hypnosis study-karl h. pribram-1990

7/30/2019 Hypnosis Study-Karl H. Pribram-1990

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III/emational Jou",al of Psychophysiology. 10 (1990) 125-142Elsevier

PSYCHO 00314

EEG' correlates of hypnotic susceptibility and hypnotic trance:spectral analysis and coherence

D-/ltt125

Michel E. Sabourin 1, Steven D. Cutcomb 2, Helen J. Crawford 3 and Karl Pribram 2.4I Departmenl of Psychology. Uninrsiry of ."'ontreal."'ontreal. Que. (Canada). 1 Neuropsychology LAboratories.

Department of Psychology. Stanford University. Stanford, CA (U.S.A.). J Department of Psychology.Virginia Polytechnic Imtltute and State University. Blacluburg. VA 14061 (U.S.A.) and 4 Centerfor Brain Research

and Informational Sciences. Department of Psychology. Radford University. Radford, VA 14/41 (U.S.A.)(Accepted 19 June 1990)

Key words: Electroencephalogram: Hypnotism; Hypnotic susceptibility Trance; Spectral analysis

EEG was recorded monopolarly at frontal (F3. F4). central (Cl. C4) and occipital (01. 02) derivations during ABA conditionsof waiting rest. hypnosis (rest. ann immobilization. mosquilo hallucination. hypnotic dream). and waking rest. Stringenlly screenedon several measures of hypnotic susceptibility. 12 very low hypnotizable and 12 very highly hypnotizable. right-handed undergraduate. SUbjects participated in one session. Evaluations were FastFourier spectral analysis. EEG coherence between selectedderivations and maximum spectral power within EEG bands. In eyes open and closed conditions in waking and hypnosis. highlyhyPnotizable subjects generated substantially more mean theta power than did low hypnotizable subjects at aU occipital. central andfrontal locations in almost aU conditions of waiting and hypnosis. with a larger difference in frontal locations. Both low and highhypnotizables showed increased mean theta power in hypnosis. suggesting an intensification of allentional processes and imageryenhancement. Mean alpha power was never a predictor of hypnotic susceptibility. Interactions with hypnotic susceptibility showedthat highly susceptible subjects had more beta activity in the left than right hemispheres. while low susceptible subjects showed onlyweak asymmetry. No main effects for or interactions between waking/ hypnosis and hypnotic level were found for coherence betweenderivations or maximum spectral power ~ t h i n theta, alpha and bela EEG bands.

INTRODUCfIONCurrent developments in EEG recording and

analytic techniques permit a reexamination of themany attempts that have been made to identifyelectrocortical correlates of hypnosis (for review.see Crasilneck and Hall, 1959; Sarbin and Sable.1979; Sabc;>urin. 1982). These studies have examined (1) EEG frequency differences betweenlow and highly hypnoti2.able individuals. and/or(2) EEG differences between waking and hypnosis

CO".Spond.nCl: M.E. Sabourin. DepL of Psyehology. University of Montreal. Montreal. Quebec, CanadL

conditions as moderated by hypnotic level. Overcoming many previous methodological limitations,the present study examined EEG frequency banddifferences in waking and hypnosis conditions in

~ u b j e c t s st ringently screened for low and highhypnotic susceptibility levels.

EEG DIFFERENCES BETWEEN LOW ANDHIGH HYPNOTIZABlES

Alpha frequency differencesWhen electroencephalographic (EEG) alphadensities were being emphasiZed. several early

studies reported that highly hypnotizable subjects

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126produced a higher proportion of occipital alphawaves (8-12 or 13 cIs) than those who were notsusceptible to hypnosis (e.g. London et aI., 1968;Bakan and Svoi'ad, 1969; Engstrom et aI., 1970a,b; Morgan et aI., 1971, 1974; Ulelt et aI., 1972a;Edmonston, 1975; Barabasz, 1980, 1982), althoughother studies reported no such relationship (e.g.Edmonston and Grotevant, 1975; Meszaros andBanyai, 1975; Cooper and London, 1976; Dumas,1977; Evans, 1979). When Evans (1979) found norelationships between alpha and hypnotic susceptibility as estimated.by two different scales or byclinical diagnostic ratings, he concluded that thealpha/hypnotizability correlations were likely dueto situational or methodological factors not re-lated to hypnosis per set While Dumas (1977)concluded that these correlations were due to subject selection biases (drafted vs volunteer subjects),Barabasz (1983) presented further data that thecorrelations are 'not simple covariates of subjectself-selection'.Comparisons between eyes-open and eyescloseq conditions within the ~ a m e subjects havefound significantly positive correlations betweenalpha amplitude and hypnotic susceptibility ineyes-closed rest conditions, but no such relationships within eyes-open rest conditions (Galbraithet aI., 1970; Morgan et al., 1974; MacleodMorgan, 1979; DePascalis and Palumbo, 1986) oreyes-closed while perfonning tasks (DePascalis andPalumbo, 1986). The condition of eyes-closed cannot explain differences across earlier studies sincemost had subjects close their eyes during EEGrecording.DePascalis et at. (1988) found significant correlations between hypnotic susceptibility and integrated amplitude, but not alpha density scores.When temporal and parietal alpha were evaluated,without consideration of occipital alpha, no correlations with hypnotic susceptibility were found(DePascalis and Imperiali, 1984). Thus, the location of the recording derivations may be of greaterimportance than previously thought.ThelQ frequency differencesGalbraith et a1. (1970) reported that theta, notalpha, components (within the range from 3

