International Journal of Psychophysiology 29 Ž1998. 255]275
EEG activity and heart rate during recall of emotional
events in hypnosis: relationships with hypnotizability and
suggestibility
V. De Pascalis a,U , W.J. Ray b , I. Tranquillo a , D. D’Amicoa
a
Department of Psychology, Via dei Marsi 78, Uni¨ ersity of Rome, Rome 00185, Italy
Department of Psychology, Penn State Uni¨ ersity, Uni¨ ersity Park, PA 16802, USA
b
Received 25 March 1997; received in revised form 10 November 1997; accepted 22 December 1997
Abstract
The purpose of the present research was to find physiological and cognitive correlates of hypnosis, imaginative
suggestibility and emotional experiences. After the administration of a standard hypnotic induction, the EEG and
heart rate ŽHR. were recorded during self-generated happy and sad emotions using a relaxation condition as a
control. Physiological recordings were also obtained during three eyes-open and eyes-closed baseline periods: Ž1.
waking rest; Ž2. early-rest in hypnosis Žjust after the hypnotic induction.; Ž3. late-rest hypnosis Žat the end of hypnotic
condition.. EEG was recorded at frontal ŽF3, F4., central ŽC3, C4., and posterior sites Žmiddle of O1-P3-T5 and
O2-P4-T6 triangles .. Using log transform of mean spectral amplitude, eight EEG frequency bands Ž4]44 Hz. were
evaluated. High hypnotizable subjects, as compared to the lows, produced a higher theta1 amplitude Ž4]6 Hz. across
both left- and right-frontal and right-posterior areas. These subjects also produced smaller alpha1 amplitude
Ž8.25]10 Hz. over both left and right frontal recording sites. High suggestible subjects, during resting conditions,
disclosed higher theta2 Ž6.25]8 Hz. and alpha1 amplitudes in eyes-closed as compared to an eyes-open condition
than did low suggestible subjects. High suggestible subjects also showed, in hypnosis-rest condition, higher 40-Hz
amplitudes Ž36]44 Hz. and HR activity than did low suggestible subjects. Hypnotizability and not suggestibility was
found to moderate emotional processing: high hypnotizable individuals self-reported greater levels of emotional
experiences than did low hypnotizables especially in terms of negative emotion. High hypnotizables, during
processing of emotional material, also disclosed opposite 40-Hz hemispheric asymmetries over anterior and posterior
regions of the scalp. These subjects during happiness showed an increased production of 40-Hz activity in the left
frontal and central regions of the scalp, while during sadness they showed an increased activity in the right central
and posterior regions. The hemispheric asymmetries for relaxation condition were similar, but less marked, to those
U
Corresponding author. E-mail: [email protected].@eit
0167-8760r98r$19.00 Q 1998 Published by Elsevier Science B.V. All rights reserved.
PII S0167-8760Ž98.00009-9
256
V. De Pascalis et al. r International Journal of Psychophysiology 29 (1998) 255]275
obtained for happiness. No significant interactions involving both hypnotizability and imaginative suggestibility were
found for physiological variables considered in this study. This demonstrates that hypnotizability and suggestibility
reflect different underlying psychophysiological activities. Q 1998 Published by Elsevier Science B.V. All rights
reserved.
Keywords: Emotion; EEG; 40-Hz EEG; Heart rate; Laterality; Hypnosis; Hypnotizability; Sensory suggestibility
1. Introduction
Two dominant themes in contemporary hypnosis over the last 30 years can be shown: Ž1. the
question of the existence of a hypnotic state Žstate
vs. non-state. and Ž2. hypnotic susceptibility as a
stable individual difference Ži.e. trait vs. situational manipulation. Žsee Fellows, 1990; Hilgard,
1973; Kirsch and Lynn, 1995; Kihlstrom, 1997..
However, upon closer examination, hypnosis research has addressed a variety of dimensions that
do not fall easily into these two camps. Recently
Kirsch and Lynn Ž1995. suggested a convergence
of position with many of the positions best viewed
as lying on a continuum. However, Kihlstrom
Ž1997. points to a number of important distinctions which separate the more cognitive theoretical explanations including the neodissociative one
from the more sociocognitive ones. According to
Kihlstrom Ž1997., hypnosis is a complex phenomenon, simultaneously consisting of a social interaction between subject and hypnotist and of a
state of altered consciousness involving basic cognitive mechanisms underlying perception, memory and thought. In the present study, we address the question of the relationship of hypnosis
and suggestion and the manner in which these
heighten emotional experiences.
1.1. State ¨ s. non-state positions
One of the best articulated state positions is
the neodissociative approach ŽHilgard, 1977, 1992.
which suggests that hypnosis reflects the activation of subordinate cognitive control systems
operation in a hierarchical order. According to
Hilgard’s neodissociation theory, when a hypnotic
suggestion becomes operant, the suggestive behavior is characterized by a reduced influence of
executive control and volition and by an increased
activation of subordinate systems of control whose
automatic activity produces the suggested response. The result is a division in consciousness,
i.e. the control executive system becoming ignorant of the intentional activities performed by
the subordinate system and, therefore, experiences these activities as involuntary. This model
assumes that hypnosis alters normal cognitive
functioning and that such alteration is reflected in
both behavior and experience Že.g. amnesia, analgesia. as well as psychophysiological functions. In
line with this view Kihlstrom Ž1987., in his excellent review on ‘cognitive unconscious’, defines
hypnosis phenomena as a category of ‘subconscious declarative knowledge’ and invokes Janet’s
concept of dissociative phenomena ŽHilgard, 1986.
to describe hypnosis. In certain phenomena of
hypnosis in which dissociation is operating, the
mental representations fully activated by perceptual inputs, are resident in working memory and
therefore available to introspection, they appear
to be temporarily set aside and seem nevertheless
inaccessible to conscious awareness. The alternative position, commonly named the social-psychological approach ŽSpanos and Coe, 1992., views
hypnosis as a social situation in which hypnotic
behavior may be described in terms of suggestibility, expectations, and demands present in the
hypnotic situation. In particular, a number of
researchers Že.g. Barber, 1969; Kirsch, 1997;
Weitzenhoffer, 1963, 1980. propose that suggestion, rather than hypnosis per-se, is the basic
phenomenon that may describe hypnotic behavior. Recently Kirsch Ž1997. indicated that hypnotic susceptibility scales are better measures of
waking suggestibility than they are of hypnotizability. He proposed a definition of hypnosis as
changes in experience following suggestions by
reducing the hypnotic induction rituals to being
nothing more than a particular set of suggestions,
rather than the means to establish a hypnotic
V. De Pascalis et al. r International Journal of Psychophysiology 29 (1998) 255]275
state. He proposed to reinterpret hypnotizability
scores as indexes of non-hypnotic imaginative
suggestibility.
Assuming that hypnotic phenomena are accurately described by individual differences in imaginative suggestibility than by individual differences in hypnotizability, then we would expect to
find, after the administration of an induction,
more suggestion-induced differences in physiological responding between high and low suggestible
subjects than between high and low hypnotizable
ones. Anyway, if suggestibility and hypnotizability
refer to similar behavior then they should share
similar patterns of physiological activity.
