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Enhancement of suggestibility and imaginative ability with nitrous oxide

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Imaginative suggestibility, a trait closely related to hypnotic suggestibility, is modifiable under some circumstances. Nitrous oxide (laughing gas) is commonly used for sedation in dentistry and is reported to be more effective when combined with appropriate suggestions. The aim of this study was to determine whether nitrous oxide inhalation alters imaginative suggestibility and imagery vividness. Thirty participants were tested twice in a within-subjects design, once during inhalation of 25% nitrous oxide and once during inhalation of air plus oxygen. Before the study, participants' expectancies regarding the effects of nitrous oxide were assessed. Participants were blinded to drug administration. During each session, participants were verbally administered detailed measures of imagination and suggestibility: the Sheehan-Betts Quality of Mental Imagery scale and the Stanford Hypnotic Susceptibility Scale Form C, minus the hypnotic induction. Imaginative suggestibility and imaginative ability (imagery vividness) were both elevated in the nitrous oxide condition. This effect was unrelated to participants' expectations regarding the effects of the drug. Nitrous oxide increased imaginative suggestibility and imaginative ability. Possible explanations of these findings are discussed with respect to the effects of N-methyl-d-aspartate antagonists and to other pharmacological effects upon suggestibility and imagination.
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ORIGINAL INVESTIGATION
Enhancement of suggestibility and imaginative ability
with nitrous oxide
M. G. Whalley &G. B. Brooks
Received: 27 May 2008 / Accepted: 19 November 2008
#Springer-Verlag 2008
Abstract
Rationale Imaginative suggestibility, a trait closely related
to hypnotic suggestibility, is modifiable under some
circumstances. Nitrous oxide (laughing gas) is commonly
used for sedation in dentistry and is reported to be more
effective when combined with appropriate suggestions.
Objective The aim of this study was to determine whether
nitrous oxide inhalation alters imaginative suggestibility
and imagery vividness.
Methods Thirty participants were tested twice in a within-
subjects design, once during inhalation of 25% nitrous
oxide and once during inhalation of air plus oxygen. Before
the study, participantsexpectancies regarding the effects of
nitrous oxide were assessed. Participants were blinded to
drug administration. During each session, participants were
verbally administered detailed measures of imagination and
suggestibility: the SheehanBetts Quality of Mental Imag-
ery scale and the Stanford Hypnotic Susceptibility Scale
Form C, minus the hypnotic induction.
Results Imaginative suggestibility and imaginative ability
(imagery vividness) were both elevated in the nitrous oxide
condition. This effect was unrelated to participantsexpect-
ations regarding the effects of the drug.
Conclusions Nitrous oxide increased imaginative suggest-
ibility and imaginative ability. Possible explanations of
these findings are discussed with respect to the effects of N-
methyl-d-aspartate antagonists and to other pharmacologi-
cal effects upon suggestibility and imagination.
Keywords Suggestibility .Nitrous oxide .Imagination .
Imaginative ability .Imagery .Hypnosis .Hypnotisability .
Suggestion .Vividness .NMDA
Introduction
Nitrous oxide (laughing gas) inhalation is a form of
conscious sedation and is an analgesic commonly used in
dentistry and also in obstetrics. It has long been noted by
dental practitioners that patients under nitrous oxide
sedation are particularly suggestible and a number of
investigators have noted the clinical advantages of using a
hypnotic voice when administering nitrous oxide (Lippe
1944; Seladin 1947). Bingham (1964) describes a case of
rapid hypnosis by using nitrous oxideand Allen (1972)
notes that during nitrous oxide sedation patients respond
well to suggestions given in a quiet, hypnotic manner.
Hilgard and Hilgard (1975) and Eysenck and Rees (1945)
also informally note that sub-anaesthetic doses of nitrous
oxide will heighten the hypnotic responsiveness of the
patient. If there is a synergistic relationship between nitrous
oxide inhalation and responsiveness to suggestions, then
strategic use of appropriate suggestions for relaxation and
analgesia should enhance the overall clinical effectiveness
of nitrous oxide sedation procedures (Simons et al. 2007).
Suggestibility can also be assessed in the absence of
hypnosis and is termed imaginative suggestibility(Kirsch
and Braffman 2001). Hypnotic inductions are commonly
used to modestly increase suggestibility (Kirsch and
Braffman 2001), and suggestibility can also be modified
by either changing peoples expectations (Vickery and
Kirsch 1991), labeling a situation as hypnotic (Gandhi
and Oakley 2005) or through training (Gorassini and
Spanos 1989). There is disagreement, however, about the
Psychopharmacology
DOI 10.1007/s00213-008-1424-0
M. G. Whalley (*):G. B. Brooks
Hypnosis Unit, Department of Psychology,
University College London,
Gower Street,
London WC1E 6BT, UK
e-mail: matwhalley@gmail.com
magnitude to which suggestibility can be affected through
training programs (e.g. Benham et al. 1998; Spanos 1986).
One study that has systematically examined the effect
of nitrous oxide upon suggestibility involved 20 volun-
teers randomly divided into two groups (Barber et al.
1979). One group received nitrous oxide supplemented
with oxygen and the other group received oxygen alone. In
the nitrous oxide group, the dose was increased until a
baseline level giving paraesthesia and generalised warmth
was reached (2040%). When signalled by the anaesthetist,
both groups were read an identical list of suggestions
involving analgesia in one leg, compulsive behaviour
(picking up a pen) and amnesia. The group receiving
nitrous oxide responded to more suggestions, and the
authors report that this effect was unrelated to subsequently
measured hypnotic susceptibility. Unfortunately, it is not
clear whether nitrous oxide was administered throughout
the test session, or just for the period prior to the
suggestions, a detail that could particularly affect scores
on the analgesia item. In addition, the use of a between-
subjects design with a small N of hypnotically unselected
participants increases the likelihood of any effects being
due to uncontrolled pre-existing variations in responsive-
ness to suggestion.
The primary aim of the present study was to assess
whether nitrous oxide sedation is associated with changes
in suggestibility and imaginative ability, both measured in a
within-subject design using standardised scales. Non-
hypnotic imaginative suggestibility, rather than hypnotic
suggestibility, was chosen in order to simplify the experi-
mental procedure and to avoid any complications arising
from participantsexpectations regarding hypnosis. We
formally assessed participantsbeliefs about the effects of
nitrous oxide in order to determine whether any increase in
suggestibility was expectancy mediated.