through 12 Hz) in the occipital location were themost important predictors of hypnotic susceptibility. These authors used a step-wise multiple re-gression analysis: 5 Hz and 6 Hz components ineyes-closed resting-and 5 Hz through 9 Hz components in eyes-open resting conditions. The bestpredictors were those in the 5 and 6 Hz range.Using analog frequency analyzer data, Apkinar etal, (1971; Ulelt et al, 1972a, b) found significantcorrelations between hypnotizability and the 3-4.5and 5-7 Hz range frequency in the right occipitalderivation. Macleod-Morgan (l979) reported nodifferences between high and low susceptible subjects in an eyes-closed condition in theta frequencies in occipital-parietal derivations of eitherhemispheres. However, she did find that the highlysusceptible subjects generated more theta in aneyes open condition. Tebecis et a!. (l975) reportedthat highly hypnotizable subjects, well practiced inself-hypnosis, generated more theta activity in theparietal location of both hemispheres, during waking and hypnosis, in conditions of eyes open andeyes closed, than did a second group who hadnever been hypnotized and scored low in susceptibility. Finally, DePascalis and Imperiali (1984)reported no correlations between hypnotic susceptibility and theta generation in the temporaland parietal locations, aU bipolarly referenced tovertex.

Differences in electrode placement across thesestudies make it difficult to compare these findings,but they do indicate the need for simultaneousevaluation of electrocortical activity in posterior,central and anterior locations. 'Since hypnosis and meditation have both beenconsidered to produce alternate states of consciousness often resulting in altered awareness, it isinteresting to note that increased theta duringquiescent meditative states have also been re-ported, often with a greater increase in experienced than in naive meditators (Kasamatsu andHirai, 1969; Banquet, 1973; Wallace et al., 1977;Elson et al., 1977; Corby et aI., 1978; Hebert andlehmann, 1979; TaneH and KIahne, 1987; Saletu,1987). However, the increase in theta activity ismore often frontal than occipital or parietotemporal. : .


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Higher frequency differencesWhile most studies are limited in their spectralanalyses by band pass filters that cut off at 35 Hzor lower, some studies have been able to expandthe Hz range. Analyzing from 0.5 to 70 Hz,Akpinar et aI. (1971; Ulelt et aI., 1972a, b) reported that regression analysis ~ e m o n s t r a t e d hypnotizability to be related to greater beta EEGactivity at the right occipital (02) derivation inwaking. DePascalis and Imperiali (1984) found norelationships between beta and hypnotic susceptibility at temporal and parietal derivations bipolarly referenced to vertex (Cz) in eyes-open andeyes-closed conditions.

EEG CHANGES ACCOMPANYING HYPNOSISAnother line of research has searched (or EEGcorrelates oC the hypnotic state per se, also withrather mixed results. Ulett et al. (1971) reported

that both low and high hypnotizable subjectsshowed similar changes during hypnosis: decreased low Crequency; increased alpha, increasedlow beta, and increased very high frequency betaat the right occipital (02) derivation. Saletu (1987)reported increased delta and theta, but decreasedalpha and beta in left and right occipital locationsduring hypnosis that correlated positively withhypnotizability. Tebecis et al. (1975) r e p o ~ t e d nochanges during hypnosis in the parietalloca.tion ofboth hemispheres, and Meszaros and Banyai(1975) Cound no changes in alpha and beta generation in fronto-occipital bipolar derivations acrosswaldng and hypnosis or as moderated by hypnoticlevel. Once again the results are inconsistent, possibly due to methodological differences, differences in derivations used, or difCering subjectcharacteristics.A commonly espoused hypothesis is that hy-pnosis involves greater right hemisphere involvement as evidenced by enhanced imagery and/orholistic processing that are commonly believed tobe right hemisphere functions (e.g. Springer andDeutsch, 1989; but see Farah (1988) who presentsevidence that the leCt hemisphere may be involved

