IL~ _ ........ _

Journal uf Abnormal Ps)choioBY1985. Vol. 94. No. l. 249-lSS

D-IIfJ-C"Ill'righl 1985 b) Ihe American Psychological A"""'a'ion. Inc.

0021·84lX/8S/SOO.7S

Hypnotic Hallucination Alters Evoked Potentials

David Spiegel, Steven Cutcomb, Chuan Ren, and Karl PribramStanford University

Brain electrical potentials evoked by visual stimulation were analyzed to studythe neurophysiological mechanism associated with hypnotic hallucination. Thevisual evoked responses of 6 high- and 6 low-hypnotizable subjects were comparedin three hypnotic conditions: stimulus enhancement. stimulus diminution, andstimulus elimination (obstructive hallucination). High-hypnotizable individualsdemonstrated significant suppression of the later components of the evokedresponse (N2 and Pl ) while experiencing obstructive hallucinations, indicating achange in information processing. This effect was significantly greater in the right.as compared to the left, occipital region.

Hypnosis involves intense concentration(Hilgard, 1965; Spiegel & Spiegel, 1978; VanNuys, 1973) and can, therefore, be a usefultool for exploring the neural mechanismsinvolved in focused attention. The hypnoticstate can be considered one extreme of acontinuum in which focus is accomplishedat the exPense of the range of ambient aware­ness. In animals, neural controls on inputprocessing have been demonstrated (Lassonde.Ptito, & Pribram, 1981; Spinelli & Pribram,1966, 1967). The nature of these controlssuggests that they operate much as does azoom lens in providing a trade-off between awide-angle view (ambience) and resolution(focus; Marg & Adams, 1970; Pribram, 1966,1967). Individuals vary in hypnotizability,and there is some evidence that in hypnotiz­able individuals the amplitude of the evokedresponse (ERP) is diminished in response tohypnotic suggestion that the stimulus is at­tenuated (Clynes, Kohn, & Lifshitz, 1964;Galbraith, Cooper, & London. 1972; Guer­rero-Figueroa & Heath, 1964; Hernandez­Peon & Donoso. 1959; Wilson, 1968), butothers have not confirmed this relation(Amadeo & Yanovski, 1975; Andreassi, Bal­insky, Gallichio, De Simone, & Mellers, 1976;Beck & Barolin, 1965; Beck, Dustman, &

We thank Walton T. Roth. James C. Corby, Ernl:st R.Hilgard, and Helen M. Blau. of Stanford University lorcritically reviewing the manuscript. Helena C. Kraemerprovided statistical consultation and review.

Requests for reprints should be sent to David Spiegel,Department of Psychiatry, Stanford University MedicalCenter, Stanford, California 94305.

249

Beier, 1966; Halliday & Mason, 1964; Sera­fetinides, 1968; Zakrzewski & Szelenberger,1981). In contrast to these earlier studies,Barabasz and Lonsdale (1983) recently dem­onstrated significant amplitude increasesrather than decreases of the P300 componentof olfactory-evoked potentials among high­hypnotizable subjects experiencing anosmia.This is the only study to report such aninverse relation between hypnotic perceptualexperience and ERP amplitude.

This study was designed to determinewhether the amplitude of the visual evokedpotential is reduced during hypnotic hallu­cination in which the subject is instructed toperceive visual stimuli as diminished inbrightness or obstructed from view. The spe­cific effect of hypnosis was tested by compar­ing the visual evoked responses of extremelyhigh-hypnotizable versus extremely low-hyp­notizable subjects asked to perform identicaltasks. Using this design, low-hypnotizablesubjects are asked to attempt to experiencehypnotic hallucination. Differences' betweenhigh- and low-hypnotizable groups in identicaltasks should be attributable to the differencesin their hypnotizability. The hypothesis testedwas that low-hypnotizable subjects instructedto simulate a hallucination obstructing theirview of the monitor should show no alterationin the amplitude of their evoked potentialsas compared with other attention conditions,whereas high-hypnotizable subjects were pre­dicted to show a diminution in amplitude ofthe evoked response in the condition in whichthey were instructed that they would be un-

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250 SPIEGEL. CUTCOMB. REN. AND PRIBRAM

able to see the visual stimulus. Because dif­ferences in perception of the stimuli could beexpected to influence button-pressing respon­ses to the ~timuli. an additional control groupwas included to assess the effects of motorresponse on the evoked potential.