Materials and methods
Participants
Participants in the study were 30 adults recruited from a
dental surgery in Manchester, England. A notice requesting
volunteers for a study of suggestibility and nitrous oxide
was posted in the waiting room and the first 30 participants
who volunteered and met the inclusion criteria were
accepted. Inclusion criteria were that participants should
be aged between 21 and 55 and not have any medical
problem contraindicating the administration of nitrous
oxide. Ethical approval was received from a University
Research Ethics Committee. Thirty participants completed
the study (ten males). Their average age was 40.06
(standard deviation (SD) =12.75).
Measures
All participants completed the SheehanBetts Quality of
Mental Imagery Scale (QMI; Sheehan 1967) and the
suggestions from the Stanford Hypnotic Susceptibility
Scale Form C (SHSS:C; minus the hypnotic induction;
Weitzenhoffer and Hilgard 1962). The QMI is a 35-item
test assessing the vividness of different modalities of mental
imagery (e.g. auditory, olfactory, tactile). It is a shortened
version of Betts(1909) original 150-item questionnaire.
The scale was modified by the authors for verbal
administration, but participants rated each item using the
original seven-point scale (1 = as clear and vivid as the real
thing; 7 = no image present at all, you only knowyou are
thinking of an object). Testretest reliability for the
SheehanBetts QMI has been found to be 0.75 for females
and 0.72 for males after 2 weeks (Westcott and Rosenstock
1976). Factor analyses of the questionnaire items have
revealed the presence of a large unitary factor cor-
responding to general vividness of imagery and modality
specific factors (Sheehan 1967; Wagman and Stewart 1974;
White et al. 1974).
The SHSS:C (Weitzenhoffer and Hilgard 1962) is a 12-
item test, individually administered according to a stand-
ardised procedure. The researcher administering the scale
objectively assesses participants responses to each item.
Suggestions on the test are progressively more difficult,
enabling the researcher to terminate the test after the failure
of three consecutive items. The SHSS:C was chosen for a
number of reasons. Firstly, it contains proportionately more
difficult items than other scales (Perry et al. 1992; Bertrand
1989), which is important to minimise any potential ceiling
effects. Secondly, it contains a high proportion of cognitive
items which we theorised might be less affected by
lethargic feelings brought about by nitrous oxide sedation.
As well as removing the hypnotic induction, we made a
number of other modifications to the SHSS:C in order to
increase its suitability for administering to participants
inhaling nitrous oxide. For the age regression item, the
American reference to school grades was omitted and
replaced with the terms junior schooland infant school.
The anosmia to ammonia item was omitted to avoid having
to remove nose mask used for administration of the nitrous
oxide. A test of post-hypnotic suggestion modified from the
Stanford Hypnotic Susceptibility Scale Form A (SHSS:A;
Weitzenhoffer and Hilgard 1959) was added to replace the
anosmia item. Post-hypnotic amnesia was assessed by joint
criteria considering both initial amnesia and subsequent
reversibility. Amnesia was scored as present only if the
participants recalled both three or fewer critical items
initially and two or more additional items following the
reversal cue. The instructions for the reversibility test ask
subjects to report all items they remember. These mod-
Psychopharmacology
ifications have been used by other groups (Kihlstrom
2007). For the dream item (where participants are invited
to have a dream), the word hypnosiswas replaced by
relaxation. The post-experimental interview was omitted
after the first visit because it was thought that these
questions might have affected the amnesia test for the
second visit. However, after the second visit, these
questions were included and the subject was asked to draw
comparisons between the two visits.
Procedures
Participants made two visits to the dental surgery, with a
gap of approximately 2 weeks. At the first visit, participants
were given information about the study and provided
informed consent. Participants were told that the study
was an investigation of responses to imaginative sugges-
tions, which would produce changes in sensation and
perception. They were told that two sessions would be
involved but that nitrous oxide would only be administered
on one visit. Participants were explicitly told that the study
was not a test of hypnosis.
Half of the participants received the nitrous oxide on the
first visit and half on the second, randomised by a dental
nurse who was blind to the study hypothesis. At the start of
the first session, participants were asked whether they
thought the administration of nitrous oxide would affect
their suggestibility, which direction any effect might be in,
and the extent of this expectation on a scale ranging from
0% to 100%. The mask was then fitted to the participants
nose and they were given instructions to breathe through
the nose. Participants were made aware that breathing
through the mouth would stop the gas from working.
Nitrous oxide was delivered by a McKesson 882 continu-
ous flow machine (McKesson Equipment Company,
Chesterfield, UK). The nose mask was scented in order to
disguise the sweet smell of the nitrous oxide. During the
non-nitrous oxide visit, the mask was placed in position and
the air intake valve left open, with oxygen still delivered at
3 L/min. This gave a mixture of oxygen diluted with air so
that the subject could still feel and hear gas flowing through
the mask. For the nitrous oxide session, the flow rate was
increased slowly until 25% nitrous oxide was being
delivered. The nitrous oxide and oxygen were always
administered by the experimenter (GB).
At both sessions, the SheehanBetts Quality of Mental
Imagery Scale (Sheehan 1967) and the suggestions from the
Stanford Hypnotic Susceptibility Scale Form C (minus the
hypnotic induction) (Weitzenhoffer and Hilgard 1962) were
administered by the same experimenter (GB) who was not
blind to the drug administration condition. For the QMI,
participants were asked to close their eyes to listen to the
description of each item and to imagine it as vividly as
possible, then to open their eyes and rate the item using the
scale. After the final item of the QMI, participants were
asked to open their eyes and the investigator checked that
they were feeling alright.
In the present investigation, the hypnotic induction that
normally precedes the suggestions on the SHSS:C was
omitted, making it a test of imaginative suggestibility
(Kirsch and Braffman 2001). Administration of suggestions
was conducted according to the manual. During the count
down to normal alertness at the end of the SHSS:C, which
was timed to last for 2 min, 100% oxygen was adminis-
tered. This was to prevent diffusion hypoxia during
emergence since the inhaled air (21% oxygen) is diluted
by the rapid excretion of nitrous oxide, nitrogen being
absorbed only slowly (Cass and Cass 1994).
The QMI was always delivered first because of the
relative complexity of the SHSS:C. During the alerting
phase of the final item of the SHSS:C, the delivery of
nitrous oxide or oxygen was stopped and the participant
was questioned about their memory for items in the test,
followed by the SHSS:C post-experimental interview.
Counterbalanced administration of the tests would have
required restarting delivery of nitrous oxide after this
alerting phase, and it was considered more feasible to
deliver the tests in a single order with nitrous oxide delivery
uninterrupted. At the end of the second session, participants
were asked during which session they thought had received
the nitrous oxide.