127in image generation as well). Influenced by findings of lateral differences in electtophysiologicalcorrelates of cognitive processing, studies of differences in hemispheric activity during hypnosiswere initiated in the 1970s and have continued tothe present. Thus, Chen et al. (1981) examined theintegrated amplitude within various Hz bands inbipolar recordings Crom Crontc-parietal derivations in a patient undergoing dental surgery withhypnosis as the sole anesthetic. They found thetotal energy output of the left and right hemispheres to diminish during hypnosis, with a greaterdiminution in the left than right hemisphere inalpha and theta bands. The greater inhibi tion ofthe left hemisphere during hypnosis has also beenreported in studies of electrodermal shifts (e.g.Gruzelier et aI., 1984; Gruzelier, 1987). Crawford,MeszAros and their colleagues (Crawford, 1989;MeszAros et al., 1989; Crawford et aI., ,1989c)reported enhancements in alpha and beta powerin the right parieto-occipital location during hypnosis in rest baseline and task performance byhighly susceptible patients but not by those whowere low in susceptibility.Hemispheric differences during hypnosis havebeen reported in terms of a laterality quotient.This is computed by taking the difference betweenthe integrated amplitude of the alpha frequencyband or total frequency band recorded from theleft and right hemispheres and divimng it by theirsum: left - Right / Left + Rieht. There are, however, inherent difficulties in interpret ing ratioscores since one cannot gauge the relative contribution of left and right hemisphere activity to theratios (e.g. Gevins and Scharrer, 1980; Gevins,1983; Beaumont et aI., 1984). Using occipitalvertex bipolar derivations, LaBriola et al. (1987)reported greater overaU total amplitude in theright hemisphere during hypnosis, while two otherstudies (Morgan et al., 1974; Meszaros and Banyai,1975) did not. Shifts toward greater right hemisphere relative to left hemisphere involvement d u r ~ing hypnosis have been reported for bipolar derivations oC occipital-parietal alpha (Macleod-Morgan, 1982), occipital-temporal total power(Karlin et al., 1980b; Karlin et al., 1981; LaBriolaet al., 1987), and fronto-occipital alpha and beta(Banyai et al., 1985; Meszaros et aI., 1986).


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PRESENT STUDYThe purpose of the present study was to validate, clarify and extend previous research in this

area of investigation. More stringent criteria wereestablished for sllbject selection. Most previousstudies have used a single testing procedure forhypnotic susceptibility, which did not permit theplateauing of stable hypnotic scores. Further, exclusive use of the Harvard Group Scale of Hypnotic Susceptibili ty (Shor and Orne, 1 9 ~ 2 ) or theStanford Hypnotic Susceptibility Scale, Form A(WeitzenhoCCer and Hilgard, 1959), emphasizesmotor and challenge suggestions rather than moredi(ficult cognitive suggestions (such as hallucinat ions and cognitive distortions) found in scalessuch as the Stanford Hypnotic Susceptibility Scale.Form C (Weitzenhoffer and Hilgard, 1962). In thepresent study, subjects were stringently screenedon multiple measures of hypnotic responsiveness,two of which are cognitive, in order to achieveplateaued hypnotic scores. The final selection ofsubjects was limited to subjects who consistentlyscored either very high or very low in hypnoticsusceptibility across screening sessions.Within the same experimental session, subjectshad their EEG recorded during rest baselines inan ABA design of waking, hypnosis and wakingconditions with eyes open and closed, so that wecould evaluate possible diCCerences in relationshipsbetween spectral power and hypnotic susceptibility across eyes open and closed conditions. AdditiOnal EEG recordings were made during thepresentation of 3 hypnotic suggestions involvingmotor and imagery/hallucinatory factors. Thus,we are able to evaluate both state (waking vshypnosis) and trait (high vs low hypnotic susceptibility) main effects and interactions.Choice of 6 monopolar EEG derivations,located bilaterally in the major areas of the brain(frontal, central and occipital) referencing toa non-active site, permitted reasonable evaluationof regional activity. As one physiological indicatorof arousal or activation (see Pribram and McGuinness, 1975. for definition of arousal as usedhere; also see Lacey and Lacey. 1970). concurrentheart rate measures were taken. Fast-Fourier spectral analysis was performed for each EEG deriva-

tion and then mean spectral power withinbandwidths (theta. alpha and beta) were subjto appropriate repeated analyses of variance.there is evidence for increased coherence dmeditation (e.g. Banquet, 1913), we felt iportant to determine whether coherence betderivations might also increase during hypnomodera ted by hypnotic level. We evaluatetrahemispheric and interhemispheric coherenassess the degree of similarity or covariance iEEG from two chosen derivations.

METHODSubjectsSubjects were 24 (14 women and 10 men).came from a pool of approximately 600 StaUniversity undergraduate students which hadbeen given a modified 10-item version oHarvard Group Scale of HyPnotic Suscepti(HGSHS; Shor and Orne, 1962). Right-hasubjects scoring either 9 or 10 (high susceptand 0 or 1 (low susceptibles) on the HGSHSasked to volunteer for further hypnotic tewith the Stanford Hypnotic Susceptibility SForm C (SHSS: C: WeitzenhofCer and Hil1962). Approximately 100 students were thusvidually tested. Only those scoring 11 0 ; 12.or 1 on SHSS : C, and who previously belongthe same category, were kept for the EEG reing session. These subjects reported stronghandedness in writing and other activitiesditionally, they demonstrated right-eyednesssighting and right-footedness when kicking aThus, a sample of 26 subjects, 14 highs alows, were selected; two high susceptiblesthen left out to equalize group numbers.study reports on 12 highs (4 men, 8 women

12 lows (6 men, 6 women). Due to thenumbers we were unable to assess for pomoderating effects due to gender. Six high sutible subjects were further tested with the ReStanford Profile Scales, Forms I and II (Wehoffer and Hilgard. 1967): the mean scorForm I was 23.8/27.0. whereas it was of 24.3for Form II. Since the objective of obtaininferential profiles amongst the highs was unat