Method

Subjects

For the h'igh. and low·hypnotizable portion of thestudy. 16 right·handed volunteers (9 women and 7 men)between the ages of 18 and 33 (AI =23.0) were recruitedon the basis of having scores of O-~ or 9-12 on theHarvard Group Scale of H)'pnotic Susceptibility (HGSHS:Shor & Orne. 1962). They were then administered theHypnotic Induction Profile (HIP: Spiegel & Spiegel.1978). Only 12 subjects who were classified as comparablyhigh or low hypnotizable on both scales (2 were not) andwho provided reliable scalp recordings (2 others did notbecause of excessive eye·blink rates) were included in thestudy. The mean hypnotizability scores for the highs were9.3 on the HIP (range = 8-10) and 10.3 on the HGSHS(range = 9-12). The mean scores for the lows were 2.8on the HIP (range = 0-5) and 1.7 on the HGSHS(range = 0-3). The 6 bUllon·pressing control subjectswere selected from a similar pool of psychology students.Two others were dropped because of excessive eye blinks.They were not hypnotized during the study, but theirmean hypnotizability scores on the HIP was 6.0 (range =2-10). '

Experimelllal Conditions

The visual evoked potentials of the high· and low­hypnotizable subjects were compared in three hypnoticconditions: (a) stimulus enhancemenl. in which subjectswere instructed that one of two colored stimuli wouldappear unusually bright and interesting; (b) stimulusdiminll/ivlI. in which subjects were instructed that thealternate color stimulus would appear drab. dull. anduninteresting: and (c) ubstructil'e halludna/iun. in whichsubject~ were instructed to visualize a cardboard boxthat blocked viewing of the TV monilOr on which thestimuli were presented. making perception of anythingon Ihe screen impossible.

All stimuli were colored gratings presented within an8 em x 8 cm square area on a color television I m infrom of the: subject. The stimulus series. analogous tothe auditory paradigm of Hansen and Hillyard (1980).contained stimuli of two colors: 50% blue vertical gratingsand 50% pink horizontal gratings. Each series contained512 stimuli. For each color. 224 stimuli-which were theslandards for evoked·potential recording-were presentedfor a brief duration (133 ms). and 32 stimuli-whichwere the targets-for a long duration (399 ms). To assureanemion. subjects were asked to press a button withtheir right hand in response to each target of onedesignated color. The stimuli of each color were presentedin random order with equal probability. with the constraintthat one targel of each color appeared at"least once every16 ~timuli Presentation rate averaged onc stimulus per

1.4 s. with an interstimulus interval ranging from 1.0 to1.8 s in a rectangular distribution for a total of 12 minper run. The same stimulus series was used for each ofthe three hypnotic conditions and two control tasks.which were presented to the subjects in counterbalancedorder. The specific color to be enhanced or diminishedwas similarly varied.

Because of the difficulty of the obstructive hallucinationtask. it was thought that it would be confusing to askthese subjects to be unable to see any stimuli on the onehand. and to button press to certain target stimuli on theother. Thus. they were told to press to any stimulus theyhappened 'to see. This meant that all stimuli becamepotential targets in this condition. raising the possibilitythat any observed differences might be due to the effectof motor potentials when they pressed. or the absence ofany expectation that they would have to press when theydid not. A button·pressing/passive·attention control groupW,lS therefore added to control for the amount of variancein response that could be accounted for by both of thesefactors. excluding differences in hypnotic hallucinatoryexperience. Only results that proved significant beyondthose found in this Janer control group were anributedto an effect of hypnotic hallucination. The 6 additionalbehavioralcontrol subjects selected for intermediate hyp­notizability were not hypnotized. but were asked toperform two tasks: (a) to press a button after eachstimulus presented and (b) to allend passively to thescreen without pressing a button to any stimuli. Thus.we had three control conditions: (a) for attentional de­mands. comparing the performance of high hypnotizablesin the obstructive hallucination versus the hypnotic stim­ulus enhancement condition. (b) for hypnotizability. incomparing the high hypnotizables in the obstructivehallucination condition versus the low hypnotizables inthe same condition. and (c) for button-pressing behavior.comparing the performance of the high hypnotizables tothat of control subjects in press versus no-press conditions.