Results
Four patients dropped out after completing one session;
they were replaced and their data were not used in the
analysis. Imaginative ability measured by the Sheehan
Betts QMI was greater in the nitrous oxide condition
(85.83, SD = 37.63) than the oxygen alone condition
(111.63, SD= 37.77; lower scores on the QMI indicate
higher imaginative ability). Imaginative suggestibility mea-
sured by the SHSS:C was greater in the nitrous oxide
condition (7.33, SD=2.80) than the oxygen alone condition
(6.16, SD=2.47).
To test for potential order effects, we included order of
drug administrations as a factor in our analysis (group 1:
first session = drug, second session = no drug; group 2: first
session = no drug, second session = drug). Groups did not
differ with respect to age (t(28) =0.253, p=0.802). A
mixed-model analysis of variance with drug as a within-
subjects factor (nitrous oxide vs. oxygen) and group as a
between-subject factor was conducted for scores on the
SHSS:C and the QMI. For scores on the SHSS:C, there was
no main effect of group (F(1,28) =0.483, p=0.493) and no
drug ×group interaction (F(1,28)=1.128, p=0.297). For
Psychopharmacology
scores on the SheehanBetts QMI, there was no main effect
of group (F(1,28)=2.075, p=0.161) and no drug ×group
interaction (F(1,28)=0.806, p=0.377). For scores on the
SHSS:C, there was a main effect of drug (F(1,28)=11.418,
p=0.002, effect size (partial η
2
)=0.290). For scores on the
QMI, there was a main effect of drug (F(1,28)= 42.957,
p<0.001, effect size (partial η
2
)=0.605).
To see whether nitrous oxide affected certain classes of
suggestion more than others, response rates for each
suggestion were assessed. The percentage pass rates for
each are given in Table 1.
Participantsscores on the QMI were broken down by
the modality of each item in order to assess whether any
particular modality was more affected by the administration
of nitrous oxide. Average scores per item are given in Fig. 1
(lower scores equate to higher imagery vividness). Items in
all categories (visual, auditory, touch, movement, taste,
smell, sensation) were significantly more vividly imagined
in the nitrous oxide than in the oxygen condition (by
modality, respectively: t(4)=5.41, p=0.003; t(4) = 15.5,
p<0.001; t(4)=5.46, p= 0.002; t(4) = 6.35, p=0.001; t(4)=
30.45, p<0.001; t(4)=4.18, p= 0.007; t(4) = 4.9, p=0.004).
Only 11 participants (36.7%) correctly judged which
session they had received nitrous oxide. A binomial test
revealed that this proportion does not differ significantly
from what would be expected by chance (p=0.201) and
indicates that participants were unaware when they were
being administered the nitrous oxide. In order to formally
assess whether changes in suggestibility were affected or
driven by participantsexpectancy, we examined changes in
suggestibility as a function of participantsexpectations.
Two participants expected the nitrous oxide to have no
effect upon suggestibility, 19 did not know and nine
participants expected the nitrous oxide to increase suggest-
ibility. The participants who expected an increase were
asked to estimate the size of the effect on a 100-mm visual
analogue scale bounded by the terms no changeand large
change. The average magnitude of expected increase was
67.19 (standard deviation 11.27). The correlation between
expected suggestibility change (Dont knowresponses
assumed to have a magnitude of zero) and actual sug-
gestibility change (difference in scores between the nitrous
oxide and oxygen conditions) was small and non-signifi-
cant (r=0.039, p=0.838 (two tailed)).
Change scores for imaginative suggestibility and imag-
inative ability (differences in scores between the nitrous
oxide and oxygen conditions) were calculated by subtract-
ing the oxygen score from the nitrous oxide score. These
two change scores for each participant were then correlated
to determine whether there was an association, revealing a
significant relationship (r=0.511, p=0.004 (two tailed)).
Lower scores on the QMI indicate higher imaginative
ability; thus, there exists a positive relationship between
changes in imaginative ability and changes in imaginative
suggestibility.
In the oxygen-alone condition, scores for imaginative
ability and suggestibility were significantly correlated (r=
0.312, p=0.0465 (one-tailed)). However, in the nitrous
oxide condition, the relationship was non-significant (r=
0.194, p=0.152 (one-tailed)).
Discussion
This investigation demonstrates, for the first time with a
standardised test, that inhalation of 25% nitrous oxide
produces objectively assessed increases in imaginative
suggestibility. Nitrous oxide also increased imaginative
Table 1 Percentage pass rates and Zand pvalues (Wilcoxon-related samples) for items of the SHSS:C when participants were inhaling either
nitrous oxide or oxygen
SHSS:C item Item class Percentage pass
oxygen
Percentage
pass N
2
0
Difference
(N
2
0oxygen)
Zp(1-tailed)
Auditory hallucination Cognitive (production) 6.6 3.3 -3.3 0.277 0.282
Moving hands apart Motor (inhibition) 96.6 96.6 0 1 0.5
Post-hypnotic suggestion Cognitive 30 33.3 3.3 0.333 0.3695
Hand lowering Motor (production) 90 93.3 3.3 1 0.1585
Dream Cognitive 56.6 63.3 6.7 0.577 0.282
Taste hallucination Cognitive 43.3 50 6.7 0.707 0.24
Arm rigidity Challenge 63.3 70 6.7 1.414 0.0785
Post-hypnotic amnesia Cognitive 40 53.3 13.3 1.155 0.124
Negative visual hallucination Cognitive (inhibition) 10 26.6 16.6 2.236 0.0125*
Age regression Cognitive 56.6 73.3 16.7 2.236 0.0125*
Mosquito hallucination Cognitive 63.3 80 16.7 1.89 0.0295*
Arm immobilisation Motor (inhibition) 53.3 80 26.7 2.53 0.0055**
N
2
0nitrous oxide
*p< 0.05, **p< 0.01
Psychopharmacology
ability, a change strongly correlated with the increase in
suggestibility. The magnitude of the change in suggestibil-
ity induced by the inhalation of 25% nitrous oxide
approached 10%. It is difficult to estimate a similar statistic
for the imaginative ability scores because of the non-linear
nature of the QMI scale, although with nitrous oxide
inhalation average item ratings on the QMI improved from
approximating moderately clear and vividtowards very
clear and comparable in vividness to the actual experience.
Increases in imaginative ability under nitrous oxide
sedation were not greater in any particular sensory type,
indicating enhancement across the range of imaginative
modalities. It is difficult to gauge the relative effects of
nitrous oxide inhalation on different types of suggestion
(motor, challenge, cognitive) because of the relative
weighting of the SHSS:C towards cognitive items. The
item demonstrating the single largest effect of nitrous oxide
was the arm immobilisation (motor inhibition) item,
although it is probable that this was due to participants
feeling relaxed and heavy whilst inhaling the gas. Only one
challenge item (arm rigidity) was included and demonstrat-
ed a modest increase. The nitrous oxide induced increase in
suggestibility for cognitive items ranged from 3.3% for an
auditory hallucination (friends voice speaking) item to
+16.7% for the mosquito hallucination and age regression
items.
