received only one test, but in order to study the consistency of vrious of the properties of the electroencephlogrm, 45 of the series were given 4 to 5 tests ech; in ll, 176 tests were mde on this smller series of 45. RESULTS (1) Dominnt frequency () Types of dominnt frequency. On first inspection, the most outstnding feture of n electroencephlogrm is the dominnt rhythm, 303
304 z 0 3: 20%- A 0 G, o OF O 54- AGE O I w RANGE 9i0 1'15%3.1.1 FRQEC IN CYLSPRSCN 095%.I1 516. 1 4 MARY A. B. BRAZIER AND JACOB E. FINESINGER FREQUENCY IN CYCLES PER SECOND FIG. 1. GRAPH COMPILED FROM THELECTROENCEPH- ALOGRAM OF ONE NORMAL ADULT>IHOWING"THE,PERCENT- AGES OF EACH FREQUENCY PRESENT, OMITTING THE BETA RANGE The dominnt frequency is defined s the one present in the gretest mount, i.e., t the mode of the curve. (In this cse, it is 9.5 cycles per second.) i.e., the frequency in cycles per second which is present in greter mounts thn ny other frequency. This frequency is usully pprent on rough inspection nd most electroencephlogrphers hve dopted the clssifiction into two groups, s first suggested by Berger, nmely, the lph nd the bet types. These hve usully been defined s the frequencies between 8.0 nd 13.0 cycles per second for the lph group, nd frequencies fster thn 18.0 cycles for the bet group. This clssifiction is not stisfctory since, in fct, ll electroencephlogrms consist of mixture of these rhythms in some degree, nd no record consists wholly of either lph or bet wves. Hence, if this clssifiction is to be used, some criterion must be defined s to the percentge of bet ctivity which must be present in record before tht record should be clssified s bet type. For the purposes of the present study, records re clssed s bet type only if there is present less thn 20 per cent of other ctivity, i.e., of wves slower thn 18 cycles per second. Also, in the present work, third clssifiction is used, since it hs been found in this series of 500 norml dults tht there is evidence for regrding the dominnt frequencies in the intermedite bnd (13.5 to 17.5 cycles per second) between the lph nd the bet rnges s seprte entity (Figure 2). A detiled sttisticl nlysis ws mde of the ctivity slower thn bet (i.e., slower thn 18.0 cycles) in the records of 500 norml dults. The men for ll the dominnt frequencies in this rnge ws 10.5 cycles per second, with stndrd devition of 0.9. Thus, ny dominnt frequency slower thn 8.0 or fster thn 13.0 cycles is outside 3 times stndrd devition for normls, nd is therefore, by definition, excluded from the lph rnge. Further reson for regrding records with dominnt frequency in the intermedite rnge s seprte group is found in study of the distribution curve of the dominnt frequencies of 500 norml dults (Figure 3). In this grph, there ppers to be norml distribution curve dominting the picture but with some outlying strgglers in the fster frequencies. The min curve consists of 474 ALPHA RHYTHM 8.0 TO 13.0 PER SECOND INTERMEDIATE RHYTHM 135 TO 17.S PER SECOND KrA RHYTHM 19.0 TO 35.0 PER SECOND _- _ or--i --%j FIG. 2. TYPES OF NoRMAL ELECTROENCEPHALOGRAMS The upright line to the right of ech trcing is the clibrtion for 100 mv. I
ELECTROENCEPHALOGRAMS IN THE NORMAL ADULT 305 subjects whose dominnt frequencies give norml distribution round mode of 10.0 cycles per second, with 26 subjects outside the norml distribution curve. Hence, it is felt tht those frequencies in the rnge 13.5 to 17.5 should not be included in the lph group. Frequencies flling within this rnge re referred to s intermedite rhythms. In this group of 500 norml dults, exmined in this lbortory, the following distribution of the three types (lph, intermedite, nd bet) ws found: Number of subjects: 500 per cent Number of lph type (8 to 13.0 cycles 474 94.8 per second): Number of intermedite (13.5 to 17.5): 18 3.6 Number of bet type (18.0 nd over): 8 1.6 (b) Consistency of the dominnt frequency. Tht the dominnt frequency remins constnt within nrrow limits for the sme individul over long periods of time cn be demonstrted by repeted observtions on the sme person. A discussion of some fctors which my, in certin circumstnces, lter the dominnt frequency will be reported in subsequent pper. Electroencephlogrms repeted on the sme subjects over period of few yers reveled only smll fluctutions in the dominnt frequency