The fixation of each subject's eyes on the center of thetelevision screen was monitored continuously using ac1osed-circuit TV camera placed directly on top of thetelevision set. In only one case did it deviate. when thesubject became drowsy. He was aroused and the runresumed.

EEG MeasurementThe electroencephalogram (EEG) was recorded from

monopolar leads at Fz. Cz. Pz. 0" and 0 20 all referredto linked ears. A I-A2. Electrooculogram (EOG) wasrecorded as a bipolar channel. using two electrodeslocated on the lower orbital ridge and on the outercanthus of the right eye. The EEG was amplified 40.000times and the EOG was amplified 4.000 times. usingTektronix FM·122 AC preamplifiers and AC power am­plifiers. The standard preamplifier time constants weremodified to yield a system band pass with - 3dB downat .08 Hz (time constant = 2.0) and 50 Hz. The EEGwas digitized on line at 200 Hz for 55 ms preceding. and655 ms following. the onset of each stimulus. Fivestandard ERP components were measured as amplitudemaxima and minima within selected latency ran~s asfollows: PI at 65-150 ms. N, at PI-210 ms. P2 at N.­300 ms. N2 at P2-380 ms. and PJ at 290-550 ms. Themean wave forms for each subject from each electrode

.~

HYPNOTIC HALLUCINATION ALTERS EVOKED POTENTIALS

A. HIGH HYPNOTlZA8US 8. LOW HYPNOTlZA8LES C. CONTROLS FORBUTTON PRESSING

I---...,,~--~ EOG

251

'3JJV Ol-_:>r-~...",..---I

+3

o 250 500

TIME (mlOC)

o 250 500

TIME (mlOCl

o 250 500

TIME Im... l

Figure J. Effect of hypnotic obstructive hallucination on visual evoked potentials. (Visual evoked potentials(VEPs) recorded at leads Fz. Cz. Pz. 0,. and 0 1 are expressed as the mean of recordings in each conditionfrom 6 individuals per group yielding approximately 1.800 VEPs per waveform. In A and B. hIghhypnotizable and low hypnotizable group data shown are VEPs to stimuli observed in the hypnoticenhancement condition (thick solid lines). the hypnotic diminution condition (thin solid lines). and thehypnotic obstructive hallucination condition (dolted lines). In C. control subjects for bUllon pressing. ~lidlines are VEPs to stimuli that were all treated as bunon-pressing targets. Dolted hnes arc VEPs In apassive anention condition in which all stimuli were treated as standards and required no bunon pressing.)

were visually inspected to insure that the computer couldselect an identifiable peak. especially at Pj' In approxi­mately 10% of the cases distributed randomly acrossgroups and conditions. the computer selected either thebeginning (290 ms) or the end (550 ms) of the window.In these cases. a latency was selected for amplitudemeasurement. which was determined by the mean Pjlatency for that group of subjects (high hypnotizables.low hypnotizables. or bUlton-pressing control subjects) atthat electrode site. The overall average Pj latency wasgreater than 400 ms. The amplitudes at these five standardERP latency ranges were then utilized as variables forstatistical analysis of group by condition effects. Anexperimentally derived heuristic was used to excludefrom data analysis visual evoked potentials (VEPs) thatwere EOG contaminated. During off-line analysis. theaverage EOG signal amplitude was computed for eachepoch. If the EOG signal deviated for more than 30 JJVfrom the average value for that epoch for more than 45(of 655) ms. then the YEP for each channel during thatepoch was excluded from analysis.