Previous studies have reported modest and uneven
associations between imaginative ability and hypnotic
suggestibility using a variety of measurement instruments
(Sutcliffe et al. 1970; Crawford 1982; Glisky et al. 1995;
for a review see de Groh 1989). The present study is novel
in that it measures suggestibility in the absence of a
hypnotic induction, yet the correlations found here are
within the range found in previous studies, with a
significant relationship demonstrated only in the non-drug
condition.
Alterations in suggestibility are most commonly brought
about by the induction of hypnosis (Kirsch and Braffman
2001) but have also been demonstrated via manipulations
in expectancy (Vickery and Kirsch 1991), labelling a
situation as hypnotic(Gandhi and Oakley 2005) or
undergoing a skills training programme (Gorassini and
Spanos 1989). In order to assess whether the effects of
inhaling nitrous oxide on suggestibility were due to an
expectancy effect, we measured participantsexpectancies
at the start of the experiment. Only one third of participants
expected the drug to increase suggestibility, with the
majority answering that they did not know. Participants
were not very accurate in identifying when nitrous oxide
was being administered, with just over a third identifying in
which condition they received the drug, further supporting
our conclusion that expectancies did not mediate the
observed effect. Lastly, we did not find expectancies to be
related to the magnitude of changes in suggestibility. These
results indicate the presence of a genuine drug effect upon
suggestibility. Unlike other modifiers of suggestibility such
as response expectancy (Kirsch 1985), the present effect
seems to be independent of participants expectations about
the effect of the drug.
In the absence of an expectancy-mediated effect, it is
interesting to speculate upon possible mechanisms by
which nitrous oxide might mediate changes in suggestibil-
ity and imaginative ability. The strong correlation between
changes in suggestibility and imaginative ability could
indicate a drug effect upon a mechanism common to both
of these changes. Nitrous oxide is one of the most widely
used yet least well understood anaesthetic gasses (Franks
and Lieb 1998) and until recently, relatively little was
known about the mechanism of its action. Jevtovic-
Todorovic et al (1998) found that nitrous oxide, like
ketamine, acts as an antagonist at glutamatergic N-methyl-
d-aspartate (NMDA) receptors, which are found throughout
the brain but are densely located in the hippocampus and
cerebral cortex (Morgan et al. 2004). NMDA receptor
antagonists seem to preferentially block NMDA receptors
on inhibitory GABAergic inter-neurons and thus increase
0
1
1.5
2
2.5
3
3.5
4
0.5
Nitrous Oxide
Oxygen
Average Vividness Score Per Item
QMI Item Modalit
y
Visual Auditory Touch Movement Taste Smell Sensation
Fig. 1 Scores on the QMI bro-
ken down by modality and drug
administration. Note: Lower
scores indicate higher imagina-
tive ability
Psychopharmacology
glutamatergic transmission via non-NMDA Glu receptors
(such as α-amino-3-hydroxy-5-methyl-4-isoxazole propio-
nate and kainite), which increases excitatory activity
generally.
A brain-wide excitation might explain the present nitrous
oxide induced increase in imagery vividness. The current
consensus is that most of the neural processes that underlie
like-modality perception are also used in imagery (Kosslyn
et al. 2001), and a brain-wide excitation could explain the
increases in vividness observed here across the full range of
imagery modalities. Modality-appropriate sensory activa-
tion has also been observed in response to hypnotic
suggestion (e.g. Szechtman et al. 1998; Kosslyn et al.
2000; Derbyshire et al. 2004), although the mechanism by
which suggestions are experienced with an involuntary
quality is thought to be the product of frontally mediated
executive control systems (e.g. Brown and Oakley 2004;
Dienes and Perner 2007). The suggestibility enhancement is
more difficult to explain via a global excitation and it is
useful to consider the effects of other drugs upon
suggestibility.
Little research has investigated the effects of other drugs
upon suggestibility in a controlled manner. Sjoberg and
Hollister (1965) administered lysergic acid diethylamide
(LSD), mescaline and psilocybin separately and in combi-
nation to participants and measured imaginative suggest-
ibility before and after drug administration. Gibson et al
(1977) measured the effect of benzodiazepine administra-
tion upon hypnotic suggestibility, and Kelly et al (1978)
tested the effect of cannabis intoxication upon the
imaginative suggestibility of participants initially scoring
low to medium on a standardised scale. Details of these
studies and the resulting changes in suggestibility are
given in Table 2. The greatest changes in suggestibility, in
order of decreasing size, are evident after administration of
nitrous oxide, cannabis, LSD, mescaline, combination of
[LSD+mescaline+psilocybin] and diazepam.
Sjoberg and Hollister cautiously interpreted the increases
they observed for LSD, mescaline and psilocybin as a result
of the withdrawal from active reality testing as a result of
drug administration. Despite the advancement of cognitive
models of response to suggestion (e.g. Brown and Oakley
2004; Dienes and Perner 2007), not enough is known about
specific neuroanatomical correlates of such effects to allow
prediction of physiological effects upon suggestion (Ott
2007). The fact that such a wide range of drugs with
varying pharmacological properties have been demonstrat-
ed to affect suggestibility argues for a non-specific effect of
drug upon suggestion: It is plausible that feeling sedated,
dissociated, or in an altered state of consciousness with
reality-testing impaired might lead participants to think they
are less in control of their actions and cognitions, in turn
leading to stronger endorsement of suggested effects
Table 2 Details of previous studies investigating pharmacological manipulation of suggestion
Study Drug Dose Number Suggestibility scale Hypnotic
induction?