from one test to the next: Subject 1, ged 35, femle Subject 2, ged 24, femle uly 1940 9.5 Oct. 1940 9.5 Dec. 16, 1940 9.5 June 1941 10.0 Dec. 26, 1940 9.5 April 1942 10.0 Sept. 1942 9.5 Sept. 1942 10.0 Subject 3, ged 36, femle Dec. 1940 20.0 June 1941 20.0 July 1941 18.0 Aug. 1941 20.0 This degree of fluctution is of the sme order s tht reported by other workers (Loomis, Hrvey, nd Hobrt (1), Jsper nd Cruikshnk (2), nd Jsper nd Andrews (3)). Tht there is lso only smll fluctution in the dominnt frequency of n individul when exmined severl times during the sme dy, hs been estblished on lrger group. One hundred nd seventy-six observtions were mde on 45 norml subjects, ll of whom were exmined t non-fsting blood sugr levels (bove 70 mgm per 100 cc.), i.e., 4 to 5 tests were mde on the sme individul t intervls during the sme dy. z20%- 500 SubJects U.I 25%- o 10%- '5%- CI * I \ J ZI ALPHA RANGE FREQUENCY IN INTERMEDIATE CYCLES PER SECOND FIG. 3. THE DISTRIBUTION CURVE OF THE DOMINANT FREQUENCIES FOUND IN THE OCCIPITAL RHYTHMs OF 500 NORMAL ADULTs Ten of the 45 subjects showed no vrition in their dominnt frequency. In 1 subject only did the dominnt frequency vry from his own men vlue by more thn 7 per cent. The men vrition for the whole series of 45 subjects ws under 1 per cent (176 observtions). (c) Reltion between consistency of the dominnt frequency nd ge. If the consistency of person's dominnt frequency be studied, it is found tht ge is fctor in the degree of vribility found in the electroencephlogrm t non-fsting blood sugr levels (bove 70 mgm). This degree of vribility is determined by finding the coefficient of vrition for the dominnt frequency of ech individul. (The coefficient of vrition = 100 Xstndrd devition men A group of 45 young dults between the ges of 17 nd 38 were thus exmined for consistency of. dominnt frequency t norml blood sugr levels. By rough observtion, it ppered s though the dominnt frequency in records of those subjects over the ge of 20 ws less stble thn of those under tht ge, nd for this reson the division into two groups ws mde t this ge level. In the series exmined, there were
306 MARY A. B. BRAZIER AND JACOB E. FINESINGER 13 subjects under the ge of 20, nd 32 were ged encephlogrm of dults up to the ge of 47. 20 or over. Clcultions of the coefficient of We hve no dt on norml individuls over vrition gve the following result: this ge. Under 20 20 ndover (e) Dominnt frequency nd its reltion to other Men of coefficient of vrition 1.2 3.2 physiologicl fctors. The dominnt frequencies Stndrd devition of coefficient 1.3 2.4 in this group were exmined for of vrition ny possible correltion with sex, weight, height, or height- The difference between these two mens ws weight rtio, but no reltion ws found with tested for relibility by determining the stndrd error of the difference: D= 3.52, where D equls the difference between the two mens, nd oid, the stndrd error of tht difference. The result is indictive of significnt difference between these two groups, i.e., the chnces of this being true difference re over 1000 to 1. It would pper therefore tht the dominnt corticl frequency becomes less stble with incresing ge, nd this is demonstrble even in series which contins no one over the ge of 38. (d) Dominnt frequency nd the ge fctor. The totl group of 500 young dults, between the ges of 17 nd 47, ws studied for correltion between their ge nd the ctul frequency of their dominnt rhythm t non-fsting blood sugr levels (in contrst to the consistency of this dominnt frequency which hs just been exmined). There hve been severl studies of this kind in reltion to ge in children (Berger (4), Loomis (5), Lindsley (6), Smith (7), nd Weinbch (8)); but the ge fctor in dults hs not received much ttention. Bernhrd nd Skoglund (9, 10) demonstrted difference of over hlf cycle in the men dominnt frequency between two ge groups of 15 to 18 nd 19 to 30, respectively. In the present series of 500 dults between the ges of 17 nd 47, with men ge of 24, the following results were obtined: Under 24 yer8 270 lphs-men dominnt frequency 10.5 7 intermedites 4 bets 24 yers old nd over 204 lphs-men dominnt frequency 10.4 17 intermedites ny of these fctors. (2) Percentge time lph This is defined s the percentge time occupied by wves of 8.0 to 13.0 cycles per second, occurring