Attention control and susceptibility to hypnosis
Cristina Iania,*, Federico Riccia, Giulia Baronib, Sandro Rubichia
aDipartimento di Scienze Sociali, Cognitive e Quantitative, Università di Modena e Reggio Emilia, Italy
bDipartimento di Discipline della Comunicazione, Università di Bologna, Italy
a r t i c l e i n f o
Received 12 September 2008
Available online 3 August 2009
a b s t r a c t
The present work aimed at assessing whether the interference exerted by task-irrelevant
spatial information is comparable in high- and low-susceptible individuals and whether
it may be eliminated by means of a specific posthypnotic suggestion. To this purpose high-
and low-susceptible participants were tested using a Simon-like interference task after the
administration of a suggestion aimed at preventing the processing of the irrelevant spatial
information conveyed by the stimuli. The suggestion could be administered either in the
absence or following a standard hypnotic induction. We showed that, outside from the
hypnotic context, the Simon effect was similar in high and low-susceptible participants
and it was significantly reduced following the posthypnotic suggestion in high-susceptible
participants only. These results show that a specific posthypnotic suggestion can alter
information processing in high-susceptible individuals and reduce the interfering effect
exerted by arrow stimuli.
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At any given moment we are confronted with a multitude of environmental inputs, many of which are irrelevant to cur-
rent behavior. Even though we are normally able to select and limit processing to relevant information – an ability known as
attentional control – there are some situations in which irrelevant information cannot be disregarded and influences our
behavior independently from our will – a phenomenon known as cognitive conflict.
In laboratory settings, two tasks widely used to study the way we deal with irrelevant information are the Stroop (Stroop,
1935; for a review see MacLeod, 1991) and the flanker tasks (Eriksen & Eriksen, 1974). In the Stroop task, participants are
asked to name the ink color of written color words (e.g., the word RED in green ink). To emit the correct response, partici-
pants need to select the relevant information (the ink color) and ignore the irrelevant meaning of the word. However, results
consistently show that they are unable to do so, being slower and less accurate in naming the ink color of incongruent color
words (e.g. BLUE in red ink) than in naming the ink color of congruent color words (e.g. BLUE in blue ink). The most common
explanation of the effect is that, since reading is automatic, the irrelevant words are read and, as a consequence, their mean-
ing interferes with color naming.
In the flanker task, introduced by Eriksen and Eriksen (1974), a central target stimulus is presented simultaneously with
two distractor stimuli (flankers) that can have the same identity as the target or a different identity and participants are in-
structed to respond accordingly to the target’s identity by pressing one of two keys. To make the correct response, partici-
pants need to select the relevant information and inhibit the surrounding irrelevant information (the flankers). Reaction
times are typically faster when the flanker letters are congruent with the response assigned to the target letter (e.g. TTT)
1053-8100/$ - see front matter ? 2009 Elsevier Inc. All rights reserved.
* Corresponding author. Address: Dipartimento di Scienze Sociali, Cognitive e Quantitative, Università di Modena e Reggio Emilia, Via Allegri 9, 42121
Reggio Emilia, Italy. Fax: +39 0522 532 205.
E-mail address: email@example.com (C. Iani).
Consciousness and Cognition 18 (2009) 856–863
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and slower when incongruent (e.g. FTF), thus suggesting that the irrelevant stimuli are not fully excluded from processing.
This effect has been interpreted as a failure of focused attention: because of spatial proximity and similarity between target
and flankers, attention cannot prevent irrelevant information to access response selection processes.
Interestingly, in high-susceptible individuals – that is individuals who show a high responsivity to hypnosis, as measured
by means of standardized scales (e.g., Shore & Orne, 1962; Weitzenhoffer & Hilgard, 1959) – both Stroop and flanker inter-
ference can be minimized by means of specific suggestions administered during hypnosis (from now on, posthypnotic sug-
gestions). Specifically, in the case of the Stroop task, there are investigations showing that in high-susceptible individuals the
interference exerted by the meaning of the color word can be reduced or even eliminated either by providing attentional
focusing instructions (e.g., Nordby, Hugdahl, Jasiukaitis, & Spiegel, 1999; Sheehan, Donovan, & MacLeod, 1988) or by provid-
ing specific posthypnotic suggestions to avoid attributing meaning to the color word (Raz, Fan, & Posner, 2005; Raz, Kirsch,
Pollard, & Nitkin-Kaner, 2006; Raz, Moreno-Íñiguez, Martin, & Zhu, 2007; Raz, Shapiro, Fan, & Posner, 2002; Raz et al., 2003).