1.2. Hypnotic susceptibility
The second major question directing hypnotic
research is that of hypnotic susceptibility Žfor an
overview of hypnotic susceptibility research see
Fromm and Nash, 1992.. In this line of research
various measures of hypnotic susceptibility have
evidenced reliable correlation coefficients above
0.60 ŽBowers, 1983.. Even more impressive is the
finding that hypnotic susceptibility has been shown
to be as stable a measure of individual differences
as measures of I.Q. or various personality inventories with a test-retest correlation of 0.71 when
considered over 10-, 15-, and 25-year periods
ŽMorgan et al., 1974; Piccione et al., 1989.. Given
this stability, it is surprising that there exist few
individual difference variables that can reliably
predict hypnotizability ŽKirsch and Council, 1992;
Silva and Kirsch, 1992.. Even conceptually similar
measures such as dissociative experiences show
an orthogonal relationship with hypnotic susceptibility ŽFaith and Ray, 1994; Ray and Faith, 1995;
Ray, 1996.. One possibility in predicting hypnotizability may lie in psychophysiological measures as
have been shown in other contexts Že.g. Tomarken et al., 1990..
Initial studies on psychophysiological indicators
of hypnotic susceptibility Žcf. London et al., 1968;
Morgan et al., 1974; Nowlis and Rhead, 1968.
have reported a positive relationship between the
time-percentage occurrence of EEG alpha waves
in resting Žeyes closed. conditions and hypnotizability. However, Evans Ž1972. did not find this
257
difference and suggested that previous results
were affected by demand characteristics. In his
review, Dumas Ž1977. suggested that the alpha
hypnotizability relationship resulted from biased
subject selection, a conclusion not supported by
Barabasz Ž1983.. In their critical review of alpha
and hypnotizability, Perlini and Spanos Ž1991.
concluded that there is little support for an
alpha-hypnotizability relationship.
According to Crawford and Gruzelier Ž1992.
the choice of alpha band ranges Že.g. 7]12 Hz or
8]13 Hz. may have influenced findings since recent studies have revealed two alpha bands Žslow
parietal alpha: 7]10 Hz and fast occipital alpha:
10]13 Hz. that are functionally different ŽCoppola and Chassy, 1986; Coppola, 1986.. The slow
alpha activity has been found to be related to a
relative cognitive inactivity or low alertness Že.g.
Bosel, 1992; Crawford et al., 1995; Gale and Edwards, 1983. and the fast alpha activity to mental
workload Že.g. Pfurtscheller and Klimesch, 1991;
Sterman et al., 1994. and memory performance
Že.g. Klimesch et al., 1990..
The most consistent relationship between EEG
activity and hypnotizability exists in the 4]8 Hz
theta band Žsee Crawford and Gruzelier, 1992..
Crawford Ž1990, 1991. found that high hypnotizables generated greater power than low hypnotizables in the high theta Ž5.5]7.5 Hz., but not the
low theta Ž3.5]5.5 Hz. range from frontal to posterior regions of the scalp. Recently, Crawford et
al. Ž1996. reported that high hypnotizables, in
comparison to the lows, showed significantly
greater right hemisphere activity compared to the
left hemisphere in parietal region in high theta
Ž5.5]7.5 Hz., high alpha Ž11.5]13.45 Hz., and
beta activity Ž16.5]25 Hz.. Vogel et al. Ž1968.
describe two inhibitory processes reflected in the
slow EEG activity: ‘class I inhibition’ and ‘class II
inhibition’. Class I inhibition is characterized by
gross inactivation of a global excitatory process
such as that which would result in relaxation or
drowsiness. Class II inhibition is associated to
selective inactivation as present during the performance of automatized behaviors. According to
Vogel et al. Ž1968. class II inhibition is reflected
in the EEG frequency which differentiates theta
from alpha, designated for normal adults at about
258
V. De Pascalis et al. r International Journal of Psychophysiology 29 (1998) 255]275
6 Hz. In other studies theta has been associated
with continuous concentration of attention and
selective attention Že.g. Basar-Eroglu et al., 1992;
Bruneau et al., 1993; Schacter, 1977.. In the field
of hypnosis research, Galbraith et al. Ž1970. reported the highest significant positive correlation
between 5 Hz theta and hypnotizability. The authors interpreted their findings by suggesting that
an increase of theta activity in high hypnotizables
was reflecting the ability of these subjects to focus
attention on relevant stimuli and to shut off the
irrelevant ones. In general, a number of subsequent studies have reported a strong relationship
between theta Ž4]8 Hz band. and hypnotizability
Že.g. Graffin et al., 1995; Sabourin et al., 1990..
Barabasz Ž1990. reported that high but not low
hypnotizable subjects generated more theta power
just after a condition of restricted environmental
stimulation.
1.3. Hypnosis and suggestion
It is generally assumed that suggestibility is not
a unitary phenomenon ŽEysenck, 1947, 1991. and
the variety of procedures used to measure suggestibility is somewhat diverse. Consequently, the
results among studies are often contradictory Žsee
Gheorghiu Ž1989. for an overview.. Recently a
test measuring sensory suggestibility was constructed ŽGheorghiu et al., 1993.. Sensory suggestibility refers to a variety of imaginal situations
characterized by the individual tendency to judge
simulated stimuli as real. With the construction
of this reliable test, it is now possible to measure
sensory suggestibility.
Evans Ž1989., after a historical and empirical
review, concluded that suggestibility and hypnotizability can be considered as the product of different processes and therefore one is not an
exhaustive expression of the other. Hilgard Ž1973,
1991. also concluded that hypnotic behavior cannot be defined simply as a response to suggestion
since there are several forms of suggestion including impersonal suggestions, conformity and
placebo responses which do not belong to the
hypnosis condition, and vice versa, hypnotic
phenomena which cover more than specific responses to suggestion.
Further, although there exists a large literature
examining psychophysiological concomitants of
hypnosis and hypnotic susceptibility Žsee Crawford et al., 1996; Graffin et al., 1995; Sabourin et
al., 1990; for a review see Crawford and Gruzelier, 1992., there are few, if any, studies examining psychophysiological contributions in the area
of suggestibility. This may result from the lack of
integrative theoretical approaches to direct research in relation to the variety of suggestive
phenomena ŽGheorghiu, 1989.. However, one hypothesis to be tested is that individual differences
in sensory suggestibility and hypnotizability are
expressions of the same psychophysiological
process.
1.4. Attention and emotion
A variety of studies have focused on site specific differences during emotional processing. For
example, Davidson et al. Ž1979, 1990. and Davidson Ž1992., using alpha activity, found that cortical activation Žless alpha activity. in the right
frontal site and anterior temporal area correlated
with disliked segments of film clips and a relative
frontal left hemisphere activation with liked ones.
In experiments in which subjects used a mood
induction procedure to recall past emotional experiences Žsad, happy, depression or sexual emotions. there was an activation of parietal region of
the cortex ŽTucker et al., 1981; Tucker and Dawson, 1984.. Stenberg Ž1992. found an increased
theta activity in the right lateral frontal region
during recall of pleasant and unpleasant events.
Ray and Cole Ž1985. engaged subjects in positive
and negative emotional tasks which required internal vs. external attentional focus. They found
more abundant right temporal beta Ž16]24 Hz.
activity was associated with positive as compared
to negative emotional tasks. The role of hemispheric asymmetry in emotional processing is generally accepted but it is still not clear how each
hemisphere contributes in the generation of emotional state. This is since a number of other
variables such as personality traits or emotion
intensity may affect hemispheric asymmetry Žsee
e.g. Davidson et al., 1990; Kline et al., 1994;
Tomarken et al., 1990; Tucker et al., 1981; Wexler
V. De Pascalis et al. r International Journal of Psychophysiology 29 (1998) 255]275
et al., 1992.. Recently, Heller Ž1993. carried out
an interesting model of emotion in which the
right parietotemporal regions of the cortex not
only are specialized for the processing of autonomic and behavioural arousal in emotional
states. In the context of this model it is suggested
that the right parietotemporal system operates in
conjunction with a system localized to the frontal
lobes that is involved in the modulation of the
emotional valence of experience.
