....

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-/ltt

125

Michel E. Sabourin 1, Steven D. Cutcomb 2, Helen J. Crawford 3 and Karl Pribram 2.4

I 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 Center for Brain Researchand 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 A·B·A 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 under·graduate. SUbjects participated in one session. Evaluations were Fast·Fourier 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 ~thin theta, alpha and bela EEG bands.

t,..

INTRODUCfION

Current developments in EEG recording andanalytic 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. Univer·sity 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~ubjects stringently screened for low and highhypnotic susceptibility levels.

EEG DIFFERENCES BETWEEN LOW ANDHIGH HYPNOTIZABlES

Alpha frequency differencesWhen electroencephalographic (EEG) alpha

densities were being emphasiZed. several earlystudies reported that highly hypnotizable subjects

!

0167-8760/9O/S03.'0 <01990 Elsevier Science Publishers B.V. (Biomedical Division)

".."

126

produced 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 re­lated 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 ~ame 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 greater·importance than previously thought.

ThelQ frequency differencesGalbraith et a1. (1970) reported that theta, not

alpha, 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 re­gression 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 re­ported, 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 spectral

analyses 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 ~emonstrated 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 HYPNO­SIS

Another line of research has searched (or EEGcorrelates oC the hypnotic state per se, also withrather mixed results. Ulett et al. (1971) reportedthat 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) repo~ted 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 hy­pnosis 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

127

in 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 inhibition 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 interpreting 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 dur~

ing hypnosis have been reported for bipolar de­rivations oC occipital-parietal alpha (Macleod­Morgan, 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).

128

PRESENT STUDY

The purpose of the present study was to vali­date, clarify and extend previous research in thisarea 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 Susceptibility (Shor and Orne, 19~2) or theStanford Hypnotic Susceptibility Scale, Form A(WeitzenhoCCer and Hilgard, 1959), emphasizesmotor and challenge suggestions rather than moredi(ficult cognitive suggestions (such as hallucina­tions 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) ~th 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 within 3 Ibandwidths (theta. alpha and beta) were subjectto appropriate repeated analyses of variance. Sinthere is evidence for increased coherence duri:meditation (e.g. Banquet, 1913), we felt it ifportant to determine whether coherence betwe:derivations might also increase during hypnosismoderated by hypnotic level. We evaluated j

trahemispheric and interhemispheric coherenceassess the degree of similarity or covariance in t!EEG from two chosen derivations.

METHOD

SubjectsSubjects were 24 (14 women and 10 men). wi

came from a pool of approximately 600 Stanfo:University undergraduate students which had firbeen given a modified 10-item version of tl­Harvard Group Scale of HyPnotic Susceptibili'(HGSHS; Shor and Orne, 1962). Right-handtsubjects scoring either 9 or 10 (high susceptibleand 0 or 1 (low susceptibles) on the HGSHS weasked to volunteer for further hypnotic testirwith the Stanford Hypnotic Susceptibility SealForm C (SHSS: C: WeitzenhofCer and Hilgar.1962). Approximately 100 students were thus inavidually tested. Only those scoring 11 0; 12. andor 1 on SHSS : C, and who previously belonged t

the same category, were kept for the EEG reconing session. These subjects reported strong righhandedness in writing and other activities. A'ditionally, they demonstrated right-eyedness whesighting and right-footedness when kicking a bai

Thus, a sample of 26 subjects, 14 highs and 1lows, were selected; two high susceptibles weIthen left out to equalize group numbers. Thstudy reports on 12 highs (4 men, 8 women) an12 lows (6 men, 6 women). Due to the smanumbers we were unable to assess for possiblmoderating effects due to gender. Six high suscertible subjects were further tested with the Revise'Stanford Profile Scales, Forms I and II (Weitzerhoffer and Hilgard. 1967): the mean score fcForm I was 23.8/27.0. whereas it was of 24.3/27.for Form II. Since the objective of obtaining diiferential profiles amongst the highs was unattaina

:.;.