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129TABLE 1EEG "cording sessionE.tperimemaJ condi/ian1 Eyes-open baseline2 Eyes-closed baselineJ Hypnotic induction (eyes-open)4 Hypnotic induction (eyes-closed).s Trance deepening instructions6 Trace deepening7 Arm immobilization instructions8 Arm immobilization challenge9 Mosquito hallucination instructionsto Mosquito hallucination

11 Hypnotic dream instructions12 Hypnotic dream13 Awakening procedure14 Eyes-open baselineIS Eyes-closed baselineTotal

Duru/ion.s min.s min10 min approx.10 min npprox.2 min2 min2 min

10 s1min10 s2 min2 minJ min.s min.s min

S4 min approx.

Number of epochs101010. io444121446to10

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ble (all subjects scored very high in almost everycategory), this lengthy procedure was soon discontinued. However, the scores on these scales clearlyconfirmed that the subjects selected were indeedvery highly hypnotizable subjects.Certain factors (like meditation experience. cof

fee and alcohol drinking. smoking, sleeping habits,drug intake), which may have potential influenceson the EEG record were evaluated and found tobe either absent or not different in the two groupsof subjects. No brain damage was evident.Experimental procedure

The EEG recording session lasted approximately 90 min. including preliminary instructionsand electrode placement. Great care was taken todevelop rappon with the subjects and put them atease. Subjects had previously been administeredthe SHSS; C and Profile hypnotic scales by thesame experimenter in the same room. The EEGrecording procedures were described clearly to thesubjects and aU questions were answered beforethe session. Each subject benefited from a shortadaptat ion period (approx. 10 min), while therecording equipment was adjusted. Postexperimental reports of tension' during the experiment

was uniformly low, with no differences betweenlow and high hypnotizables.After eyes-open and eyes-closed baseline recordings, subjects were submitted to a s tandardized taped hypnotic induction procedure involving eye-fixation and suggestions of relaxation andeye-closure_ Immediately after hypnotic induction,trance deepening instructions were given, and subjects were left to themselves for a period of twomin. This was followed by hypnotic testing basedon a motor item (arm immobilization), a hallucinatory item (mosquito hallucination), and afantasy item (hypnotic dreaming); these specificitems were selected in order to probe differenthypnotic abilities, and because in these casessuccess or failure did not require differentterminating instructions. The awakening procedure was then initiated, and final baselines wererecorded. An outline of the procedure is providedin Table I. Immediately after removal of the electrodes, a short postexperimental written questionnaire was given in order to collect the differentsubjective reactions to the experience. Questionsabout situation-related anxiety, self-scoring of thehypnotic items, and the content of the suggestedhypnotic dream were included. Finally, all ques-


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130tions raised about the experience were answeredand all subjects were asked not to discuss thesemallers with other potential subjects; each subjectwas given a chart record of his EEG, EOG andEKG as a token of appreciation.EEG recordingSix monopolar EEG derivations were used,located according to the 10-20 System (Jasper,1958) at 01, 02, C3, C4 and F3, F4. All recordings were done at an amplification of 50 IlV /cm.Ground was attached at a point 3 cm above thenasion; the reference was linked ears (A1-A2),balanced for impedance. A lead I EKG was re-corded and a bipolar EOG record was.also taken,with electrodes at the outer canthus and sub-orbitalto the right eye. EEG and reference electrodeplacements were tested to insure contact resistanceof 10 K or less, and balanced for impedance levelas closely as possible.Eight channels of a Beckman type R Dynograph were used to record these physiological sig-nals. EEG and EOG were recorded using a timeconstant of 1 with low-pass filters set to HI OUTand High frequency ... 2. The empirically derivedlow-pass frequency response function at these setting had a -3 dB point at 15 Hz. with 50%allenuation at 28 Hz.A time event marker was encoded by latchingtwo Hunter timers, such that the Beckman eventchannel was high for 8 s (duration of an epoch)and low for 7 s, in a cycle which produced 4 highperiods (or epochs) per min. All 8 channels plusthe event signals were recorded on chart paper,and the 6 EEG channels plus the events channelwere recorded on a 7 channel Ampex SP-300 FMtape recorder (a Tektronic 120 4-channel scopewas used to monitor the Beckman power amplifierbackplane outputs and the signals were tape-recorded).The recording system was calibrated after test

ing every 6 subjects by feeding a 10 c/s, 100 IlVsinewave into each of the EEG couplers. whilerecording the output to FM tape. An averagepeak-to-peak amplitude of the calibration signalwas calculated, upon playback of the record intothe analog-to-digital converter, to generate 6 floating-point scale factors for adjusting raw EEG in