Hypnosis is often viewed as a condition of
heightened experience and attention Že.g. Crawford, 1982; Crawford et al., 1996; De Pascalis,
1993; Gruzelier and Warren, 1993. in that hypnotically responsive individuals in the waking as
well as in hypnosis conditions, both in cognitive
task performances and self-report measures, demonstrate more controlled attention, and greater
absorption in everyday events ŽCrawford, 1982;
Crawford et al., 1993; Karlin et al., 1979; Tellegen
and Atkinson, 1974.. The assertion that high hypnotizable persons have a greater ability to control
the activity of the attentional system is supported
by a growing number of neurophysiological findings. One of the most reliable findings is that
there are changes in ERP components ŽN1, P2,
N2 or P3. during hypnosis when high hypnotizables are able to reduce or eliminate the conscious awareness of incoming auditory ŽCrawford
et al., 1996; Jutai et al., 1993., visual ŽDe Pascalis,
1994; Jasiukaitis et al., 1996; Spiegel et al., 1985.,
olfactory ŽBarabasz and Lonsdale, 1983; Spiegel
and Barabasz, 1988. or somatosensory ŽKropotov
et al., 1997; De Pascalis and Carboni, 1997; Spiegel
et al., 1989; Zachariae and Bjierring, 1994. stimuli.
A robust finding in the hypnosis literature is
that high hypnotizables in waking, as well as in
hypnosis conditions Že.g. Bower, 1981; Bryant and
McConkey, 1989; Crowson et al., 1991; Crawford
et al., 1995; De Pascalis et al., 1987, 1989., usually
report more intense affect when experiencing
positive and negative emotions. Using 40-Hz EEG
as a physiological indicator of focused arousal as
suggested by Sheer Ž1975., some evidence has
been provided that such greater ability in accessing emotions is due to the greater ability of these
subjects to focus their attention to relevant emo-
259
tional events and to shut off the irrelevant ones.
In a previous study carried out in our laboratory
high hypnotizable subjects, while they were in
hypnosis, experienced greater feeling ratings of
self-generated negative events than they did in a
waking condition ŽDe Pascalis et al., 1989.. These
emotional patterns were paralleled by an increased level of 40-Hz activity in hypnosis as
compared to waking state.
The purpose of the present research is to bring
together the areas of hypnosis, suggestibility, and
emotional experiences. Given the current state of
research in hypnosis and suggestibility, we suggest
three major hypotheses and a number of pilot
questions. Firstly, we expect to find greater theta
activity in highly hypnotizable persons compared
to the low hypnotizable ones. Secondly, emotionality should be experienced more strongly by
high as compared with low hypnotically susceptible individuals. And, thirdly, based upon prior
hypnosis research on electrocortical processing of
emotional material ŽCrawford et al., 1996; De
Pascalis et al., 1987; Heller et al., 1995; Ray and
Cole, 1985., we expected greater hemispheric
asymmetries during induced emotions in high
hypnotizables compared to low hypnotizables.
High subjects were expected to show greater EEG
asymmetries associated with focused attention i.e.
high frequency theta, high alpha, beta and 40-Hz
EEG activity. Further, using a more advanced
measure of 40-Hz EEG spectral amplitude, we
sought to examine EEG differences in high and
low sensory suggestible subjects in an attempt to
replicate and extend our previous 40-Hz EEG
correlates of emotionality. In particular, we expected to find that such asymmetries should be
manifested in a reciprocal relationship between
frontal and posterior sites with left frontal and
right temporo-parieto-occipital sites associated to
positive affect and with posterior right site associated with negative one. Assuming that suggestion
and hypnosis are part of the same phenomenon,
then we expected to find a common physiological
substrate between individuals characterized by extreme levels of sensory suggestibility and hypnotizability dimensions.
Past research ŽKahneman et al., 1969; Lacey,
1967; Lacey and Lacey, 1970. had demonstrated
260
V. De Pascalis et al. r International Journal of Psychophysiology 29 (1998) 255]275
that cognitive effort in generating imagery and
other internal activity is accompanied by heart
rate ŽHR. increase. However, HR increases have
also been found to reflect emotionality. Many
studies have found HR modifications during almost every emotional state Žfor a review see Cacioppo et al., 1993; Levenson, 1992; Palomba et
al., 1997.. Thus, HR has some claim to be considered as a reasonable index of cognitive and
emotional work. Whether HR increases reflect
cognitive effort or emotional arousal may be dependent on individual differences and specific
experimental factors. Assuming that highly hypnotizable subjects become more emotionally involved in their imagining than lows ŽHilgard, 1979;
Tellegen and Atkinson, 1974., the last but not
least aim of the present study was to evaluate the
validity of the suggestion that self-generated emotional events produce a greater HR increase in
high than low hypnotizable subjects. A similar
HR trend was expected for high and low sensory
suggestible subjects.
2. Methods
2.1. Subjects
Twenty right-handed females between the age
of 20 and 25 years were selected from a sample of
85 psychology students. The subjects were all
volunteers who were tested in small groups of 3]6
persons. They were informed that the experiment
involved evaluating their sensibility to perceive
sensory stimuli, but were unaware that a standard
suggestibility scale was being administered, the
SSKG ŽGheorghiu et al., 1993.. This is a group
scale consisting of 10 items designed to measure
an individual’s level of suggestibility based on
sensory judgement with scores ranging from 0 to
40. A second was held in which subjects were
administrated individually the Stanford Hypnotic
Susceptibility Scale, Form C ŽSHSS: C; Weitzenhoffer and Hilgard, 1962.. The SHSS: C scale was
administered by tape-recorder after the experimenter had established rapport with the subject.
The experimenter was a woman. The experimenter stopped the tape if extra time was necessary to the subject to comply with task sugges-
tions. The hypnotic induction was administered
by tape for standardization purposes and because
there is evidence of little or no difference between
the effects of live versus taped inductions in
voluntary students Že.g. Shor and Orne, 1963;
Ulett et al., 1971.. Following the administration
of the SHSS, the subjects were asked to write
down two emotionally-meaningful situations in
their lives, one of happiness and another of sadness, and one neutral-relaxation control situation
Ži.e. with no markedly positive or negative emotional valence.. Handedness was evaluated with the
Italian version of the Edinburgh Inventory Questionnaire ŽSalmaso and Longoni, 1985.. Subjects
were also administered the Tellegen Absorption
Scale ŽTAS: Tellegen and Atkinson, 1974. and the
Betts’ questionnaire of mental imagery ŽSheehan,
1967.. The subjects were all women since it is
reported that women are significantly more susceptible to hypnosis than men ŽBowers, 1971. and
that sex is a moderating variable in the relationship between hypnotic susceptibility and hemispheric asymmetry ŽGur and Gur, 1974.. Moreover, there is experimental evidence for a greater
task-induced hemispheric asymmetry in males
compared to females Žsee e.g. Glass et al., 1984;
Ray et al., 1976..