129

TABLE 1

EEG "cording session

"E.tperimemaJ condi/ian

1 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 instructions

to Mosquito hallucination11 Hypnotic dream instructions12 Hypnotic dream13 Awakening procedure14 Eyes-open baselineIS Eyes-closed baseline

Total

Duru/ion

.s min

.s min10 min approx.10 min npprox.2 min2 min2 min

10 s1 min

10 s2 min2 minJ min.s min.s min

S4 min approx.

Number of epochs

101010

. io444121446

to10

90

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. cof­fee 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 procedureThe EEG recording session lasted approxi­

mately 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 shortadaptation 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 standar­dized 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-

;.

l..,..:

130

tions 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 re­corded 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 sig­nals. 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 test­ing 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 lossof 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 digi~izing: 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,

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. Finally, 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 xI_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).

Autospectral estimates were then ensembleaveraged, within each of 15 blocks for each sub­ject, to produce smoothed power spectrum esti­mates for each condition and for each group.

Data analysisThe EEG of low and high hypnotizables in

waking 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 analysesare therefore not detailed. Ordinarily, the main

131

effects and interactions of the repeated measuresanalyses of variances will be reported if subse­quent Tukey mean comparisons verified signifi­cant 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 beta bandwidth wasattenuated due to the filters employed; thus, betaanalyses were limited to the range below 28 Hz.

RESULTS

Section I: spectral analysis

Eyes-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, ~.176'" 3.26, P < 0.01(see Fig. 1). Highly hypnotizable subjects hadsubstantially more mean theta power (P < 0.01)than did low hypnotizable subjects in occipital,central and frontal regions in aU conditions butone, the central location 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. I. Theta mean power at rrontal (Fl. F4). central eCl. C4) and occipital (01. 02) locallons ror low and hiply hypnotizablesubjccts across eyes-closed conditioll$ or initial waiting baseline. hypnotic induction arter eye closure. hypnosis baseline. awalteninll

and final waking baseline.

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133

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Fig. 2. Alpha mean power :u frontallF3. F4). central IC3. C4) and occipital (01. 02) locations for low and highly hypnotizablesubjects across eyes-closed l:onditions of initial waiting baseline. hypnotic induction after eye dosure. hypnosis baseline. awakening

and final waking baseline.

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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 eyes~losed conditions of initial wwng baseline. hypnotic induction aller eye closure. hypnosis baseline. awakening

and final waking baseline.

',.

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

l3S

central 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 substantially 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 CLOSED

0.8 0.8

~~HIGHS:;)

I0.8 0.8

~HIGHSm ill LOWS

! 0.4 0.4

ill mLOWS~.

i 0.2 0.2

'I0 0

LEFT RIGHT LEFT RIGHT

HEMISPHERE HEMISPHERE ...

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

':,

"

136

ble subjects did nolo There was a non-significanttrend, FI 3Z'" 4.17, P < 0.10, for a similar interac-

. tion bet~een 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 <0.05.

Hypnotic suggestionsFor each of the bandwidths, repeated measures

ANOVAs were performed separately: 2 (hypnoticsusceptibility: low vs high) X 3 (intrahemisphericlocation: frontal, central and occipital) X 2 (inter­hemispheric location: left and right) x 4 (hypnoticsuggestions: trance deepening, arm immobiliza­tion, mosquito hallucination, and hypnotic dream).

Theta. Consistent with the previous analyses,highly susceptible subjects (M ... 1.38) had signifi­cantly more theta power than did low susceptibles(M - 0.94), fi.n'" 4.31, P < 0.05, across aU 3 lo­cations.

Alpha. There was no significant main effect orinteraction for hypnotic susceptibility. An in-

. trahemispheric location by condition interaction,f6.m - 47.14, P < 0.001, found the 3 intraherni­spheric locations equal in mean ~lpha power forthe deepening suggestion, but during each of thesubsequent hypnotic suggestions there was signifi­cantly more mean alpha in the occipital locationthan in the frontal and central locations, whichdid not differ from one another. In addition. inthe occipital location the deepening suggestion

. had substantially less alpha mean power than thesubsequent 3 hypnotic suggestions (P < 0.01),while in the central and frontal locations therewere' no' si~ficant difterences in mean alphapower between the 4 conditions.