order to correct for interchannel differences priorto the Fast-Fourier Transform (FF1).Primary signal analysis consisted of analog-todigital conversion of the tape-recorded EEG, editing of the digitized EEG, performing the FFT onselected EEG epochs. power and coherence spectral computation and ensemble averaging of thesespectra.All the analyses were done on a PDP-1l/34aminicomputer. Tape-recorded EEG was playedback to the ARll real-time analysis peripheral foranalog-to-digital conversion at either the recording speed (1 7/8 ips) for the first couple of subjects or at 4 times this speed (7 1/2 ips) for mostsubjects, the latter chosen for speed without loss

of precision. The conversion rate was 64 cis perchannel real-time. which meant digitizing at 256c/s/ch. when playback speed was 4 times therecording speed (for about half the subjects inboth groups). Each sampling epoch was of an 8-sduration. creating an integer array of 512 pointsper channel per epoch. One hundred and eightyepochs were digitized for each' subject. representing approx. 45 min of the original recordingsession.EEG editing was based on several criteria. Thefirst selection occurred during d i g i ~ i z i n g : althoughmore than 180 EEG epochs had often been recorded, only 180 were kept for conversion. Therejected epochs were those containing obviousmuscle or eye-blink artifact. Next, the raw EEGpaper record corresponding to each digitized epochwas visually scanned by two observers to identifyless obvious eye-movement or muscle artifact contamination of that epoch. The number of eachunwanted epoch was entered into a computer fileto effect rejection prior to performing the FFT.This editing procedure was necessary to ensurelow spectral energy in the 32-64 Hz frequencyband, since energy in this band would alias to the0-32 Hz frequency l>and of interest (Bendat andPiersol, 1971). A third form of editing was programmed by computer, and thus done automatically. Since the AR11 data buffer is 10 bits in size,the digitized data are represented by values in therange 0-1023, corresponding to an input sigpalvoltage range of - 2.5 to 2.5 V. This 5 V voltagewindow accommodates EEG of 150llV amplitude,


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but large transient artifacts cause digitized valuesto be extreme, either 0 or 1023. For each epoch, acount was kept for each channel, and if thethreshold of 10 extremes was exceeded on anychannel, that epoch was tagged for rejection priorto performing the FFT. Final ly, only 90 of theremaining digitized epochs representing the different experimental conditions were kept tor the FFTprocessing stage.

The selected 90 epochs of digitized EEG datawere then subjected to the FFT, thus transformingthe time-series into 'frequency representation. Digitizing at rate X allows the frequencies representedto be from DC to 1/2 X so our data yielded DCto 32 cis in spectral representation. This procedure is equivalent to subjecting the raw time-seriesto a cosine bell data window; this is a convolutionof the Fourier results with a 3-point Hanning filter(weights om x I_ I ' " 0.25, XI ' " 0.50, x l + l ... 0.25),which is equivalent to having subjected the original data to convolution with a cosine beU filter(Bendat and Piersol, 1971). The FFT was implemented by using a fixed-point assembly-language program available from Digital EquipmentCorporation, and 'the first and second halves ofeach epoch were transformed separately; thus, 256point transforms were accomplished, creating aresolution of 1/4, cis in the resulting spectra.Ensemble averaging, of the two non-overlappingsegments per channel per epoch created a peridogram with at least 4, df, and perhaps more due tothe Hanning convolution (Bendat and Piersol,1971).Autospec tral estimates were then ensemble

averaged, within each of 15 blocks for each subect, to produce smoothed power spectrum estimates for each condition and for each group.Data analysis

The EEG of low and high hypnotizables inwaking and hypnosis conditions were evaluated by3 different approaches: (1) Fast-Fourier spectralanalysis, (2) EEG coherence between selected derivations, and (3) maximum spectral power withineach EEG band. There were no shifts in maximumspectral power during hypnosis even when moderated by hypnotic susceptibility. These analysesa re therefore not detailed. Ordinarily, the main

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effects and interactions of the repeated measuresanalyses of variances will be reported if subsequent Tukey mean comparisons verified signifi cant differences (P < 0.05). Comparisons addressing hypnotic susceptibility level and differences incondition are emphasized. The effects of eyes openor closed during rest conditions were separatelyassessed but because very few significant differences were obtained between these conditionsin terms of the relevant comparisons, only theeyes-closed data are reported unless they arevariant with eyes open. Instruction periods priorto the actual suggestions were excluded. Afterspectral analysis of 0-35 Hz, the mean powerestimates for 3 Hz bands, theta (4.00-7.75 Hz),alpha (8.0-12.75 Hz) and beta (13-28 Hz), foreach condition were extracted. Although 0-35 Hzwas spectral analyzed, the be ta bandwidth wasattenuated due to the filters employed; thus, betaanalyses were limited to the range below 28 Hz.

RESULTSSection I: spectral analysisEyes-closed rest conditionsFor each of the bandwidths repeated measures

ANOVAs were performed separately: 2 (hypnoticsusceptibility: low vs high) X 3 (intrabemisphericlocation: frontal, central and occipital) x 2 (interhemispheric location: left and right) X 5 (condition: initial waking baseline. hypnotic inductionafter eye closure, hypnosis per se during trancedeepening instructions, awakening procedure before opening eyes, and final waking baseline).