In the case of the flanker task, interference can be reduced by means of a posthypnotic suggestion influencing attentional
focusing (Iani, Ricci, Gherri, & Rubichi, 2006).
Taken together these results suggest that in high-susceptible individuals attentional control abilities may be enhanced by
means of specific posthypnotic suggestions. To note, the idea of a connection between attentional abilities and hypnosis is
not new. Indeed, it was first developed in the neuropsychological model of hypnosis proposed by Gruzelier (1988, 1998; see
also Crawford, 1994; Crawford & Gruzelier, 1992). According to this model, the process of hypnotic induction consists of
three stages characterized by different patterns of brain activation and inhibition and by a different attentional involvement.
In the initial stage, attention has to be focused on the hypnotist’s voice and distracting information has to be filtered out, a
process accompanied by the activation of frontal areas. In the second and third stages, the hypnotic state is reached and
attentional functioning is decreased due to inhibition of frontal areas and activation of posterior areas. From this model de-
rive two main predictions. First, high-susceptible subjects should show better baseline attentional functioning compared to
low-susceptible subjects. Second, their attentional functioning should be impaired after hypnotic induction.
While the second prediction has been widely confirmed, more debated is whether, outside from the hypnotic context,
attentional functioning is more efficient in high-susceptible individuals than in low-susceptible ones. Even though several
attentional tasks have been used, we will focus our discussion only on those studies employing the Stroop and flanker par-
adigms, which specifically test attention control abilities.
As concerns performance after a generic hypnotic induction, there are several studies reporting impaired performance on
the Stroop task for high-susceptible individuals (e.g., Blum & Graef, 1971; Egner, Jamieson, & Gruzelier, 2005; Jamieson &
Sheehan, 2004; Kaiser, Barker, Haenschel, Baldeweg, & Gruzelier, 1997; Nordby et al., 1999; Sheehan et al., 1988). Neuroim-
aging investigations have helped to shed some light on the cause of this impairment, showing that in high-susceptible indi-
viduals hypnosis disconnects the two main areas involved in handling the conflict, the anterior cingulate cortex (ACC), and
the lateral prefrontal cortex (LPFC). There are indications that the ACC serves as a conflict detection mechanism, while the
LPFC is thought to implement cognitive control, by sending the appropriate signals to posterior areas once it receives inputs
from the ACC (e.g., Stürmer, Redlich, Irlbacher, & Brandt, 2007). By assessing activity levels in the ACC and LPFC areas while
high- and low-susceptible participants performed the Stroop task, Egner et al. (2005) found that after hypnotic induction
neural activity in the ACC was higher for high-susceptible participants compared to both baseline and low-susceptible par-
ticipants. Surprisingly, this higher activity was not accompanied by increased activity in the LPFC.
As for attention functioning outside from hypnosis, the results are controversial. We are aware of only two studies
employing the flanker task and they both report no difference between high- and low-susceptible individuals (e.g., Castel-
lani, D’Alessandro, & Sebastiani, 2007; Iani et al., 2006). The picture emerging when the Stroop task is considered is much
more complex. Indeed, while Egner et al. (2005) found no difference between high- and low-susceptible subjects, there
are some reports of larger Stroop interference in high-susceptible individuals (e.g., Dixon, Brunet, & Laurence, 1990; Dixon
& Laurence, 1992). In contrast to these results, using a slightly different paradigm and measuring accuracy instead of re-
sponse times, Rubichi, Ricci, Padovani, and Scaglietti (2005) found a smaller Stroop interference effect in high-susceptible
than in low-susceptible individuals and a negative correlation between amount of interference and level of susceptibility.