Ten high and 10 low hypnotically susceptible
females were selected such that each high and
low hypnotizable group included half of the subjects with high and half with low level of sensory
suggestibility. Subjects were designed as being
high hypnotizables Ž N s 10, mean s 10.1, S.D.s
0.69. when their scores on SHSS: C were 0.9 S.D.
above the group mean Ž N s 60, mean s 6.5,
S.D.s 2.33.; an equivalent but opposite deviation
designated the low hypnotizable subjects Ž N s 10,
mean s 2.9, S.D.s 1.1.. In the same way subjects
were designed as being high suggestibles Ž N s 10,
mean s 23.0, S.D.s 3.2. when their scores on
SSKG were 0.9 S.D. above the group mean Ž N s
60, mean s 13.8, S.D.s 7.1.; an equivalent but
opposite deviation designated the low suggestible
subjects Ž N s 10, mean s 5.3, S.D.s 2.2.. At the
beginning of the experimental psychophysiology
session, subjects were administered the State
Anxiety Version of the State-Trait Anxiety Inventory ŽSTAI, Spielberger et al., 1970.. Subjects who
V. De Pascalis et al. r International Journal of Psychophysiology 29 (1998) 255]275
had reported psychiatric or neurological disorders
were excluded from EEG recordings. Since menstrual cycle may affect EEG activity ŽGlass, 1968.,
no subjects were experiencing a menstrual period
during the time of the EEG recording.
2.2. Procedure
The subjects were shown the experimental setting including the psychophysiological monitoring
equipment. Neither the specific experimental hypotheses nor their level of hypnotizability was
discussed with the subjects. The experimental session was carried out between 15:00 and 18:30 h.
After that electrodes were attached, each subject
sat in a comfortable armchair in a sound-attenuated dimly-lit room. During the experimental
session the hypnotist and the subject were in
touch with another experimenter by means of an
intercom system. The hypnotist of this session was
a woman, but not the same person who previously
assessed the hypnotic susceptibility and suggestibility of all the subjects. After a 2-min relaxation period, the physiological recordings started
with two 1.5-min waking resting periods one with
eyes closed and with eyes open. After the administration of a tape-recorded hypnotic induction
Žthat included the SHCS; Morgan and Hilgard,
1978., two additional relaxation 1.5-min baselines
Žeyes open and eyes closed. were taken. After this
period the subject was told to keep her eyes
closed and to recall from memory one of three
emotional events experienced Žhappiness, sadness
or neutral relaxation events. described previously.
The hypnotist emphasized that the emotional memory should be experienced with strong visual
imagery and emotional feeling as in the reality.
Each emotional task lasted for 2-min. Each period of emotionality was preceded by a 1-min rest
period. After these three tasks were completed, a
final two 1.5-min baselines Žeyes open and eyes
closed. were completed. The order of baselines
and emotional tasks were counterbalanced across
subjects.
Subjects were told if they were unable to perform any of the tasks to inform the experimenter.
In those cases in which the subject had difficulties
261
performing the task, she was given extra time.
The subject signalled the start of the task by
raising her right index finger. At the end of each
emotional task the subject rated along a 10-point
scale the vividness of visual imagery Ži.e. 1 ‘no
image at all’, to 10 ‘perfectly clear and vivid as in
the reality’. and level of emotional feeling Ži.e. 1
‘no emotion at all’, to 10 ‘perfectly identical feeling as in reality’. and relaxation Ži.e. 1 ‘not relaxed at all’, to 10 ‘relaxed as in reality’..
2.3. EEG recordings
EEG recordings were made using silver-silver
chloride cup electrodes ŽAg-AgCl. attached by
collodion, on F3 and F4 Žanterior sites., on C3
and C4 Žcentral sites., and in the middle of O1P3-T5 and of O2-P4-T6 triangles Žposterior sites..
These anterior and posterior recording sites were
chosen according to Sheer Ž1989. in terms of
regulation of arousal. Raw EEG signals were
obtained by using an eight-channel EEG machine
Ž‘ERA-9’-OTE Biomedica Italiana .. Time constants were set at 0.3 s and filters at 75 Hz. EEG
signals were recorded on tape by an 8-channel
digital recorder ŽOTE-Biomedica Italiana, 0]80
Hz bandwidth..
The EEG was acquired in digital form, using an
IBM PC-AT computer, by sampling at 128 Hz per
channel with a 12-bit interface ŽMetrabyte Dash16.. For each condition, 20 epochs of 4 s Ž512
points per epoch per channel . were recorded on
3.50 microdisks. Forty-Hz EEG, for each electrode, was obtained off-line from the digitized
raw EEG by filtering the EEG in the 36]44 Hz
band and then correcting the filtered signal for 60
Hz EMG Ž56]64 Hz bandwidth. contamination.
The correction consisted in subtracting from the
filtered 40-Hz EEG scores the predicted EEG
values, as obtained by linear regression of filtered
40-Hz EEG on filtered 60-Hz EMG Žfor more
details see Raghavan et al. Ž1986. and Sheer
Ž1975...
Eye movements ŽEOG. were recorded with
Ag-AgCl electrodes in a bipolar arrangement,
with the superior orbit referenced to the outer
canthus of the right eye. All electrode impedances
262
V. De Pascalis et al. r International Journal of Psychophysiology 29 (1998) 255]275
were less than 7 k V. EEG epochs were excluded
from analysis when the eye movements produced
a slow potential variation greater than 30]40 mV.
The EEG was processed by Fast Fourier Transform ŽFFT. over 4 s epoch. FFT was implemented
by using a Keithley Asyst-4.0 software system.
Thus, 512 point transforms were accomplished
creating a resolution of 0.25 Hz in the resulting
spectra. To reduce discontinuities at the window
edges a Blackman window was applied to the
acquired EEG data before computing the spectrum.
Mean EEG spectral amplitudes were obtained
in the following frequency bands: theta1 Ž4]6
Hz., theta2 Ž6.25]8 Hz., alpha1 Ž8.25]10 Hz.,
alpha2 Ž10.25]13 Hz., beta1 Ž13.25]16 Hz., beta2
Ž16.25]20 Hz., beta3 Ž20.25]35.75 Hz., corrected
40-Hz EEG Ž36]44 Hz.. A log transform of spectral amplitude data was used as database for
statistical analysis.
2.4. Cardiac recording
An electrocardiogram ŽEKG. was obtained
from two Ag-AgCl electrodes attached to the
manubrium sterni and to the left rib cage. The
EKG signal was detected by a computerized system ŽSATEM, Biolab PT104SC. that measured
R-R intervals by supplying one data point for
every 100 ms. For each resting and task condition
a recording period of 80 s was obtained. The R-R
interval time series was off-line processed for
artifact correction. First, values lower than 400
ms and greater than 1200 ms were excluded for
the computation of the cardiac series. Second,
each R-R value that was less than 2.5 standard
deviations below the R-R mean and that was
greater than 3 standard deviations above the R-R
mean were considered to be errors. Errors were
then corrected only in the cases in which one or
two consecutive data points were out of the range,
by using a smoothing procedure. In the cases in
which there were from 3]6 consecutive artefacted
data points they were excluded from data analysis. R-R intervals were measured in ms and expressed as beatsrmin Žbpm..
2.5. Statistical data analyses
2.5.1. EEG bands
For each EEG band a repeated measures analysis of variance ŽANOVA. was performed across
resting and emotion conditions. The GLM procedure of the Statistical Analysis System ŽSAS. was
used with the following design: 2 Hypnotizability
Žhigh, low. = 2 Suggestibility Žhigh, low. = 2
Hemisphere Žleft, right. = 3 Intrahemispheric Location Žfrontal, central, posterior. = 3 Trial Žpositive, negative, neutral .. A similar analysis was also
performed across eyes openrclosed resting conditions.
2.5.2. Heart rate
A repeated ANOVA was performed for R-R
intervals across emotion conditions according to
the following design: 2 Hypnotizability = 2 Suggestibility = 3 Trial. A similar analysis was performed across three resting baselines by considering the eye-condition factor.