Theta. There was a 3-way interaction betweenhypnotic susceptibility, condition oC hypnosis, andintrahemispheric location, ~ . 1 7 6 ' " 3.26, P < 0.01(see Fig. 1). Highly hypnotizable subjects hadsubstant ia lly more mean theta power (P < 0.01)than did low hypnotizable subjects in occipi ta l,central and frontal regions in aU conditions butone, the central locat ion where there is a non-significant trend during the final waking baseline.While maintaining these significant differences,both lows and highs showed significant increasesin mean theta power (P < 0.01) from initial wale-

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Fig. 3. Bela mean power 81 fronlal (Fl, F4). central (Cl. C4) and occipital (01. 02 ) locations for low and hipiy hypnolizablesubjects across e y e s ~ l o s e d conditions of initial wwng baseline. hypnotic induction aller eye closure. hypnosis baseline. awakeningand final waking baseline.


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ing to hypnosis in all 3 locations. Both low andhigh hypnotizable subjects showed a substantialdecrease in mean theta power in all 3 locationswhen coming out of hypnosis. There were nosignificant interactions involving interhemisphericlocation with hypnotic susceptibility or conditionsof. hypnosis.. Alpha. There were no significant main effects orinteractions involving hypnotic susceptibility.There was a significant location by condition ofhypnosis interaction, 18.176 - 61.95, P < 0.001 (seeFig. 2). During initial waking baseline, central andfrontal locations showed significantly more meanalpha power than the occipital location (P < 0.05),while when coming out of hypnosis and in. thefinal waking baseline, the occipital location showedsU,bstantially more power than the other two locations (P < 0.05); during hypnosis, the locationswere equal. Thus, there was a significant increasein mean alpha power as the experimental progressed at the occipital locations but not at the

l3Scentral and frontal locations. Significantly moremean alpha power was found overall in the lefthemisphere, 1.22 .... 16.39, P < 0.01.Bela. There was a significant location by condition interactions, 18.176'" 47.57, P < 0.001. As isseen in Fig. 3, mean beta power decreased significantly as the experiment progressed at the QC,-.cipitallocations, but not significantly at the centraland frontal locations.

During eyes-open rest in waking and hypnosis,there was a significant interaction between interhemispheric location and hypnotic susceptibility,1.22 . .. 4.38, P < 0.05. As is seen in Fig. 4, highlysusceptible subjects had substantial ly more betapower in the left hemisphere than did low hypnotizable subjects, while subjects did not differ significantly from one another in recordings madefrom the right heniisphere. In addition, the highlysusceptible subjects showed significantly moreoverall beta power recorded from the left thanfrom the right hemisphere, while the low suscepti.

EYES OPEN EYES CLOSED0.8 0.8

~ ~ H I G H S;)I0.8 0.8 ~ H I G H Sm il l LOWS! 0.4 0.4 ill mLOWS~ .i 0.2 0.2

'I0 0LEFT RIGHT LEFT RIGHTHEMISPHERE HEMISPHERE ...

FilJ. 4. Beta mean power of left and right hemispheric locations for low and highly hypnotizable subjects across eyes-open andeycsdosed baseline measurements.


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ble subjects did nolo There was a non-significanttrend, FI 3Z'" 4.17, P < 0.10, for a similar interac-. tion b e t ~ e e n hypnotic' susceptibility and interhemispheric location for the eyes closed restingconditions. Overall, there was more mean betapower in the left hemisphere, Fl.zZ -= 4.39, P


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naire (Paivio, 1971), which assesses the main factors of (1) use of verbal abilities and (2) use ofimagery. While the low and high hypnotizablesdid not differ on verbal-related factors, the highsreported significantly higher involvement on themain imagery factor, as well as the additionalscales of mental pictures, daydreams and the use.of mental pictures (all P < 0.02).

DISCUSSIONMean theta power

The major robust finding of this study is thatmean theta power seems to be strongly and positively related to the trait of hypnotic susceptibility. This confirms what other studies based onsimilar techniques had previously reported(Galbraith et a1. 1970; Ulett et aI., 1972a; Tebeciset al., 1975). In both eyes-open and eyes-closedconditions in waking and hypnosis highly hypnoticaUy susceptible subjects generated substantiaUymore mean theta power than did low hypnotizabies. Moreover, the interactions indicate that thisdifference held up at all occipital, central andfrontal locations in aU conditions but one, with alarger difference in frontal locations. While maintaining a significant difference between groups,both low and highly susceptible subjects showed asubstantial increase in mean theta power at all 3locations after the hypnotic induction, that continued at a similar level through the various hypnotic suggestions. Both low and highly hypnotizable subjects showed a substantial decrease inmean theta power in aU 3 locations upon leavinghypnosis. The literature concerning theta and psychological phenomena (for review, see Schacter,1977) shows that increments in theta activity occur in a variety of problem-solving, perceptualprocessing and cognitive taslu. While this increment can be observed in a variety of locations, itis most prominent in the fronto-central areas.As lor state differences, hypnotic susceptibilitylevel did not playa moderating role. Enhancements in mean theta power during hypnosis occurred for both low and high hypnotizables. Whiledifferences between low and high hypnotizablesubjects were maintained in hypnosis, the percent