Results

The evoked responses among high hypno­tizab(es in the obstructive hallucination con-

dition are clearly diminished, both in com­parison to the other hypnotic conditions ex­perienced by that group and in comparisonto those of low-hypnotizable subjects in theanalogous condition. The averaged VEPs tostandard stimuli for all conditions are shownin Figure I.

The initial statistical comparison of interestwas between high- and low-hypnotizable sub­jects in the three conditions to which theywere exposed: enhancing the brightness ofthe stimulus. diminishing the brightness ofthe stimulus. and having an obstructive hal­lucination blocking the stimulus. As Table,lindicates. enhance-diminish differences aresmall and comparable in high- and low­hypnotizable groups. whereas enhance-ob­structed view differences are large at all peaksamong high-hypnotizable subjects only.

However. because the instructions weredifferent in the obstructive hallucination con­dition. that is. the subjects were told to treat

252

",." ;'j..

SPIEGEL, CUTCOMB. REN. AND PRIBRAM

Table IDijferr:nc(' Be/ween Hypnotic Conditionsill I'EP Ampli/lldes

Conditions compared

Enhance vs.Enhance vs. obstructed

diminish view

Visual evoked Hypnotizability Hypnotizabilitypotential peak High Low High Low

PI 0.09 0.19 1.42 0.02N, 0.13 0.19 1.26 0.41p, 0.43 0.2:! 3.05 0.12N: 0.40 0.55 1.65 0.38Pl 0.98 0.42 3.28 0.75

NOll'. Visual evoked potentials (VEPs) were recorded atleads Fl. Cz. Pz. 0 •. and 0:. The amplitude differenceat each of five VEP peaks between hypnotic conditions is .compared among the 6 high- and 6 low-hypnotizable sub­jects. Numbers are standardized differences in themean VEP amplitudes in microvolts (M, - M:/Vs.e.,l + s.e./l between the hypnotic enhancement andboth the hypnotic diminution and hypnotic obstructivehallucinalion conditions.

all stimuli as targets in this condition. thecomparability of these findings in relation tothe other two conditions is open to questionbecause of possible contamination by motorpotentials when subjects button pressed. or apossible difference in expectancy when theydid not; that is. the high hypnotizables couldhave differentially interpreted their instruc­tions and could have seen the targets, butsimply failed to report seeing them. In fact.they pressed to only 5% of the stimuli, whereasthe low hypnotizables in the same conditionpressed to 96% of the stimuli. This is abehavioral indication that the highs werecomplying with the hypnotic instruction and

. is consistent with their reports that they wereunable to see the stimulus generated becauseof a cardboard box blocking it. The lowhypnotizables, given identical instructions.reported a failure to see any image strongenough to obstruct their view of the monitor.However. because these hallucination instruc­tions could conceivably have produced a dif­ferential expectancy regarding the need tobutton press, a third group was included thathad been instructed in one condition to treatall stimuli as targets and in the other conditionto observe them but treat them all as stan-

dards, that is. not to button press to any ofthem. Because the diminished condition wasnot significantly different from the enhancedcondition (see Table I) and there was nostrictly analogous condition for the button­pressing control group, it was decided to dropthe diminished condition from subsequentanalysis.

Data were then analyzed using a two-way,repeated measures analysis of variance (AN­OVA) comparing the YEP difference betweenthe standard stimulus enhancement conditionand the obstructive hallucination conditionfor the high and low hypnotizables. and theanalogous button-pressing target response taskfor the control subjects. Statistically significantdifferences among the three groups using datafrom all five .leads were obtained at NI' F(2,IS) = 4.2. P < .05; P2 , F(2, IS) = 8.9, p <.005: and P l , F(2, IS) = 3.9. p < :05, whereasthose at PI and N2 just missed ,significance(p < .1). In all cases. the differences in evokedresponse between the two conditions weresmallest among the low hypnotizables.