An ANOVA was also performed for self-rating
scores of visual imagery and emotional feeling.
For EEG, R-R time, and self-rating scores significant effects were assessed using the Greenhouse-Geisser epsilon ŽVasey and Thayer, 1987..
A rejection region with at least a value of P- 0.05
was selected and used throughout. Multiple comparisons of the means were carried out by Tukey’s
HSD test ŽKirk, 1968, p. 88.. Comparisons addressing hypnotizability and suggestibility with
hemisphere functioning are emphasized.
3. Results
3.1. Self-report measures
No significant differences for state anxiety were
found between hypnotizability groups or between
high and low suggestible subjects Ž t s 0.65 and
t s 0.78, P) 0.05, respectively..
The ANOVA for visual-imagery ratings demonstrated a main effect for Hypnotizability Ž F1,16s
10.5, MSes 157.7, P- 0.005. that indicated a
higher level of visual imagery for the high hypno-
V. De Pascalis et al. r International Journal of Psychophysiology 29 (1998) 255]275
tizables compared to the low hypnotizables across
all emotional task conditions Žhighs: 7.3, 7.8, and
8.1; lows: 4.2, 5.0, and 6.1; respectively for neutral, positive and negative emotion conditions..
ANOVAs involving emotional self-report ratings showed the following effects: Ž1. Hypnotizability Ž F1,16s 6.63, MSes 170.0, P- 0.02.; Ž2.
Trial Ž F 2,32 s 9.5, MSe s 1.8, P - 0.001.; Ž3.
Hypnotizability = Trial Ž F 2,32s 3.44, MSes 1.8,
P- 0.05.. The first effect indicated that high hypnotizables experienced higher levels of emotionality compared to the low hypnotizables Ž7.4 vs. 5.3,
respectively for high vs. low hypnotizables.. The
second effect indicated that high hypnotizables
experienced more pronounced negative emotional feelings compared to positive emotionality
and relaxation. The low hypnotizables, in contrast, did not evidence differences between positive and negative emotions, but both emotional
ratings were higher than relaxation ratings Žhighs:
8.2 vs. 7.1 and 7.0; lows: 6.1 vs. 6.0 and 3.7;
respectively for negative vs. positive and relaxation feelings..
In order to evaluate the relationships between
the personality dimensions, a correlation coefficient was calculated for the entire subject pool
with complete data Ž N s 60.. Hypnotizability was
significantly related with sensory suggestibility Ž r
s 0.447, Ps 0.0003., with vividness of mental
imagery Ž r s y0.294, Ps 0.0225., and with absorption Ž r s 0.270, Ps 0.0369.. Absorption was
also found related to Bett’s vividness of visual
imagery Ž r s y0.323, Ps 0.0118.. The other correlation coefficients between personality variables
were not significant. The negative sign in this case
indicates that absorption increases with vividness
of visual imagery, since the level of imagery was
scored in the way that at higher scores were
corresponding lower levels of vividness.
3.2. EEG bands
3.2.1. Resting conditions
There were significant site effects for alpha2,
beta1 and beta3 amplitudes Žalpha2: F 2,32s
15.01, MSes 0.786, P- 0.0001; beta1: F 2,32s
11.01, MSes 0.197, P- 0.0002; beta3: F 2,32s
6.10, MSes 0.312, P- 0.01. that indicated greater
263
amplitudes over posterior sites compared to
frontal and central sites. Common to alpha2,
beta1, beta2 and beta3 activity was a significant
effect for the eyes openrclosed-condition Žalpha2:
F1,16 s 49.37, MSe s 2.12, P - 0.0001; beta1:
F1,16s 5.58, MSes 0.309, Ps 0.03; beta2: F1,16
s 9.13, MSes 0.341, Ps 0.0081; beta3: F1,16s
8.19, MSes 0.407, P- 0.01.. This effect indicated
that alpha2, beta1 and beta2 amplitudes were
higher in eyes-closed condition compared to
eyes-open condition, while an opposite trend was
found for beta3 amplitude.
3.2.1.1. Theta1. The following effects were
found for theta1 amplitude: Ž1. Eye-condition
Ž F1,16s 18.7, MSes 0.286, P- 0.0005.; Ž2. Location Ž F 2,32s 77.9, MSes 0.139, P- 0.0001.;
Ž3. Eye-condition = Location Ž F 2,32s 37.6, MSe
s 0.07, P- 0.0001.; Ž4. Location = Hemisphere
Ž F 2,32s 68.6, MSes 0.02, P- 0.0001.; Ž5. Hypnotizability = Hemisphere = Location = Trial
Ž F4,64s 3.7, MSes 0.004, Ps 0.009.. The first
effect indicated that there was a greater theta
amplitude in eyes-open compared to an eyesclosed condition Ž1.56 vs. 0.983, for eyes-open vs.
eyes-closed condition.. The second effect indicated that there was a monotone decrease of
amplitude from frontal to central and posterior
leads. The third effect showed that this decrease
was more pronounced in eyes-open compared to
the eyes-closed condition Žeyes-open: 1.484, 1.085
and 0.879; eyes-closed: 1.012, 0.925 and 0.914; for
frontal, central and posterior leads, respectively..
The fourth and fifth effects disclosed that during
all three baseline conditions, high hypnotizable
subjects showed greater theta1 amplitude over
left frontal compared to the right hemisphere and
in the posterior areas, an opposite effect across
the first two conditions. During the initial baseline and the first hypnosis baseline there was
greater theta1 amplitude in the right hemisphere
compared to the left in posterior recording sites.
The highly hypnotizable subjects in comparison to
the lows demonstrated more theta1 activity in the
left and right frontal areas and in the right posterior area. The low hypnotizables did not show
significant hemisphere differences over posterior
sites. However, they showed greater amplitude
over left frontal sites compared to the right sites
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Fig. 1. Theta1 Ž4]6 Hz. spectral amplitude in left and right hemispheres at frontal, central and parieto-temporo-occipital ŽPTO.
scalp leads for high and low hypnotizable subjects during waking-rest ŽW., hypnosis-rest1 ŽH-1., and hypnosis-rest2 ŽH-2. conditions.
Data natural log transformed.
across all conditions Žsee Fig. 1.. No effects involving sensory suggestibility were found.
3.2.1.2. Theta2. Effects for Location, Eye-condition, and Eye-condition = Location were significant Ž F 2,32s 27.47, MSes 0.205, P- 0.0001;
F1,16s 4.97, MSes 0.823, Ps 0.04; and F 2,32
s 23.73, MSes 0.065, P- 0.0001.. The Location
effect disclosed that there was a greater theta2
amplitude over frontal scalp area as compared to
central and posterior sites Ž1.35, 1.12 and 1.06,
respectively.. The other effects indicated that in
eyes-closed compared to eyes-open condition
there was a significantly greater production of
theta2 activity over posterior scalp sites Žfrontal:
1.34 vs. 1.36; central: 1.06 vs. 1.17; parietal: 0.9 vs.
1.23, respectively for eyes-open vs. eyes-closed
conditions.. Finally, the Suggestibility = Eyecondition = Trial interaction was also significant
Ž F 2,32s 3.82, MSes 0.083, Ps 0.03.. This effect
indicated that highly suggestible subjects, compared to the lows, during waking-baseline and
hypnosis-baseline1 produced more theta2 in waking eyes-closed than in eyes-open condition. In
hypnosis, with eyes-closed conditions, highly sug-
gestible subjects produced more theta2 than the
lows Žsee Fig. 2a..