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increase from the initial waking baseline to thehypnosis baseline was somewhat larger for thelows than the highs: respectively, 86% vs 71% atthe occipital locations, 76% vs 53% at the centrallocations, and 76% vs 43% at the frontal locations.It is possible that the theta observed in ourhighly susceptible subjects renects common underlying cognitive mechanisms that differentiatethem from low susceptible subjects. One suchmechanism could be related to attentional components. Highly hypnotizable subjects often reportgreater absorptive attentional skills on questionnaires as weU as demonstrating greater attentionalskills in experimental tasks (e.g. Tellegen andAtkinson, 1974; Crawford et aI., 1989a). The hypnotic induction is thought to intensify selectiveattention or inat tention. In the same vein, we canalso refer, as Galbraith et a1. (1970) have done, tothe Class II concept of Vogel et a1. (1968), whichpostulates that slow waves represent 'a selectiveinactivation 01 particular responses so that a continuing excitatory state becomes directed or patterned (p. 172)'. It should be noted that slow EEGwaves represent two kinds of behavioral inhibition. The enhanced theta of the high hypnotizabies in the present study is thought not to beassociated with'Class I inhibition', which is correlated with general inactivity or drowsiness. butrather to be associated with 'Class II inhibition',which is correlated with more efficient and attentive performance (Vogel et aI., 1968).Further support for the relationship betweentheta and problem solving (Vogel et a1.. 1968;Schacter, 1977) comes from recent work by Crawford (1990) which examined the EEG correlates ofcold pressor pain in hypnosis, with and withoutsuggested analgesia. In low theta (3.5-5.5 Hz)' there were no differences in power between lowand high hypnotizables, but in high theta (5.5-7.5Hz) highs generated substantially more power thanlow hypnotizables at frontal, temporal. parietaland occipital locations. Crawford et al. (1986; see :!also Crawford, 1989) found substantial increases(as much as 28% in comparison to waking conditions) in regional cerebral blood now. an i n d i c a t o ~of cortical metabolism accompanying cognitivearousal/ performance, in anterior, parietal. tem":poral and temporal-posterior regions. during hyp-


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138notic rest and ischemic pain (with and withoutsuggested analgesia) conditions in high, but notlow, hypnotizables.A complementary hypothesis can be based onreports involving the hypnagogic state, a state alsocharacterized by the presence of low voltage thetaactivity (for review, see Scbacter, 1977). In suchresearch, many experiments, like those of Foulkesand Vogel (1965) and Stoyva (1973), have shownthat psychological phenomena related to imageryproduction accompany the low voltage theta EEG."It follows that those individuals who producemoretheta may be higher in imagery abilities. Supporting such a hypothesis in the present study were thepositive relationships found between hypnotic susceptibility and greater reports of imagery mediated mentations in a subset of the sample.Mean alpha powerMean alpha power was never a predictor ofhypnotic susceptibility in this study. Since oursubjects were all familiar with the experimenterand the research situation, due to prior hypnotictests and precautions taken to make the subjectsfeel at ease. we believe situational variables hadless of: a differential effect upon low and highhypnotizables than in some prior studies (for review, see Evans, 1979). Contrary to many findingsthat report no hemispheric asymmetry for alphaor greater right hemisphere alpha production (e.g.for review, see Butler and Glass, 1973; Gevins andSchaffer, 1980), in our very quiescent conditions,mean al,pha power was significantly greater in the'left hemisphere across locations and conditionsfor total subjects.Unexpectant distributions of mean alpha poweramplitude across locations in the waking, hypnosis, wakinS series were found. During preliminarywa!dnS measurements 3feater power was found infrontal and central, than occipital, locations. lbisis contrary to the common flDdin3 in the literaturethat alpha is 3feater in the occipital than in themore anterior locations. As the experiment proceeded. 'si3J1ificant shifts occurred: during hypnosis the locations were equal in mean alpha power.while in the post-waking measurement the accipitallocation had significantly more mean alphapower. These unexcepted distributions cannot pre-

sently be fully explained; however, recent findingsregarding a parietal location for an alpha genera. tor at the lower end of the band may provide aclue.In the present study we examined the broadalpha range of 8.0-12.75 Hz. Recent findings (e.g.Herrmann. 1982; Coppolo, 1986; Coppolo andShassy, 1986) indicate that low and high alpha aredifferentially distributed across individuals, andthat there are different alpha generators in theoccipital (high alpha) and parietal (low alpha)locations. Crawford et at. (1989b) found differential changes in integrated amplitude power in lowand high alpha bands at frontal and parietallocations across induced happy and sad emotionalstates in waking and hypnosis, as moderated byhypnotic level. Thus, future research shouldevaluate separately low and high alpha bands (aswell as low and high theta bands).As the experiment progressed, mean alphapower increased while mean beta power decreasedsignificantly in the occipital locations (Figs. 2 and3). Yet, there were no significant increases ordecreases of alpha and beta, respectively, at thecentral and frontal locations. Given the substantial increases in the theta band in hypnosis, theseresults suggest the need to examine the patterns oflocation changes across Hz bands as they relate,perhaps differentially, to both cognitive andphysical arousal levels.Mean beta power