No.
items
Suggestibility Suggestibility change
Non-
drug
Drug Absolute Percentage
Sjoberg and Hollister
(1965)
Mescaline 5 mg/kg 24 SSS:A/B
a
No 17 3.33 5.5 2.16 12.7
LSD-25 1.5 mcg/kg 24 SSS:A/B
a
No 17 3.62 5.16 1.54 12.8
Psilocybin 225 mcg/kg 24 SSS:A/B
a
No 17 3.45 3.54 0.08 0.66
Combination 1/3 of each of above 24 SSS:A/B
a
No 17 3.33 4.75 1.41 11.75
Gibson et al (1977) Diazepam 5 mg 34 SHSS:A/B
b
Yes 12 4.44 4.55 0.11 0.91
Placebo (nicotinic
acid)
50 mg 37 SHSS:A/B
b
Yes 12 5.24 5.29 0.05 0.41
Kelly et al (1978) Cannabis Until quite a bit high, average=
834 mg
17 HGSHS
c
No 11 1.69 4.18 2.49 22.6
Control (no drug) NA 18 HGSHS
c
No 11 2.5 2.56 0.06 0.54
Barber et al (1979) Nitrous oxide 2040% 20 Purpose built No 5 1.5 3.3 1.8 36
Nitrous oxide 2040% 20 Purpose built (excluding
analgesia)
No 4 1.3 2.4 1.1 27.5
SSS:A/B Stanford Suggestibility Scale Form A/B, SHSS:A/B Stanford Hypnotic Susceptibility Scale Form A/B, HGSHS Harvard Group Scale of Hypnotic Susceptibility
a
Weitzenhoffer and Sjoberg (1961)
b
Weitzenhoffer and Hilgard (1959)
c
Shor and Orne (1962)
Psychopharmacology
(c.f. Sjoberg and Hollister 1965). Hilgard (1986) describes
the effect of a hypnotic induction (which increases
suggestibility by similar amounts to the drug effects seen
here) as a manipulation which impairs memory and reduces
reality testing, meaning that response to the stimulation
provided by the hypnotist takes precedence over planned or
self-initiated action, resulting in the voice of the hypnotist
becoming unusually persuasive (cf. Gorasini 2004). Future
studies of the effects of drugs upon suggestibility should
formally assess experiences of dissociation, relaxation,
sedation and alterations in consciousness (Pekala and
Kumar 2007).
Numerous studies have demonstrated powerful euphoric
and dysphoric effects of nitrous oxide administration (for a
review, see Walker and Zacny 2005) and it is therefore
worth briefly considering the possibility of a mediating
effect of emotion upon the present increases in imaginative
ability and suggestibility. Holmes and Matthews (2005)
briefly review the sparse literature on imagery and emotion
and note that despite much theory and speculation, there is
little empirical research. They found that instructions to
imagine aversive events led to greater increases in reported
anxiety than instructions to focus on the verbal meaning of
the same descriptions. Whether anxiety or other emotional
states led to increased vividness of imagery is currently
unresolved, but future studies should closely assess the
subjective effects of drug administration upon affect and
look for interactions with imagery vividness.
There is an important caveat to the aforementioned
results. The present study was single blind; although
participants were unaware of whether they were receiving
the drug, the experimenter who delivered the drug also
administered the tests of imaginative ability and suggest-
ibility. The QMI and SHSS:C were scripted and manual-
ised; nevertheless, it is possible that the observed
differences are due to some degree to demand character-
istics and may not solely represent genuine drug effects
(Orne 1962). Such a factor means that the present results
should be considered preliminary and require confirmation
by a double-blind study. The present results raise a number
of other questions for future research. In this study, nitrous
oxide concentration was fixed at 25%, low enough to avoid
causing noticeable side effects, and fixed to avoid having
the experimenter make a necessarily subjective judgment
about how much effect the drug was having (as in Barber et
al. 1979). Since drug concentration in the current study was
fixed at a relatively low 25%, it will be important to
determine whether there is a doseresponse relationship
between drug administration, suggestibility and imaginative
ability. Additionally, we are concerned that the objective
scoring criterion for each suggestibility item used in the
present study may have masked more subtle effects in how
strongly each suggestion was perceived and recommend the
additional use of subjective assessment of response to
suggestion. Similarly, a measure of imaginative ability
using an interval linear scaling might be preferable to the
ordinal responses required by the QMI, and a number of
theoretically derived measures of imagery have recently
been developed (McAvine and Robertson 20062007).
Practically, since the objective of clinicians is to maximise
responsiveness to suggestion, it will be interesting to see
whether nitrous oxide administration plus a hypnotic
induction would increase suggestibility additively. Finally,
since nitrous oxide also has analgesic effects, it will be
interesting to see how these interact with suggestions for
analgesia since many clinicians informally report a syner-
gistic effect between the two (Simons et al. 2007).
Hypnosis in combination with conscious sedation has been
shown to be clinically superior to conscious sedation plus
alternative psychological approaches (Faymonville et al.
1997), and it will be important to test the analgesic
properties of nitrous oxide alone and in combination with
hypnotic suggestion.
Acknowledgements The authors would like to thank David Oakley,
Irving Kirsch, and Celia Morgan for their helpful comments during the
preparation of this manuscript.
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Psychopharmacology
... Multiple early studies that lacked rigorous controls, and numerous clinical observations, suggest that N 2 O augments suggestibility, that suggestion can be used to shape the response to N 2 O, and that N 2 O-induced dissociative states parallel the phenomenological effects of an induction (Dworkin, Schubert, Chen, & Clark, 1986;Parbrook, 1967). Inspired by these preliminary results, at least two controlled studies have shown that N 2 O inhalation augments (non-) hypnotic suggestibility (Barber, Donaldson, Ramras, & Allen, 1979;Whalley & Brooks, 2009). Barber et al. (1979) reported that 20-40% N 2 O inhalation was associated with greater hypnotic suggestibility than placebo (O 2 ) inhalation. ...
... Barber et al. (1979) reported that 20-40% N 2 O inhalation was associated with greater hypnotic suggestibility than placebo (O 2 ) inhalation. A subsequent placebo-controlled study similarly found that 20% N 2 O-inhalation was associated with greater non-hypnotic suggestibility (Whalley & Brooks, 2009). Interestingly, N 2 O inhalation was also associated with increased imagery vividness that correlated with increases in suggestibility, which implies that suggestibility-augmentation is driven by greater imagery vividness or vice versa (see also Terhune & Oakley, 2020). ...
... This study itself lacked a control condition so the precise factors underlying these induced effects are unclear but alongside real-life cases of suggestion producing psychedelic responses (Moore & Ramirez, 1998), it demonstrates that contextual factors are at least partly responsible for psychedelic responses in some individuals. This issue can be addressed by administering lower doses that are difficult to detect (Polito, Stevenson, & Arnone, 2019;Yanakieva et al., 2019) coupled with concurrent measurement of response expectancies (Whalley & Brooks, 2009). ...