3.2.1.3. Alpha1. The alpha1 amplitude displayed a number of significant main and interaction effects: Ž1. Eye-condition Ž F1,16s 27.29,
MSes 3.69, P- 0.0001.; Ž2. Suggestibility = Eyecondition = Trial Ž F 2,32s 4.63, MSes 0.150, P
s 0.02.; Ž3. Hemisphere = Location Ž F 2,32 s
37.13, MSes 0.040, P- 0.0001.; Ž4. Hypnotizability = Hemisphere = Trial Ž F 2,32s 3.21, MSes
0.009, P - 0.05 . ; Ž 5 . H ypnotizability =
Hemisphere = Location = Trial Ž F 2,32 s 3.20,
MSes 0.004, Ps 0.02.. The first effect indicated
that there was more alpha1 in eyes-closed than in
eyes-open condition; the second effect indicated
that the difference in alpha1 amplitude between
eye conditions was more pronounced for highly
suggestible subjects across waking and hypnosisbaseline1 conditions Žsee Fig. 2b.. The third and
fourth effects indicated that highly hypnotizable
subjects had more alpha1 activity in the left hemisphere over frontal region during a waking-rest,
the hypnosis-rest1 and even more hypnosis-rest2.
This hemispheric trend demonstrated a mono-
V. De Pascalis et al. r International Journal of Psychophysiology 29 (1998) 255]275
265
change significantly across baseline conditions for
low hypnotizable subjects. Comparisons of the
means between hypnotizability groups evidenced
that high subjects had smaller alpha1 amplitude
compared to the lows over both left and right
frontal recording sites Ž P- 0.01. Žsee Fig. 3..
These subjects, however, in waking-rest and hypnosis-rest conditions displayed a relative right
hemisphere activation Ži.e. lower alpha1 amplitude. over frontal sites and a relative left-hemisphere activation over posterior sites. Low hypnotizable subjects in waking-rest and hypnosis-rest
conditions also displayed a relative right hemisphere activation over frontal sites Žsee Fig. 3..
3.2.1.4. 40-Hz. The following significant effects
were found: Ž1. Suggestibility = Trial Ž F 2,32s
3.60, MSes 1.098, Ps 0.039.; Ž2. Suggestibility =
Hemisphere Ž F1,16 s 6.76, MSe s 0.063, P s
0.02.. The first effect showed that 40-Hz amplitude in highly suggestible subjects progressively
increased from waking to hypnosis1 and hypnosis2 conditions. In low suggestible subjects, in
contrast, there were no differences in 40-Hz amplitude across baseline conditions Žsee Fig. 4..
The second effect displayed a right-hemisphere
activation for highly suggestible subjects while, in
contrast, low suggestible ones displayed a hemisphere balancing Žhigh suggestibles: 0.70 vs. 0.81;
low suggestibles: 0.59 vs. 0.57; for left vs. right
hemisphere, respectively..
Fig. 2. Ža. Theta2 Ž6.25]8 Hz. amplitude for high and low
sensory suggestible subjects during waking-rest ŽW., hypnosisrest1 ŽH-1. and hypnosis-rest2 ŽH-2. in eyes-open and eyesclosed conditions. Žb. Alpha1 Ž8.25]10 Hz. amplitude for high
and low suggestible subjects during waking-rest ŽW.,
hypnosis-rest1 ŽH-1. and hypnosis-rest2 ŽH-2. in eyes-open
and eyes-closed conditions.
tonic increase of frontal alpha1 from waking to
hypnosis conditions with the increase being more
pronounced over the left frontal site. Over posterior sites there were, for high hypnotizables, more
alpha1 in the right hemisphere, but this difference disappeared in the hypnosis-baseline2 due
to a monotone increase of alpha1 in the left
hemisphere from waking to hypnosis1 and hypnosis2 conditions. The hemispheric trend did not
3.2.2. Emotion conditions
For theta1, theta2, alpha1, beta1, beta2, and
beta3 there were no significant effects involving
Hypnotizability, Suggestibility and Trial. Common
to these EEG bands were the significant effects
for Location and Hemisphere = Location. These
effects indicated that there were higher amplitudes over posterior recording sites and that these
amplitudes were greater over left-frontal scalp
sites and over right-posterior scalp sites.
3.2.2.1. Alpha2. A main effect for Hypnotizability was evidenced Ž F1,16s 5.19, MSes 4.09, Ps
0.037.. This effect indicated that across emotion
conditions highly hypnotizable individuals had a
greater alpha2 amplitude than low hypnotizable
individuals. The Location and Hemisphere =
Location factors were also significant Ž F 2,32s
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Fig. 3. Alpha1 Ž8.25]10 Hz. amplitude in left and right hemispheres at frontal, central and parieto-temporo-occipital ŽPTO. scalp
leads for high and low hypnotizable subjects during waking-rest ŽW., hypnosis-rest1 ŽH-1., and hypnosis-rest2 ŽH-2. conditions.
Fig. 4. 40-Hz Ž36]44 Hz. EEG amplitude for high and low
sensory suggestible subjects during waking-rest ŽW., hypnosisrest1 ŽH-1. and hypnosis-rest2 ŽH-2. conditions.
17.69, MSe s 0.754, P - 0.0001; F 2,32 s 8.14,
MSes 0.102, Ps 0.0025, respectively.. The first
effect indicated that alpha2 amplitude was greater
over posterior sites compared to frontal and central sites; the second effect indicated that alpha2
was greater over the left-frontal compared to the
right-frontal site of the scalp, while there were no
hemispheric differences across central and posterior sites.
3.2.2.2. 40-Hz. A number of significant effects
were evidenced for 40-Hz amplitude: Ž1. Hemisphere = Location Ž F 2,32s 10.09, MSes 0.083,
Ps 0.0004.; Ž2. Hypnotizability = Hemisphere =
Location Ž F 2,32s 3.72, MSes 0.083, Ps 0.035.;
Ž3. Hypnotizability = Hemisphere = Trial Ž F 2,32
s 4.00, MSes 0.147, Ps 0.045.; Ž4. Hemisphere
= Location = Trial Ž F4,64s 4.51, MSes 0.052,
P s 0.014.; Ž5. Hypnotizability = Hemisphere =
Location = Trial Ž F4,64s 3.05, MSes 0.052, P0.05.. The first and second effects indicated that
high hypnotizables had a significantly greater 40Hz amplitude over the left compared to the right
frontal site, no hemispheric differences over central sites and a right hemisphere prevalence over
posterior sites; low hypnotizables displayed a left
hemisphere prevalence over frontal sites and no
hemisphere differences across central and posterior sites Žsee Fig. 5..
The third effect indicated that high hypnotizables had a differential hemispheric asymmetry
across emotion and neutral-relaxation conditions
and that this hemispheric trend was different
from that displayed by low hypnotizable subjects.
The fourth and fifth effects disclosed for highly
hypnotizable subjects: Ža. in the happiness emotion condition a left-hemisphere prevalence over
V. De Pascalis et al. r International Journal of Psychophysiology 29 (1998) 255]275
267
Fig. 5. 40-Hz Ž36]44 Hz. EEG amplitude for high and low hypnotizable subjects at frontal ŽF3-F4., central ŽC3-C4. and
parieto-temporo-occipital ŽPTO. scalp leads averaged across emotion conditions.
frontal and central scalp sites, and a right-hemisphere activation for posterior sites; Žb. in the
sadness emotion, as compared to the other conditions, there was a pronounced increase of 40-Hz
in the right-hemisphere at frontal, central and
posterior sites which showed more power in the
left frontal site and right central and right posterior sites; Žc. in the neutral-relaxation condition a
left hemisphere activation at frontal sites and no
hemispheric differences at central and posterior
Fig. 6. 40-Hz Ž36]44 Hz. EEG amplitude in the left and right hemispheres at frontal, central and parieto-temporo-occipital scalp
leads for high and low hypnotizable subjects during happiness ŽH., sadness ŽS. and neutral-relaxation ŽN. emotion conditions.