An interesting interaction between h y p n ~ t i csusceptibility and interhemispheric location' occurred only in the beta band. Highs had substantially more mean beta power in the left hemisphere across the 3 locations than did low hyp,no-. Itizables, while they did not differ significantlyfrom one another in the right hemisphere. Whilethe mean beta power was essentially the same inboth hemispheres for the low hypnotizable subjects. the high hypnotizable subjects showed significantly more overaU beta power in the left thanin the right hemisphere. This was significant in theeyes-open rest conditions, and showed non-significant trends in the same direction for eyes-closedconditions. Thus. regardless of condition, highhypnotizable subjects seem to show greater asym-


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,metry between the two hemispheres in the betaband than do low hypnotizable subjects.The results suggest that there are differences in

c ~ a r a c t e r i s t i c patterns of hemispheric arousal associated with hypnotic susceptibility. The highlyhypnotizable subjects in the present study demonstrated characteristically higher left hemispherearousal in beta, the cause of which is unknown. .Highly stable individual differences in asymmetries of electrocortical activity over time have beenreported (Morgan et aL, 1971; Ehrlichman andWeiner, 1979). These differences were found to becorrelated with differential behavioral performance (e.g. Furst, 1976; Glass and Butler, 1917;Rebert, 1917; Davidson et aL, 1979; levy et al.,1983). Crawford, Meszaros and their associates(Crawford et a!., 1988; Crawford, 1989; Meszaroset al., 1989) reported differential bipolar frontocentral and parieto-occipital asymmetries in thelow' and high alpha bands during waking andhypnosis in low and highly hypnotizable subjects.Using either the alpha band or overall total power,several studies (Karlin et aI., 1980a; Macleod-Morgan and lack, 1982; Meszaros and Banyai,1985) have demonstrated that high hypnotizablesubjects have greater hemispheric specificity during the performance of tasks during waking; thatis, highs show greater left hemisphere activationwhen performing analytical tasks and greater righthemisphere activation when performing visuospatial, holistic tasks. While DePascalis et al. (1988)found no evidence supportive of occipital-parietalhemispheric specificity differences, a fotlow-upstudy (DePascalis and Palumbo, 1986) foundhemispheric asymmetry differences for low butnot high difficulty taslcs. While the above hypnosisstudies used bipolar recordings of different derivationS; and are difficult to compare with the prese'ntstudy's monopolar recordings, recent researchusing monopolar recordings have found highs toshow greater hemispheric asymmetries in certainEEG bands than lows, in induced emotional statesduring wa1cing and hypnosis (Crawford et al.,1989b) and in cold pressor pain with and withoutsuggested analgesia (Crawford, 1990).Worthy of further investigation is the suggestedhypothesis in all of these studies that differencesin hypnotic susceptibility may accompany dif-

139ferential patterns of asymmetric hemisphericarousal. like DePascalis and Palumbo (1986), wefound these differences to occur more in the lefthemisphere. Interestingly, when hemispheric elecotrocortical activity differences have been reponedacross cognitive tasks, often it is the left hemisphere which appears to shift in power more than- ' the right hemisphere (e.g. Gevins, 1983): - .Heart rale changesWhile highly susceptible subjects tended to havesomewhat higher heart rates across all conditions,only highly susceptible subjects had significantlyhigher heart rates during the mosquito and: hypnotic dream suggestions during hypnosis. .Tasks which require acceptance of the input ofexternal stimuli are accompanied by heart ratedeceleration while tasks which require the rejection of external stimuli and the focusing uponinternal mental processes are accompanied byheart rate acceleration (lacey et aI., 1963; van derMolen et aI., 1984; Cole and Ray, 1985). It is ofinterest, therefore, that the two hypnotic suggestions that required the strongest focusing uponinternal mental processes, with a giving up otreality testing, are the ones which discriminatedbetween low and high hypnotizable subjects. Thesignificant increase in heart rate among highly

susceptible subjects may indicate deeper involvement by the subjects in the internal mentalprocesses. Future research could test the hypothesis of greater attentiona! involvement in highlysusceptible subjects by evaluating heart rate differences while perfonning input acceptance andrejection tasks, such as those used by Ray andCole (1985). .

CONCLUSIONIn summary, the results of the current s tudydemonstrate that highly hypnotizable subjects hadmore mean theta activity in frontal, central andoccipital locations than did low hypnotizable subjects in both waking and hypnosis conditions.Interactions with hypnotic susceptibility show that. ,highly susceptible subjects had more beta activityin the left than right hemispheres, while sus-


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le subjects s h o ~ e d only weak asymmetry.low and high hypnotizable subjects showedements of theta during fijpnosis, suggestingensification of attentional processes and anement of imagery.

search support to Michel Sabourin was prod by a visiting scholarship grant by theistry of Intergovernmental Affairs of thernment of Quebec, Canada. Additional supcame from a grant from The Spencer Founn to Helen Crawford.

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hypnosis study-karl h. pribram-1990

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