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A diverse array of studies has been devoted to understanding the neurochemical systems supporting responsiveness to hypnotic suggestions, with implications for experimental and clinical applications of hypnosis. However, this body of research has only rarely been integrated and critically evaluated and the prospects for the reliable pharmacological manipulation of hypnotic suggestibility remain poorly understood. Here we draw on pharmacological, genotyping, neuroimaging, and electrophysiological research to synthesize current knowledge regarding the potential role of multiple widely-studied neurochemicals in response to suggestion. Although we reveal multiple limitations with this body of evidence, we identify converging results implicating different neurochemical systems in response to hypnotic suggestion. We conclude by assessing the extent to which different results align or diverge and outline multiple avenues for future research. Elucidating the neurochemical systems underlying response to suggestion has the potential to significantly advance our understanding of suggestion.
... The standard scales are also widely used as dependent measures. Nearly all studies that have attempted to modify hypnotic suggestibility using cognitive training (Gorassini, 2004), noninvasive brain stimulation (Coltheart et al., 2018;Dienes & Hutton, 2013), or pharmacological agents (Whalley & Brooks, 2009) have used these scales or abbreviated versions thereof. They have also been used by all studies investigating the heritability, and genetic basis, of hypnotic suggestibility (Morgan, 1973;Rominger et al., 2014) as well as its cognitive (Parris, 2017;Terhune et al., 2017) and personality correlates (Cardeña & Terhune, 2014), as well as numerous theoretical questions, such as the relation between hypnotic and nonhypnotic suggestibility (Kirsch & Braffman, 2001). ...
... They have also been used by all studies investigating the heritability, and genetic basis, of hypnotic suggestibility (Morgan, 1973;Rominger et al., 2014) as well as its cognitive (Parris, 2017;Terhune et al., 2017) and personality correlates (Cardeña & Terhune, 2014), as well as numerous theoretical questions, such as the relation between hypnotic and nonhypnotic suggestibility (Kirsch & Braffman, 2001). Such usage occasionally involves analysis of individual hypnotic suggestions (e.g., Bryant et al., 2001;Whalley & Brooks, 2009), although these scales were not developed to provide item-level measurement. ...
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The most well-established finding gleaned from decades of experimental hypnosis research is that individuals display marked variability in responsiveness to hypnotic suggestions. Insofar as this variability impacts both treatment outcome in therapeutic applications of hypnosis as well as responsiveness to suggestions in experimental contexts, it is imperative that clinicians and researchers use robust measures of hypnotic suggestibility. The current paper critically evaluates contemporary measures of hypnotic suggestibility. After reviewing the most widely used measures, we identify multiple properties of these instruments that result in the loss of valuable information, including binary scoring and single-trial sampling, and hinder their utility, such as the inclusion of suboptimal suggestion content. The scales are not well-suited for contemporary research questions and have outlived their usefulness. We conclude by outlining ways in which the measurement of hypnotic suggestibility can be advanced.
... The standard scales are also widely used as dependent measures. Nearly all studies that have attempted to modify hypnotic suggestibility using cognitive training (Gorassini, 2004), noninvasive brain stimulation (Coltheart et al., 2018;Dienes & Hutton, 2013), or pharmacological agents (Whalley & Brooks, 2009) have used these scales or abbreviated versions thereof. They have also been used by all studies investigating the heritability, and genetic basis, of hypnotic suggestibility (Morgan, 1973;Rominger et al., 2014) as well as its cognitive (Parris, 2017;Terhune et al., 2017) and personality correlates (Cardeña & Terhune, 2014), as well as numerous theoretical questions, such as the relation between hypnotic and nonhypnotic suggestibility (Kirsch & Braffman, 2001). ...
... They have also been used by all studies investigating the heritability, and genetic basis, of hypnotic suggestibility (Morgan, 1973;Rominger et al., 2014) as well as its cognitive (Parris, 2017;Terhune et al., 2017) and personality correlates (Cardeña & Terhune, 2014), as well as numerous theoretical questions, such as the relation between hypnotic and nonhypnotic suggestibility (Kirsch & Braffman, 2001). Such usage occasionally involves analysis of individual hypnotic suggestions (e.g., Bryant et al., 2001;Whalley & Brooks, 2009), although these scales were not developed to provide item-level measurement. ...
Preprint
The most well-established finding gleaned from decades of experimental hypnosis research is that individuals display marked variability in responsiveness to hypnotic suggestions. Insofar as this variability impacts both treatment outcome in therapeutic applications of hypnosis as well as responsiveness to suggestions in experimental contexts, it is imperative that clinicians and researchers use robust measures of hypnotic suggestibility. The current paper critically evaluates contemporary measures of hypnotic suggestibility. After reviewing the most widely used measures, we identify multiple measurement properties of these instruments that result in the loss of valuable information, including binary scoring and single-trial sampling, and hinder their utility, such as the inclusion of sub-optimal suggestion content. The scales are not well-suited for contemporary research questions and have outlived their usefulness. We conclude by outlining ways in which the measurement of hypnotic suggestibility can be advanced.
... Lynn and colleagues argued that inherent neurocognitive differences between "naturally" high-hypnotizable and low-hypnotizable individuals might explain the limits of a behavioral intervention in modifying hypnotizability 13 . Several clinical trials attempted modifying hypnotic responsiveness using psychoactive drugs and other chemical substances, including Lysergic Acid Diethylamide (LSD-25), mescaline, psilocybin 14 , diazepam 15 , nitrous oxide 16 , oxytocin 17 , and modafinil 18 . ...
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Hypnotizability, one’s ability to experience cognitive, emotional, behavioral, and physical changes in response to suggestions in the context of hypnosis, is a highly stable trait associated with increased functional connectivity between the left dorsolateral prefrontal cortex (L-DLPFC) and dorsal anterior cingulate cortex (dACC). We conducted a preregistered, triple-blinded, randomized controlled trial to test the ability of continuous theta-burst stimulation (cTBS) over a personalized neuroimaging-based L-DLPFC target to temporarily enhance hypnotizability. We tested our hypothesis in 78 patients with fibromyalgia syndrome (FMS), a functional pain disorder for which hypnosis has consistently been shown to be beneficial as a nonpharmacological treatment option. Pre-to-post cTBS change in Hypnotic Induction Profile scores (HIP; a standardized measure of hypnotizability) was significantly greater in the Active versus Sham group. Our findings suggest a causal relationship between L-DLPFC and dACC function and hypnotizability. Dose-response optimization should be further examined to formalize guidelines for future clinical utilization. Trial registration ClinicalTrials.gov NCT02969707
... Examples include ketamine (NMDA antagonist) and salvinorin A (a κ-opioid receptor agonist derived from the plant salvia divinorum), which have been found to interfere with true memory performance when administered before encoding (MacLean et al., 2013;Morgan and Curran, 2006). Similarly, the dissociative anaesthetic and NMDA antagonist nitrous oxide, better known as laughing gas, has been found to cause anterograde amnesia and heightened imaginative suggestibility during acute intoxication (Dwyer et al., 1992;Whalley and Brooks, 2009). Moreover, a study with the dissociative dextromethorphan (NMDA antagonist) and psilocybin (psychoactive ingredient of 'magic mushrooms/truffles', serotonin 2A receptor or 5-HT 2A agonist) indicated impairments of true memory (verbal recall) in response to both drugs (given before encoding and retrieval shortly after; Barrett et al., 2018). ...