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sites were found. The poor hypnotizables displayed, for happiness and neutral-relaxation conditions, similar hemispheric trends to that found
for high subjects across frontal and posterior sites.
The lows, in contrast with the highs, displayed
across frontal and posterior sites a left hemisphere activation for the negative emotion condition. These hemispheric trends are shown in Fig.
6.
3.3. Heart rate
The ANOVA for HR scores across resting conditions evidenced a significant effect for Suggestibility = Trial Ž F 2,32 s 3.74, MSe s 1642.7,
Ps 0.04.. This effect indicated that high suggestible subjects significantly increased their heart
rate from waking-rest to hypnosis-rest conditions;
low suggestible subjects, in contrast, displayed a
monotonic heart rate decrease from waking rest
to hypnosis rest conditions Žhigh suggestibles: 80.6,
82.3, and 81.8; low suggestibles: 77.5, 75.5, and
74.0 bpm; respectively for waking rest, first hypnosis rest, and last hypnosis rest..
The ANOVA across emotion and control conditions evidenced a main effect for Trial Ž F 2,32
s 8.93, MSes 398.4, Ps 0.0008.. This effect indicated that there was a monotonic increase of
heart rate from the relaxation-control to positive
and negative emotion conditions Ž77.5, 79.4, and
80.6 bpm, respectively.. No main or interaction
effects were found for Hypnotizability on HR
scores.
4. Discussion
Given the current state of research in hypnosis
and suggestibility, we examined three major hypotheses. In terms of the first hypothesis, we
expected to see a greater theta activity in high
than in low hypnotizable subjects. Indeed, as it
can be seen in Fig. 1, this was the case for theta1
Ž4]6 Hz. and alpha1 Ž8.25]10 Hz. activities, as
measured across resting conditions. In particular,
high hypnotizable subjects during waking and
hypnosis conditions had greater amplitudes in the
left than right hemisphere at frontal regions. The
opposite hemispheric trend was observed for these
subjects across posterior sites during waking and
the first hypnosis condition. In the later hypnosis
condition there was an increased activity for both
theta1 and alpha1 over left temporo-parieto-occipital region with no hemispheric differentiation.
The low hypnotizable subjects also produced more
theta1 and alpha1 over the left as compared to
the right frontal region, but they did not show
differences in asymmetry between waking and
hypnosis conditions Žsee Figs. 1 and 3.. For theta2
Ž6.25]8 Hz. spectral amplitude we failed to find
significant effects involving hypnotizability in resting conditions. Moreover, specific EEG patterns
in resting conditions for theta2 and alpha1
Ž8.25]10 Hz. spectral amplitudes differentiate
high and low suggestible persons Žsee Fig. 2..
Thus, results from the present study demonstrate
that there are distinctive EEG patterns among
resting baselines that characterize high and low
hypnotizable subjects and differentiate them from
high and low suggestible ones. Second, we expected emotionality to be experienced more
strongly by high as compared to low hypnotically
susceptible individuals. In terms of self-report,
high hypnotizable individuals did report greater
levels of emotional experiences than low susceptible individuals. Further, high hypnotizable individuals reported a greater experiencing of negative as compared to positive affect whereas low
susceptible individuals did not show this differentiation. Third, in terms of electrocortical processing of emotional material, we hypothesize opposite frontal and temporo-parieto-occipital EEG
differences in emotionality. As can be seen in Fig.
6 in terms of 40 Hz this was clearly the case for
high hypnotizable individuals in terms of sadness
and for low and high hypnotizable individuals in
terms of happiness. In terms of pilot work, we
sought to examine differences between subjects
showing high and low levels of imaginative suggestibility. The only differences found between
high and low suggestibility groups was in the
gamma band in terms of resting conditions and
for alpha and theta in terms of eyes openrclosed
conditions. Since this is the first study to examine
EEG differences in this group, future work is
required to fully understand these results. Another purpose of the present study was to repli-
V. De Pascalis et al. r International Journal of Psychophysiology 29 (1998) 255]275
cate and extend our previous 40-Hz EEG correlates of emotionality using a more advanced measure of 40-Hz EEG spectral amplitude.
4.1. Psychophysiological differences during rest in
waking and hypnosis
The theta1 finding is consistent with previous
hypnosis research Že.g. Galbraith et al., 1970;
Graffin et al., 1995; Sabourin and Cutcomb, 1980;
Sabourin et al., 1990; Tebecis et al., 1975.. Indeed, as with the present study, Graffin et al.
Ž1995. found the strongest theta Ž4]8 Hz. differences between susceptibility groups in the more
frontal areas of the cortex. Previous research has
evidenced that increased frontal midline theta,
especially at 6 Hz, has been associated with attentional readiness or attentive performance Žin resting condition. of cognitive tasks ŽBruneau et al.,
1993; Ishihara and Yoshii, 1972; Lang et al., 1988;
Mizuki et al., 1980.. Moreover, looking at the
Galbraith et al. Ž1970. findings, the highest correlated theta frequency with hypnotizability was at
5 Hz which is corresponding to the center frequency of our theta1 band. As in the Galbraith et
al. study, our theta1 findings may be seen as
reflecting a class II inhibition process i.e. that
high hypnotizable individuals might manifest a
heightened state of attentional readiness and focalization of attention. In the present study we
failed to find theta2 differences between high and
low hypnotizable subjects as, indeed, it was found
in the recent study of Crawford et al. Ž1996.. We
think that the apparent conflicting findings may
lie in the sometimes incorrect assumption that
low theta is associated with class I inhibition and
high theta with class II inhibition, since we cannot exactly know the band edges which define the
high and low theta activities. It may be reasonable to think that these edges are dependent from
a number of factors such as personality, sampling,
and situation-specific variables.
The alpha1 activity has been found to be associated with decreased cortical activity or with
poor sustained attentional abilities Že.g. Bosel,
1992; Gale and Edwards, 1983; Klimesch et al.,
1990.. If we assume that an increase in theta1
activity reflects an increase in focused attention
269
and that an increase in alpha1 reflects an aspecific effect of relaxation, then the increased theta1
and alpha1 amplitudes found over left temporoparieto-occipital site in highly hypnotizable subjects, indicate that these subjects, in resting hypnotic condition, were not only more relaxed, but
also more facilitated in processing relevant stimuli than they did in a waking condition. Moreover, the finding that high hypnotizables, in comparison with lows, displayed higher theta1 and
lower alpha1 amplitudes over left and right frontal
regions, is indicating that highs were actively more
involved in the hypnosis condition than did the
lows. Finally, the alpha1 trend observed across
waking and hypnosis conditions over frontal and
temporo-parieto-occipital regions of the left
hemisphere is consistent with the Gruzelier neurophysiological model of hypnosis wherein the
central inhibitory processes, particularly of the
left hemisphere, are considered as the main indicators characterizing hypnosis ŽGruzelier et al.,
1984; Gruzelier, 1988; Gruzelier and Warren,
1993..
The HR data failed to evidence significant differences among resting conditions between high
and low hypnotizable subjects. This result does
not confirm the expected finding, i.e. that hypnosis induction, being composed of relaxation suggestions, produces a decrease in HR Žsee e.g.