either singly or in chins; i.e., it is the gross lph count. In 500 subjects who were exmined t nonfsting blood sugr levels, the percentge time lph vried in the group from 9 to 93 per cent, with men of 61 per cent; There ws some lph ctivity present in ll records, even those which were predominntly bet in type. The distribution mong this group of the mount of lph ctivity present t non-fsting blood sugr levels is given in the following distribution digrm (Figure 4).,- C 40,%C 0 - U 0 50%- 10%- _ Is 20%0-. AI Ez2 10%-. under 25% 25%-49% 50%-74%' 75%-over PER CENT TIME ALPHA ACTIVITY 4 bets FIG. 4. GRAPH ILLUSTRATING 500 NoRMAL SUBJECTS (ExPRESSED AS PERCENTAGES) GROUPED AccoRDING TO Thus, it would pper tht ge hs no in- THE AMOUNT OF ALP ACTIVITY PRESENT IN THEIR fluence on the dominnt frequency of the electro- REcoRDS
ELECTROENCEPHALOGRAMS IN THE NORMAL ADULT Of the 24 subjects with less thn 25 per cent lph ctivity, 16 hd predominntly intermedite rhythm, nd 8 hd dominnt frequencies in the bet rnge. () Consistency of percentge time lph. The vribility in percentge time lph for n individul ws not so mrked in this series s hs been described by Rubin (11) in bipolr recordings, the men stndrd devition for repeted tests on n individul being 7.1. It should be pointed out tht in this work the percentge time lph quoted includes ll lph ctivity present, whether occurring in single wves or in chins, wheres Rubin's observtions re bsed on criterion of 3 wves of lph frequency occurring together; nd in Rubin's experiments, the blood sugr ws not controlled. The reltion between vribility of percentge time lph nd high or low lph percentge, s described by Rubin (12), did not hold in this series. (b) Reltion between percentge time lph nd dominnt frequency. There ws found to be n inverse reltionship between percentge time lph nd the dominnt frequency of tht lph; in other words, individuls whose percentge time lph vlues were high showed dominnt frequencies in the slower lph rnge. In the totl series of 500 subjects, the records were exmined for the reltion between the dominnt frequency of ny lph present nd the percentge time occupied by this lph ctivity. The following results were obtined: Number Men Alph frequency of tgnge Stndrd sujcs sbetime devition lph Slower thn 10.5 cycles 236 68.5 14.1 10.5 cycles nd fster 264 54.7 20.1 The difference between these 2 mens ws tested for relibility by determining the stndrd error of the difference which ws found to be 8.90. Such high stndrd error of the difference is beyond the possibility of chnce. It is therefore concluded tht n inverse reltionship exists between the dominnt frequency of the lph present nd the mount of totl lph present. (c) Reltion between percentge time lph nd 307 other physiologicl fctors. No correltion ws found in this series between the percentge time lph nd ge, sex, height, weight, or the heightweight rtio. (3) Percentge time intermedite rhythm Frequencies of 13.5 to 17.5 cycles per second, i.e., those which lie between the lph nd bet rnges, re not commonly found in more thn negligible quntities in the records of norml dults, nd re only rrely found s the dominnt frequency of the record. In this series of 500 subjects, there were 18 with dominnt frequency in this rnge, or 3.6 per cent. Rhythms in this rnge, however, normlly occur in bursts during the lighter stges of sleep (13), nd it would not be surprising to find tht they hve different physiologicl origin from the lph wves. They do not pper to be merely ccelerted lph wves, since observtions on sleep show tht they pper bruptly nd do not emerge by grdul trnsition from the higher lph frequencies. These fcts suggest tht one is here deling with dichotomy. As hs lredy been noted, the electroencephlogrm in norml dults contins very little ctivity in this intermedite frequency bnd. Wht little there is might be expected to pper in those records with the fster lph frequencies s n extreme vrition of their predominntly 12.5 to 13.0 cycle rhythms, but n exmintion of this series filed to estblish ny such correltion. There were no more wves of the intermedite frequencies in those records with dominnt frequencies in the fster lph rnge thn in those with predominntly 9.0 nd 9.5 cycle ctivity. Unlike the so-clled lph nd bet rhythms which re present to some extent in ll records, the intermedite bnd of 13.5 to 17.5 cycle ctivity is sometimes totlly bsent, fct which contributes to the impression, previously mentioned, tht one is here deling with dichotomy. (4) Percentge time bet Bet ctivity (i.e., 18.0 cycles per second nd over) ws found, to greter or less extent, in every record in this series t non-fsting blood sugr levels. As hs lredy been remrked, ll