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Alcohol and other psychoactive drugs are oftentimes implicated in legal cases. A pertinent question herein is whether such substances might adversely affect testimonies of victims, eyewitnesses, or suspects by propelling the formation of false memory and increasing susceptibility to suggestion. In the current review, we amassed all available evidence on the effects of intoxication on false memory formation and suggestibility, including the substances alcohol, cannabis, stimulants, hallucinogens, antipsychotics, benzodiazepines and anticholinergics. Our review indicated that alcohol and cannabis under certain conditions increased the susceptibility to false memories and/or suggestion with effect sizes ranging from medium to large. When intoxicated during an event, alcohol is most likely to increase this susceptibility at high intoxication levels or after a delay, whereas cannabis exerts detrimental effects during acute intoxication but not necessarily once sober. For other substances, ecologically valid research separating different memory phases is needed. Overall, differences between substances regarding false memory effects exist, suggesting that a nuanced approach is needed when dealing with intoxicated individuals in a legal context.
... Expectancy of outcome has also been offered as an explanation for the improvement seen in clinical responsiveness to cognitive behavioural therapies when these are accompanied by hypnotic procedures (Kirsch et al., 1995). There is also some evidence that the hypnotic induction procedure has a small but significant effect in increasing responsiveness to direct verbal suggestions when the response is measured in terms of brain activity (Derbyshire et al., 2009;Derbyshire, Whalley, Seah, & Oakley, 2017) Also in non-clinial samples nitrous oxide, commonly used for sedation in dentistry and often combined with suggestions of relaxation and analgesia, increases an individual's direct verbal suggestibility, as measured without a hypnotic induction using the Stanford Scale (Whalley & Brooks, 2009). Barnier and McConkey (2004) in their review identify 13 'hypnotizability scales' devised since the 1950 ′ s that share the common feature of measuring direct verbal suggestibility. ...
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Hypnotic suggestibility is part of the wider psychological trait of direct verbal suggestibility (DVS). Historically, DVS in hypnosis has informed theories of consciousness and of conversion disorder. More recently it has served as a research tool in cognitive science and in cognitive neuroscience in particular. Here we consider DVS as a general trait, its relation to other psychological characteristics and abilities, and to the origin and treatment of clinical conditions. We then outline the distribution of DVS in the population, its measurement, relationship to other forms of suggestibility, placebo responsiveness, personal characteristics, gender, neurological processes and other factors, such as expectancy. There is currently no scale specifically designed to measure DVS outside a hypnotic context. The most commonly used and well-researched of the hypnosis-based scales, the Harvard Group Scale, is described and identified as a basis for a more broadly based measure of DVS for use in psychological research.
... Most impressively, in Wickless and Kirsch et al. (1989), after the expectancy manipulation, 73% of participants scored as "high hypnotizable" and none as "low hypnotizable." Moreover, non-hypnotic and hypnotic suggestibility can be enhanced through pharmacological agents (Whalley & Brooks, 2009) and non-invasive brain stimulation (e.g., Dienes & Hutton, 2013), independently of changes in response expectancies thereby implicating executive functioning or metacognition. ...
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We present 21 prominent myths and misconceptions about hypnosis in order to promulgate accurate information and to highlight questions for future research. We argue that these myths and misconceptions have (a) fostered a skewed and stereotyped view of hypnosis among the lay public, (b) discouraged participant involvement in potentially helpful hypnotic interventions, and (c) impeded the exploration and application of hypnosis in scientific and practitioner communities. Myths reviewed span the view that hypnosis produces a trance or special state of consciousness and allied myths on topics related to hypnotic interventions; hypnotic responsiveness and the modification of hypnotic suggestibility; inducing hypnosis; and hypnosis and memory, awareness, and the experience of nonvolition. By demarcating myth from mystery and fact from fiction, and by highlighting what is known as well as what remains to be discovered, the science and practice of hypnosis can be advanced and grounded on a firmer empirical footing. This article is protected by copyright. All rights reserved.
... It has been found, for instance, that LSD can increase suggestibility and mental imagery (Carhart-Harris et al., 2015). Similarly, inhaling nitrous oxide appears to enhance suggestibility (Whalley & Brooks, 2009). Interestingly, a high dosage of psilocybin resulted in a long-lasting increase (more than one year later) in the personality trait of openness to experience (MacLean, Johnson, & Griffiths, 2011)-a trait that is consistently related to absorption (Glisky et al., 1991). ...
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Some people seem to have a 'talent' for spiritual experience: they readily sense the presence of supernatural beings, receive special messages from God, and report intense feelings of self-transcendence, awe and wonder. Here we review converging strands of evidence to argue that the trait of 'absorption' captures a general proclivity for having spiritual experiences. Participants scoring highly on the Tellegen Absorption Scale report vivid experiences of hearing God's voice during prayer, intense mystical experiences in response to psychedelics or placebo brain-stimulation , and strong feelings of presence and transcendence when confronted with natural beauty, virtual reality, or music. Several mechanisms may help to explain the relationship between absorption and spiritual experience. We suggest that absorption captures an experiential mindset that intensifies inner and outer sensory experience in ways that reflect both prior expectation and novel sensory engagement. It seems to enable that which must be imagined to feel more real.
... Overall, those having low expectancies of responding successfully to hypnosis have been found to be more accurate prognosticators than those with higher expectancies (Spanos, Burnley, & Cross, 1993). Moreover, expectancies have not accounted for the efficacy of other methods that have successfully augmented hypnotizability and nonhypnotic suggestibility, such as transcranial magnetic stimulation (Dienes & Hutton, 2013) and nitrous oxide (Whalley & Brooks, 2009). Cumulatively, these studies suggest that, although response expectancies can be manipulated, such changes do not reliably translate to increases in responsiveness to suggestion. ...