Sarbin and Slagle, 1979.. Gray et al. Ž1970. and
Bauer and McCanne Ž1980. obtained findings in
line with this prediction: they observed a decrease
in HR during a rest period after the hypnotic
induction as compared to a rest period before the
induction. However, the present result appears in
agreement with the Lamas and del Valle-Inclan
`
Ž1994. findings supporting that the induction
procedure does not produce HR deceleration in
high hypnotizable subjects.
4.2. EEG differences during emotions in hypnosis
During the emotional conditions, high hypnotizable subjects, compared to the lows, reported
greater ability in experiencing positive and negative emotions, more alpha2 activity and more
pronounced emotion-related 40-Hz EEG hemispheric shifts. Specifically, in terms of 40-Hz am-
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plitude high hypnotizables showed three main
findings Žsee Fig. 6.. Firstly, in the happiness
condition an increased activity in the frontal left
hemisphere that produced a marked hemispheric
asymmetry in favor of the left compared to the
right hemisphere; a similar but less pronounced
asymmetry in favor of the left hemisphere was
also found across central sites, while an opposite
hemispheric asymmetry was evidenced in the posterior region. This pattern of findings is consistent
with previous research showing more left frontal
Žcf. Davidson, 1992. and right posterior Žcf.
Tucker, 1981. cortical involvement in the processing of positive emotionality. Secondly, in the sadness condition, compared to the other conditions,
there was an increased activity over the right
hemisphere that produced a hemispheric balancing in the frontal region and a right hemisphere
activation in the central and even more in the
posterior regions. These results are consistent
with the predictions of the Pribram Ž1981. anterio-posterior neurophysiological model of emotions. Thirdly, the relaxation condition produced
more activity in the left than in the right frontal
region, along with more equal hemispheric activity in the central and posterior regions.
Overall findings in terms of 40-Hz activity parallels those of previous research from our lab.
One major EEG difference between positive and
negative emotional experience was an increased
production of 40-Hz activity in the left frontal
region for happiness and a significant increase in
the right posterior regions for sadness. These
results support prior research of self-generated
emotional states that demonstrate, using alpha,
activation of parietal region for positive and negative emotions ŽCollet and Duclaux, 1987; Harman
and Ray, 1977; Karlin et al., 1979; Meyers and
Smith, 1987; Smith et al., 1987, 1989; Tucker et
al., 1981; Tucker and Dawson, 1984.. These results also support the validity of the Heller Ž1993.
model indicating that parietotemporal regions
play a critical role in the experience of emotion.
Our observations are parallel to those of Stenberg Ž1992. who found the frontal lateral region
of the brain mainly engaged in emotional processing. Similar 40-Hz hemispheric trends were reported in our previous studies in which 40-Hz
density was the dependent variable ŽDe Pascalis
et al., 1987, 1989.. In our previous studies, 40-Hz
density increased in the right hemisphere during
negative emotions. The current study found an
increased activity in the left and even more in the
right posterior sites by displaying a hemispheric
asymmetry in favor of the right hemisphere. Starting from the assumption that 40-Hz EEG rhythm
is the physiological expression of ‘focused arousal’
Že.g. Sheer, 1989; Tiitinen et al., 1993., our 40-Hz
EEG findings during emotional states are consistent with the dual-attentional model of Kinsbourne Ž1982. and of Pribram Ž1981. wherein the
left hemisphere is more involved to maintain selective or focal attention and the right to maintain ‘sustained’ attention in order to form a continuous synthesis of the incoming information.
During positive emotions, as compared to a relaxation condition, the frontal and central regions in
the left hemisphere are more engaged to extract
information from the recollecting material and
the posterior-right hemisphere to form a connotational synthesis of the ongoing processed stimuli.
That is, it is the posterior area that gives color to
the emotional experience. The recollection of a
negative emotion, with respect to a relaxation
condition, requires an engagement of both right
and left frontal areas and mainly of the right
central and posterior areas of the cortex.
In terms of HR activity it was found that HR
increased during positive and even more during
negative emotional events as compared to a relaxation condition. The HR activity failed to evidence, among emotion conditions, different trends
between high and low hypnotizable subjects. This
result confirms findings from other studies using
facial configuration or relived emotion tasks. The
HR increases for sadness, fear and anger ŽEkman
et al., 1983; Levenson et al., 1991. and happiness
ŽLevenson et al., 1990.. The HR activity has been
used in hypnosis research as a physiological measure of attentional demands. It has been found to
increase with stressful active suggestions and to
decrease with relaxation ones Žsee e.g. Barber,
1965; Sarbin and Slagle, 1979.. The HR changes
should be more pronounced in highly hypnotizable subjects since they are more engaged than
the lows in the suggested tasks, as indeed it was
V. De Pascalis et al. r International Journal of Psychophysiology 29 (1998) 255]275
the case in the study of Sabourin et al. Ž1990..
Our HR changes were sensitive to the emotional
valence of the recollected material but failed to
evidence differences in trends between high and
low hypnotizable subjects. This lacking result may
lay in the fact that attentional demands and emotional valence have an additive effect on HR
increase Že.g. Cook et al., 1991; Hawk et al.,
1992.. This effect may have been sufficient to
produce an HR increase also in the low hypnotizable subjects.
4.3. Suggestibility
The HR increase during hypnosis resting conditions as compared to a waking resting condition
differentiate suggestible from non-suggestible
subjects. In terms of EEG, theta2, alpha1, and
40-Hz EEG activities were sensitive to individual
differences in suggestibility, but only between eyes
open and closed conditions. Common to theta2
and alpha1 was the tendency for highly suggestible persons to display a significantly greater
amount of these activities in resting eyes-closed
as compared to resting eyes-open conditions. In
general, high suggestible individuals compared to
low suggestible ones showed a differential increase of theta2 in the last eye-closed baseline
condition. In terms of alpha, high suggestible
individuals showed more alpha1 in the eyes closed
condition Žsee Fig. 2.. The 40-Hz EEG in highly
suggestible subjects also displayed a monotonic
increase from waking to the first and last hypnosis-rest conditions Žsee Fig. 4.. These results to do
not form a consistent picture but can be understood as related to the greater reactivity of highly
suggestible subjects to the hypnosis task per-se.
The validity of this interpretation is also supported by the more pronounced heart rate increase found for these subjects in the hypnosisrest conditions as compared to the initial waking
condition. The lack of relationship between sensory suggestibility and absorption scores as well as
a lack of EEG theta differentiation would suggest
that individual differences in the capacity to be
absorbed in imaginative activity is separate from
that of suggestibility. Since no significant interactional effects involving both hypnotizability and
271
suggestibility were found, for each of the variables considered in this study we maintained that
these two dimensions are expressions of different
underlying psychophysiological activities.
Overall, the present study replicates a variety
of EEG research, especially theta and 40 Hz
activity, in relation to hypnosis. As in previous
studies, high and low hypnotically susceptible individuals differed in EEG theta and in emotional
processing as reflected in both self-report and 40
Hz activity. The present study also supported
previous theoretical speculations suggesting
frontalrposterior differences in emotional processing. Finally, it gives psychophysiological support to the theoretical position that suggestibility
and hypnosis reflect different physiological mechanisms, but it also indicates that it might be
worthwhile to take into account suggestibility in
future research.
Acknowledgements
We greatly acknowledge the assistance of the
following students: Giovanna Carbone, Simona
Tomassetti. This research was assisted in part by
a collaborative research grant from NATO.
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