308 MARY A. B. BRAZIER AND JACOB E. FINESINGER c 40%- U0 - S 30%- ' 20% - I0 S..01 2 z. 10%- 24.5% 47% 500 Subjects A 4% under 10% 10%-24% 25%-49% 50%-over PER CENT TIME BETA ACTIVITY FIG. 5. GRAPH SHOWING 500 NORMAL SUBJECTS (Ex- PRESSED AS PERCENTAGES) GROUPED ACCORDING TO THE AMOUNT OF BETA ACTIVITY PRESENT IN THEIR RECORDS electroencephlogrms re mixture of rhythms, nd, in this series, the 8 cses which hve been clssified s bet type, ll contined some lph; their percentges of bet ctivity vried from 51 to 80 per cent. The distribution of bet ctivity in this series is given in Figure 5. Figures 4 nd 5 tken together do not, of course, give the totl picture for the group, since the time occupied by intermedite frequencies nd by slow ctivity is not represented. The mount of bet ctivity in n individul's record ws found to be remrkbly constnt in repeted runs, nd is chrcteristic for the individul. It did not vry with the degree of relxtion chieved by the subject. Although in this series there ws tendency to more bet ctivity in the records of the older subjects, no sttisticl reltion between it nd ge could be estblished. The mount of bet ctivity present did not correlte with sex, height, or weight. (5) Activity slower thn 8.0 cycles per second Wves of frequency slower thn 8.0 cycles per second re here referred to s slow ctivity. Trces of 7.0 to 7.5 per second frequencies were found in the records of 125 out of the series of 500 subjects, but only in 4 individuls ws there more thn 5 per cent of such slow ctivity in the 2-minute recording; of these 4 individuls, 1 hd 19 per cent slow ctivity, 2 hd 9 per cent, nd 1 hd 6 per cent. Six-cycle wves were found in the occipitl leds in 41 (8 per cent) of the 500 records t nonfsting blood sugr levels, but wves slower thn 6 cycles per second were found in only 4 subjects, or less thn 1 per cent. Were n investigtion of the potentils from the temporl regions to be mde in wy similr to the present detiled study of the occipitl potentils, it seems likely tht there would be higher incidence of 6- nd 7-cycle wves in norml records, this being our experience nd tht of other electroencephlogrphers (14). No wve in the rnge commonly clled delt ctivity (i.e., 4 cycles or slower) ws found. (6) Voltge This ws mesured by pir of clipers, djusted to the clibrtion mde for voltge t the beginning of ech record. The mximum voltge ws mesured in every cse nd clssified s to whether it ws under 25 mv, over 25 mv but under 50 mv, over 50 mv but under 100 mv, or over 100 mv. In the ltter cse, further brekdown in clssifiction ws mde between those who hd less thn 20 wves which reched 100 mv in mplitude nd those which hd more thn this number in 2- minute run. The distribution of voltge in this series of 500 norml subjects t non-fsting blood sugr levels ws s follows: () Voltge nd dominnt frequency. The fster frequencies tend to be of low voltge; no record with dominnt frequency fster thn 11.5 hd ny potentils which reched s much s 100 mv. In the lph rnge, the records with the slower lph frequencies were of higher voltge thn
ELECTROENCEPHALOGRAMS IN THE NORMAL ADULT the fster ones, thus following the usul chrcter of oscilltions in which the mplitude is inversely proportionl to the frequency. In the following tble, 474 norml subjects, whose dominnt frequencies were in the 8.0 to 13.0 cycle bnd, re listed ccording to their voltge. Men dominnt frequency Mximum voltge under 50 mv (105 subjects) 11.0 Mximum voltge over 50 mv (274 subjects) 10.4 but under 100 mv Mximum voltge over 100 mv (95 subjects) 10.0 (b) Voltge nd percentge time lph. The mximum voltge of the potentils in record vry directly with the percentge time lph ctivity present; i.e., those records which hve lrge mount of 8.0 to 13.0 cycle wves rech higher mximum potentil. In the following tble, voltge is relted to the men percentge time lph found in the sme series of 474 norml subjects whose dominnt frequencies were in