Beta. There was a non-significant trend for aninteraction between hypnotic level and hemi­sph~re, FI .ll co 3.8S, P < 0.10, in the same direc­tion as that found for the eyes open and closedrest conditions in waking and hypnosis. There wasa significant interaction between condition andintrahemispheric location, f6.m'" 30.41, P <0.001. At the occipital location, there was a greaterreduction in beta power over the 4 items than inthe centrid and frontal locations.

Section 1/: coherence between derivations

The calculated coherence inde" ranges from 0to 1.0, with higher. values indicating more similar­ity in spectral phase at a given frequency betweenEEG derivations. There were no main effects orinteractions which involved hypnotic susceptibil­ity. The only hypnosis per se effect was during therest baseline conditions: in theta there was morecoherence during hypnosis (0.08) than during theinitial and end waking baseline conditions (both0.06) for frontal-occipital derivations within eachhemisphere, but it continues to be quite low.

In eyes closed baseline rest conditions, in wak­ing and hypnosis, there was greater inlrahemi­spheric coherence between fronlal-central deriva­tions than occipital-central derivations, in theta(respectively, left: 0.36,0.17; right: 0.39,0.09) andalpha (respectively, left: 0.34, 0.14; right: 0,42,0.09). Beta coherence was substantiaJIy lower andonly showed non-significant trends in the samedirection (respectively, left: 0.07, 0.05; right: 0.11,0.03). Similar inlrahemispheric coherences werefound for the hypnosis suggestions.

Section Til: heart rate

While the overall heart rate mean for low hyp­notizable subjects (X ... 61.8/min) was lower thanfor the highly hypnotizable subjects (X ID

66.9/min), this difference was not significant.There was a significant hypnotic level by condi­tion interaction, FU .264 ID 31.67, P < 0.001. Subse­quent mean comparisons indicated that whilehighly susceptible subjects had somewhat higherheart rates than low susceptible subjects across aUconditions, during the following conditions thisdifference was significant: mosquito hallucination,71.1 vs 60.3; hypnotic dream, 70.4 vs 63.7 (but notduring the preceding dream instruction).

Section TV: ability to image

To assess the commonly observed relationshipbetween hypnotic susceptibility and the ability toimage (e.g. Sheehan, 1979), 19 (12 highs, 7 lows)subjects were recontacted 6 months later and ad­ministered the Individual Differences Question~

·~

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).

DISCUSSION

Mean theta powerThe major robust finding of this study is that

mean 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

137

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 inattention. 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 indicato~of cortical metabolism accompanying cognitivearousal/ performance, in anterior, parietal. tem":poral and temporal-posterior regions. during hyp-

I".'

138

notic 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 produce moretheta 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 of

hypnotic 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 powerAn interesting interaction between hypn~tic

susceptibility 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-. I

tizables, 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-

,

metry between the two hemispheres in the betaband than do low hypnotizable subjects.

The results suggest that there are differences inc~aracteristic 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; Macleod­Morgan 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-

139

ferential patterns of asymmetric hemisphericarousal. like DePascalis and Palumbo (1986), wefound these differences to occur more in the lefthemisphere. Interestingly, when hemispheric elec o

trocortical 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 have

somewhat 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 highlysusceptible 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). .

CONCLUSION

In summary, the results of the current studydemonstrate 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 lo~ sus-

,.'~,

'.1

140

ceptible subjects sho~ed only weak asymmetry.80th low and high hypnotizable subjects showedenhancements of theta during fijpnosis, suggestingan intensification of attentional processes and anenhancement of imagery.

ACKNOWLEDGEMENTS

Research support to Michel Sabourin was pro­vided by a visiting scholarship grant by theMinistry of Intergovernmental Affairs of theGovernment of Quebec, Canada. Additional sup­port came from a grant from The Spencer Foun­dation to Helen Crawford.

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