Post hoc I tests were performed only whenprotected by a significant overall ANOVA, andthen in order to include as significant onlyfindings in which differences among the highhypnotizables were ~ignificantly greater thanthose found among both the low hypnotiz­abies and the behavioral control subjects. Theamplitude differences among the high hyp­notizables were not significantly higher thanthose of the button-pressing control subjectsat N I or P2• The high hypnotizables demon­strated a unique suppression of Pl throughoutthe cortex in the obstructive hallucinationcondition. The mean P l difference for thehigh hypnotizables was twice that of thesecontrol subjects. I( 10) = 3.5. p < .01, andthree times that of the low hypnotizables.I( 10) = 5.3, p < .00 I. There were no signifi­cant Group X Condition YEP latency diller­ences: ANOVA H2, 20) = 1.1 at PI; 1.2 atNI; 1.3 at P2 ; 2.7 at N2 ; and 0.5 at Pl.

Because this was a visual task, the extentof suppression of the evoked response wasanalyzed separately using data from the oc­cipital leads only. The high-hypnotizable.group showed significantly greater suppressionat N1, H2, 15)'= 6.9. P < .01, and Pl , F(2,:15) = 5.3, p < .025, than did both othergroups confirmed by post hoc I tests.

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HYPNOTIC HALLUCINATION ALTERS EXOKED POTENTIALS 253

Table 2Laterality Differences in H.vpnotic Suppression of VEP Amplitude

Amplitude difference between conditions

High Low Controls for bullonYEP peak hypnotizables hypnotizables pressing

and occipitallead AI SD AI SD AI SD m. IS)

Pl0 1 Left 1.57 0.89 -1.12 0.51 2.29..•• 0.69

7.9·..••Ol Right 2.98"· 0.58 -0.58 0.44 1.04 0.53

INl

0 1 Left 1.72 0.52 0.89 0.55 0.47........ 0.396.3"·

Ol Right lOS.... 0.61 1.29 0.67 -0.91 0.62P,

O. Left l03 0.79 1.19 0.64 1.37···.. 0.28 7.0........Ol Right 4.37" 1.02 1.30 1.02 0.09 0.19

No/e. Significant Group X Lead interactions using 01 (left occipital) and 02 (right occipital) leads were found at Pl.N). and Pl. Mean differences in microvolts are shown between hypnotic allention enhancement and obstructivehallucination conditions among the 6 high- and 6 low-hypnotizable subjects. and between bUllon-pressing/no-bullon­pressing tasks among the 6 control subjects. Data were examined using Iwo-way. repeated measures analysis of variancewilh POSI hoc / tests. YEP = visual evoked potential... P < .05.... p < .025...... p < .01. ....... P < .005.

In view of the data suggesting a relationbetween hemispheric laterality and hypnotiz­ability, and especially a preferential use ofthe right hemisphere among high hypnotiz­abies (Bakan, 1969; Bakan & Svorad, 1969;Galin, Selig, & Ellis, 1975; Gur & Gur. 1974;Morgan, MacDonald. & Hilgard, 1974), itwas of interest to examine differences in thesuppression of response in the left, as com­pared with the right, occipital region. Therewere significant GrQup X Lead interactionsat P2 • N2 • and P3 (see Table 2). Among highhypnotizables. amplitude differences betweenenhancement and obstructive hallucinationconditions were found to be consistently andsignificantly higher on the right. using posthoc I tests; at P2 , {(to) = 3.28. p < .01; atN2• {(to) = 4.03, p < .01; and at P3 • {( 10) =2.58. p < .05. However, among the lows therewas no clear pattern, and among the controlsubjects amplitude differences comparing theanalogous press/no-press conditions were sig­nificantly higher on the left.

Discussion

Our results are consistent with the hypoth­esis that an hypnotic instruction of obstructivehallucination among high-hypnotizable sub­jects is accompanied by a decrease in the

amplitude of the P3 component of the evokedresponse throughout the brain, and of the N2and P3 components in the occipital region.This dampening of amplitude is particularlynotable among high hypnotizables in theright, as compared with the left, occipitalarea. suggesting greater inhibition of scalp­recorded response to a visual stimulus in theright hemisphere.