the 8.0 to 13.0 cycle bnd. Mximum voltge Under 25 mv Over 25 mv, under 50 mv Over 50 mv, under 100 mv Over 100 mv DISCUSSION Men percentge time lph 40.3 (10 subjects) 48.4 (95 subjects) 65.4 (274 subjects) 73.6 (95 subjects) Since the first development of electroencephlogrphy, interest hs been centered minly on its ppliction to clinicl problems, nd it is only recently tht there hs been shift of interest to the study of the norml dult. For mny resons, it would hve been preferble hd the reverse tken plce, for the development of this test to hve proceeded from bsic study of the norml to comprison of clinicl records with norm lredy well estblished. The desirbility is ptent for the estblishment of yrdstick for the norml popultion ginst which my be mesured the vribles found in pthologicl records. It is eqully desirble in ttempting to ssess the electroencephlogrms of norml subjects (s, for exmple, is being done in ir-pilot selection) to hve quntittive bsis from which one my clculte the chnces of ny observed phenomenon being norml finding. 309 In this pper, beginning hs been mde in n ttempt to find wht rnge of vrition cn be found for some of the chrcteristics of the electroencephlogrm of norml dults. At the present stge, this study hs been limited to n nlysis of the corticl potentils from the occipitl lobes, nd it cnnot be too strongly emphsized tht different set of dt would undoubtedly be obtined from the frontl lobes, nd different gin from the temporl nd prietl regions. This study is lso limited to nlysis of electroencephlogrms during norml brething. A report of n investigtion during hyperventiltion will follow this, with specil reference to the role of blood sugr nd depth of hyperventiltion. In the pst, the bulk of the work on both norml nd clinicl electroencephlogrms hs been done by the method of pprisl. The experienced electroencephlogrpher hs looked t the record, compred it in his mind with his impression of those records which hve previously come into his lbortory, nd ssessed it from this mentl comprison. Where gross differences re present, such s re found in ptients with epilepsy or with neoplsms, this method hs in the min sufficed, but when finer shdes of difference re being serched for, more finely differentited set of stndrds is necessry. Such stndrds cn only be set up on bsis of ctul mesurement, method which is time bsorbing, but essentil in ny reserch project designed to estblish norml control stndrds. Were such set of stndrds estblished on lrge enough group of individuls, it would then be possible to estimte the chnces of normlity when ny fine differences occur in the record, s for exmple, 7 cycle wves occurring singly in the occipitl leds, or trins of 14 cycle wves. The percentge of norml records in which such wves occur would be known nd the importnce of the finding could thus be ssessed. A development in the mesuring of electroencephlogrms hs been mde by Gibbs nd Grss (15) in the form of spectrum nlyzer. This pprtus gives compiltion of the mount of energy present t ech frequency. At the present stge of our experience, we hve found more mening in the frequency of wves thn
310 MARY A. B. BRAZIER AND JACOB E. FINESINGER in their voltge inf norml records, nd we therefore look for method where the number of wves present t ny frequency is not obscured by the voltge. An instrument for this purpose hs recently been designed by Wlter (16), but is not yet on the mrket. It is obvious tht the current differentition of electricl potentils into lph, bet, nd delt rhythms is rbitrry nd, in some respects, unfortunte. The clssifiction of records into lph nd bet rhythms tends to obscure the fct tht mny so-clled lph records contin potentils of fster frequencies nd tht mny bet records contin percentges of the slower lph frequencies. A more ccurte nd more complete ssessment could be mde by describing the frequency distribution on the potentils. It would seem much wiser to describe rhythms in terms of the incidence of ctully mesured frequencies until some physiologicl or sttisticl reson cn be found for grouping frequencies into certin rhythms. The fct tht significnt contributions to clinicl dignosisn epilepsy nd the locliztion of neoplsms hve been mde by the crude methods of gross inspection of records would by no mens invlidte the need for studies bsed upon creful mesurements. However, once the dt on sufficiently lrge number of cses re collected, it might well be possible to develop simpler methods of nlysis, bsed upon the known verifible distributions of frequencies. This would seem more logicl pproch nd could give more precise informtion in the study of problems in which fine differentitions occur in the records. SUMMARY The occipitl corticl potentils hve been nlyzed under conditions of controlled blood sugr in 500 subjects. Of these, the mjority received but one test, but in 45, repeted observtions were obtined. The following chrcteristics were found in these electroencephlogrms. 