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This study evaluated factors underlying individual differences in spontaneous (unsuggested) experiences during hypnosis. Participants varying in hypnotizability (low, medium, and high) completed a questionnaire about various dimensions of consciousness they would expect to experience at the "deepest level of hypnosis" (expec-tancy), an eyes-closed resting condition (baseline), and their actual experiences during "neutral hypnosis" (hypnosis). Responses during hypnosis were characterized by higher scores in dimensions related to alterations in conscious experience, affect, and imagery, and lower scores in rationality and agency. Only highs and mediums evinced increases in altered experience and body image. Across conditions, highs reported greater alterations in time experience and lower self-awareness than other groups. Participants overall tended to overestimate the changes they would experience in hypnosis. Baseline and hypnosis correlated in various dimensions, including affect, arousal, and internal dialogue. After controlling for baseline scores and hypno-tizability, expectancies correlated with some dimensions having to do with alterations in consciousness. In sum, spontaneous experiences during hypnosis are driven by response expectancies, hypnotizability, and baseline experiences, which show differential effects. A discussion by White (1941) on the importance of individuals' expectations on their responses to hypnosis was the impetus for important research and theoretical developments (e.g., Kirsch, 1991; Shor, 1971). In recent decades, as part of a broader line of research on the role of expectations in human behavior and experience (Kirsch, 1999), various studies have investigated whether
Chapter
Nitrous oxide (N2O) is an invaluable tool for managing the mild to moderately anxious child. The American academy of pediatric dentistry, among other organizations, recognizes N2O/oxygen inhalation sedation as a safe and effective technique to reduce anxiety, produce analgesia, and enhance effective communication between a patient and healthcare provider. N2O serves as an adjunct to behavior management. N2O is widely accepted as a behavior management technique in pediatric dentistry. Most discussions concerning N2O stressed the analgesic and anesthetic properties for extractions. Dental offices remained dependent upon N2O for pain control until the introduction of local anesthesia. N2O has multiple mechanisms of action that underlie its varied pharmacological properties. N2O has multiple mechanisms of action that underlie its varied pharmacological properties. The most common, though infrequent, complication found to occur with the administration of N2O to children is vomiting.
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Examined the interrelationships between the following measures: hypnotic susceptibility (Stanford Hypnotic Susceptibility Scale, Forms A and C), imagery vividness (Vividness of Visual Imagery Questionnaire; VVIQ), involvement in everyday imaginative activities (Tellegen Absorption Scale; TAS), and daydreaming styles (28 scales of the Imaginal Processes Inventory). Ss were 56 university students and staff members. Factor analysis produced a factor characterized as a positively vivid and absorptive imagination style. Hypnotic susceptibility, VVIQ, TAS, and positive-affect daydreaming styles all loaded on this factor. Two other factors were a dysphoric daydreaming style and a lack-of-attentional-control style. Stepwise multiple regressions suggested that males and females exhibit different relationships between hypnotic susceptibility and predictor variables. Similar differences were found for the VVIQ and the TAS and their daydreaming-scale predictor variables. (39 ref) (PsycINFO Database Record (c) 2006 APA, all rights reserved).
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This reprinted article originally appeared in American Psychologist, November 1962, Vol. 17 (No. 11), 776-783. (The following abstract of the original article appeared in record 1963-05917-001.) Taking part in an experiment is "a special form of social interaction." The S plays a role and places himself under the control of the E; he may agree "to tolerate a considerable degree of discomfort, boredom, or actual pain, if required to do so." The very high degree of control inherent in the experimental situation itself may lead to difficulties in experimental design. The S "must be recognized as an active participant in any experiment." With understanding of factors intrinsic to experimental context, experimental method in psychology may become a more effective tool in predicting behavior in nonexperimental contexts.
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Nitrous oxide (N2O) is a gas at room temperature and pressure. It is used primarily for anesthesia but is also used as a propellant for whipped cream or to boost octane levels in racing cars. N2O was extensively studied by Sir Humphrey Davy, who presented a detailed description of his subjective experiences under the influence of N2O, as well as self-reports by friends and colleagues of their own experiences while inhaling the gas. Sir Davy's treatise was a thorough, systematic, and extensive characterization of N2O and was a foreshadowing of two centuries of research to follow. Sir Davy's and subsequent research consisted of the dose-response assessment of subjective effects of acute and repeated N2O administration, examination of individual differences, and the study of environmental and organismic determinants/modulators of N2O effects. This chapter presents the results of such experiments in a chronological framework and attempts to detail the various characterizations of N2O across the years since its discovery.
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Reliabilities were sought for the two most frequently used imagery assessment devices, the Vividness of Imagery Scale of Betts Questionnaire Upon Mental Imagery and Gordon Test of Visual Imagery Control. Reliability was assessed by means of test-retest and Cronbach's Coefficient Alpha using 147 subjects. Betts scale test-retest reliabilities ranged from .72 to .75, and alphas ranged from .90 to .94. Gordon scale test-retest reliabilities ranged from .81 to .86 and alphas from .53 to .74. Testing implications are discussed.
Article
Attempts to measure the ability to experience hypnosis first began within the context of 19th century clinical practice (Perry & Laurence, 1980). Braid (1855/1970), for example, proposed a tripartite classification in terms of three "stages" or degrees of hypnotic depth, which he labelled as "slight hypnosis", "deep hypnosis" and "hypnotic coma". The first of these referred to what is experienced by approximately 90% of the population; at that time, given the close parallel drawn between hypnosis and nocturnal sleep a degree of lethargy was most commonly reported, as opposed to the more frequent current experience of relaxation. At the same time, the person in "slight hypnosis", then as now, had no subsequent loss of memory. The subsequent two "stages" involved different forms of post hypnotic amnesia: "deep hypnosis" involved being able to remember the events of a particular hypnotic session only in a subsequent one, while "hypnotic coma" represented a permanent amnesia for what was experienced in a particular hypnosis session. These two amnesias are rarely, if ever, seen today, mainly because current investigators see posthypnotic amnesia as a suggested phenomenon, rather than as "spontaneous". By contrast, the amnesias of a century and a half ago appear to have been the products of shared implicit views held by both hypnotic practitioners and hypnotized patients about the manner in which hypnotic response manifests itself; the belief of both patients and therapists that "spontaneous" amnesia was to be expected most likely led to its occurrence in those patients possessing the abilities of the highly hypnotizable. Viewed in this manner, such amnesias offer striking instances of how the demand characteristics (Orne, 1959) of history markedly influenced the behavior observed in hypnosis at these earlier times. A review of the literature reveals that many clinicians subsequent to Braid attempted to further categorize degrees of responsivity to hypnosis, with Bernheim (1889) proposing nine such categories, each defined in terms of behavioral responses that were thought to have subjective accompaniments. Interestingly, Bernheim's taxonomy of hypnotic responsivity bears a close resemblance to what is found today utilizing one of the currently available standardized measuring instruments of hypnotic ability (see Perry & Laurence, 1980, for a review of this literature).