These data show that while experiencingan obstructive hallucination blocking thestimulus, high-hypnotizable subjects demon­strate a change in the information-processingcomponents of the evoked response (Bari­beau-Braun, Picton, & Gosselin, 1983), ratherthan primarily in channel selection. which isreflected more by PI and N. (Ford. Roth,Dirk, & Kopell, 1978; Hillyard & Picton,1979). Although there were differences at PIand N I between high and low hypnotizables,they were not significantly greater than thoseobserved in the press/no-press control group.These observations make it possible to addressseveral alternative explanations for the find- '!ings, such as the possibility of differences in '.nonspecific arousal leading to a differentialpreparation (Naatanen, 1969), which shouldbe reflected primarily in changes in the earlycomponents, as would any. differences in pupilsize. Drowsiness or inattention in this con-

.-

254 SPIEGEL. CUTCOMB. REN, AND PRIBRAM

dition should be associated with an increase,rather than a decrease, in response amplitudes(Schacter. 1976). The possibility that highhypnotizables might have defocused their viewof the monitor (Schulman-Galambos, & Gal­ambos, 1978) is made less likely by the factthat defocusing is accompanied by increasesin PI latency (Sokol & Moskowitz, 1981),whereas there were no P I latency differencesin the obstructive hallucination condition.

These results may help explain the negativereports (Amadeo & Yanovski, 1975; Andreassiet al.. 1976: Beck & Barolin. 1965; Beck,Dustman. & Beier. 1966; Halliday & Mason,1964; Serafetinides, 1968; Zakrzewski & Sze­lenberger. 1981), in which attempts were madeto have high-hypnotizable subjects diminishthe brightness of a stimulus in the hope thatthe evoked response would mimic that elicitedby a stimulus of decreased brightness. Clearly,hypnotizable subjects are performing a dif­ferent kind of mental work when they blockout the view of a stimulus altogether, ratherthan attend to a stimulus and attempt todecrease its intensity. Diminution of the per­ceived stimulus requires attention to thestimulus, even if it is for the purpose ofdiminishing its brightness. It is not surprisingthat the time-locked evoked response duringsuch effort is more substantial than in thecondition in which the subject is instructedto focus on a hallucinated object blockingany perception of the visual stimulus. It is ofinterest that among the high-hypnotizablesubjects our results suggest a continuum ofattention from the enhancement conditionwith the greatest amplitude through dimin­ishing the stimulus with slightly lower ampli­tudes to the obstructive hallucination, whichseemed to reduce markedly cortical responseto the stimulus.

Our findings show a difference in the op­posite direction from the findings in therecent report of Barabasz and Lonsdale( 1983). In their study, negative hallucinationinstructions in the olfactory system resultedin an increase in P3 when hypnotized highsinstructed to experience anosmia were ex­posed to both weak and strong odors. In ourstudy in the visual system, high hypnotizablesdemonstrated suppression of P3 in the ob­structive hallucination condition. Barabaszand Lonsdale proposed, reasonably, that high-

hypnotizable subjects in their study may bedemonstrating a general increase in corti­cal arousal. Because the P3 is associatedwith information processing (Baribeau-Braun,Picton, & Gosselin, 1983) and is increasedwhen the stimulus is unexpected. intrusive.and defined as task relevant (Hillyard &Picton, 1979), it may be that if the hypnoti­cally instructed anosmia was incomplete, thehigh-hypnotizable subjects were surprised byany perception of odor, even though theyreportedly did experience anosmia. The in­structions given their subjects differed in oneimportant respect from those given ours.They were told "You can no longer smellanything at all" (Hilgard, 1965). The subjectsin the present study were instructed to visu­alize a cardboard box blocking the monitor.Thus. the obstructive hallucination in thisstudy was the product of a positive halluci­nation and may have more fully absorbed thehigh-hypnotizable subjects' attention in thisvisual task, whereas the instruction not tosmell anything may have left the high hyp­notizables more vulnerable to some experi­ence of the stimulus. Although this mayrepresent differences in design or the effectof hypnotic hallucinations on different sensorysystems, what may be measured by! bothstudies is the greater flexibility of high hyp­notizables in directing attention toward oraway from a given perceptual channel.

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Received September 4. 1984Revision received December 18. 1984 a

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