1. The dominnt frequency of n individul is comprtively stble, but becomes less so with incresing ge. There is sttisticlly significnt correltion between ge nd stbility of dominnt frequency (45 subjects). 2. Wves of 8.0 to 13.0 cycles per second ("lph") were present in ll records exmined. The percentge time occupied by lph wves vried inversely with the frequency of the dominnt frequency. This inverse reltionship hs been estblished sttisticlly in 500 subjects. 3. Wves of 13.5 to 17.5 cycles per second ("intermedite") re rrely found in ny quntity in norml records, nd constitute the dominnt frequency in only 3.6 per cent of ll normls exmined (500). 4. Wves of 18.0 cycles per second nd fster ("bet") were present in ll records exmined (500). The percentge time occupied by wves in this rnge is nerly constnt for n individul in repeted runs (45 subjects). 5. Wves slower thn 8.0 cycles per second re found in occipitl potentils of 25 per cent of norml subjects. Wves s slow s 6.0 cycles were found in only 41 out of 500 subjects. 6. No wves of 4.0 cycles per second or slower ("delt") were found in the occipitl recordings of ny norml subject while brething normlly (500 subjects). 7. Mximum voltges re higher in those records which contin the most lph ctivity, nd in those records whose dominnt frequencies fll in the slower lph frequencies (500 subjects). 8. The physiologicl fctors of sex, height, weight, or the height-weight rtio did not correlte with ny chrcteristic in the brin wve record. All the electroencephlogrphic trcings for this reserch were recorded in the Brin Wve Lbortory of the Msschusetts Generl Hospitl with the coopertion of the director, Dr. Robert S. Schwb. The uthors re indebted to Mrs. Frnces Cooperstein nd Miss Mrgret Gry for technicl help, nd to Mrs. Mry Newell for nlysis of the records. BIBLIOGRAPHY 1. Loomis, A. L., Hrvey, E. N., nd Hobrt, G., Electricl potentils of the humn brin. J. Exper. Psychol., 1936, 19, 249. 2. Jsper, H. H., nd Cruikshnk, R. M., Visul stimultion nd the fter-imge s ffecting the occipitl lph rhythm. J. Gen. Psychol., 1937, 17, 29. 3. Jsper, H. H., nd Andrews, H. L., Electroencephlogrphy. Arch. Neurol. nd Psychit., 1938, 39, 96. -4. Berger, H., tober ds Elektrenkephlogrmm des Menschen. Arch. f. Psychit., 1932, 98, 231.
ELECTROENCEPHALOGRAMS IN THE NORMAL ADULT 311 5. Loomis, A. L., Hrvey, E. N., nd Hobrt, G. A., Cerebrl sttes during sleep s studied by humn brin potentils. J. Exper. Psychol., 1937, 21, 127. 6. Lindsley, D. B., Brin potentils in children nd dults. Science, 1936, 84, 354. 7. Smith, J. R., Electroencephlogrm during infncy nd childhood. Proc. Soc. Exper. Biol. nd Med., 1937, 36, 384. 8. Weinbch, A. P., Contour mps, center of grvity, moment of inerti nd surfce re of humn body. Humn Biol., 1938, 10, 356. 9. Bernhrd, C. G., nd Skoglund, C. R., Recherches sur l frequence lph de l'electro-encephlogrmme chez l'enfnt. Act psychit. et neurol., 1939, 14, 223. 10. Bernhrd, C. G., nd Skoglund, C. R., On lph frequency of humn brin potentils s function of ge. Skndinv. Archiv. f. Physiol., 1939, 82, 178. 11. Rubin, M. A., The distribution of the lph rhythm over the cerebrl cortex of norml mn. J. Neurophysiol., 1938, 1, 313. 12. Rubin, M. A., A vribility study of the norml nd schizophrenic occipitl lph rhythm. J. Psychology, 1938, 6, 325. 13. Loomis, A. L., Hrvey, E. N., nd Hobrt, G., Further observtions on potentil rhythms of cerebrl cortex during sleep. Science, 1935, 82, 198. 14. Kershmn, J., Personl communiction. 15. Grss, A. M., nd Gibbs, F. A., Fourier trnsform of electroencephlogrm. J. Neurophysiol., 1938, 1, 521. 16. Wlter, W. G., An utomtic low frequency nlyzer. Electronic Engineering. June 1943.