Interestingly, recent studies assessing the relation between susceptibility and genotype found that individuals carrying
the valine/methionine Catechol-O-Methyltransferase (COMT) genotype scored higher on standardized hypnotic susceptibil-
ity scales compared to individuals carrying the valine/valine genotype (e.g., Lichtenberg, Bachner-Melman, Ebstein, & Craw-
ford, 2004; Lichtenberg, Bachner-Melman, Gritsenko, & Ebstein, 2000; Raz, Fan, & Posner, 2006) and to individuals carrying
the methionine/methionine genotype (Raz et al., 2006). This clear-cut pattern of differences was, however, not completely
replicated in a study assessing the relation between COMT genotypes and Stroop performance (e.g., Sommer, Fossella, Fan, &
These contrasting results can be taken as an indication that the relation between susceptibility and attention may be
subtle and that, as acknowledged by Rubichi et al. (2005), more sensitive tasks may be necessary to detect differences be-
tween high- and low-susceptible individuals. Another task that can be used for exploring differences in the ability of par-
ticipants to ignore irrelevant information and handle conflicts between competing task dimensions is the Simon task
(Simon & Rudell, 1967; for a review, see Proctor & Vu, 2006). In this paradigm participants are required to respond to a
non-spatial stimulus feature (e.g., stimulus color or shape) by pressing a spatially-defined response (e.g., a left or right
response key). Even though stimulus location is completely irrelevant for performing the task, responses are faster and
more accurate when stimulus and response position correspond (corresponding trials), compared to when they do not
C. Iani et al./Consciousness and Cognition 18 (2009) 856–863
correspond (non-corresponding trials). The difference in performance between corresponding and non-corresponding
responses, known as Simon effect, is attributed to the automatic pre-activation of the response that spatially corresponds
to stimulus location (De Jong, Liang, & Lauber, 1994). In corresponding trials this automatically activated response is the
same as the one indicated by the relevant stimulus feature, therefore no competition between response codes arises. In
non-corresponding trials, on the contrary, the automatically activated response and the response activated on the basis
of the relevant stimulus feature are different and the incorrect response needs to be aborted thus causing a slowing of
response time and an increased number of errors.
Although both the Stroop and Simon tasks measure attention control, the source of the conflict in the two tasks is sup-
posed to be different (cf. Egner, Delano, & Hirsch, 2007). To explain this point, we refer to the dimensional overlap model
proposed by Kornblum, Hasbroucq, and Osman (1990; see also Kornblum, 1992) which allows to classify conflict tasks
according to the degree of overlap, that is similarity, between stimulus and response dimensions. According to Kornblum
et al.’s (1990) taxonomy, in the Simon task conflict arises because of the overlap between the irrelevant stimulus dimension
and the response. Differently, in the Stroop task, conflict arises because the relevant and irrelevant stimulus dimensions
overlap with the response and with each other.
The Simon effect is rather robust and is evident also with centrally-presented stimuli which convey spatial information
through meaning, such as pointing arrows or spatial words (e.g., Masaki, Takasawa, & Yamazaki, 2000; Pellicano, Lugli,
Baroni, & Nicoletti, 2009; Ricciardelli, Bonfiglioli, Iani, Rubichi, & Nicoletti, 2007). In this case, the interference arises between
the response automatically activated by the (irrelevant) directional meaning of the arrow or word and the response activated
on the basis of the relevant stimulus dimension (i.e. the color). To the best of our knowledge, the Simon task has never been
used to assess differences in attentional control between high- and low-susceptible individuals but, based on the above con-
siderations, it may represent a sensitive tool.
The aim of the present study was twofold. First, we were interested in assessing whether outside of the hypnotic context,
the Simon effect is comparable between high- and low-susceptible individuals. Second, we were interested in assessing
whether in high-susceptible participants the conflict at the basis of the Simon effect may be eliminated by means of a specific
Participants were required to perform a Simon-like task with centrally-presented arrows after the administration of a
suggestion aimed at preventing attributing spatial meaning to the stimuli. Since it is plausible to assume that a symbolic
directional stimulus can influence performance only after its shape and meaning have been assessed, the aim of the sugges-
tion was to block the processing of the irrelevant spatial information conveyed by the arrows. The suggestion could be
administered either following a standard hypnotic induction (posthypnotic suggestion condition) or in the absence of an
hypnotic induction (non-hypnotic suggestion condition). We used centrally-presented arrows to rule out alternative inter-
pretations of the results. Indeed, investigations of the time course of the Simon effect showed that, in the case of lateralized
stimuli, the effect is present with faster response times but decreases with slower responses (e.g., Simon, Acosta, Medwaldt,
& Speidel, 1976), either because the task-irrelevant spatial code spontaneously decays with time or is actively inhibited (e.g.,
De Jong et al., 1994; Hommel, 1994; Rubichi, Nicoletti, Iani, & Umiltà, 1997). Since responses tend to be slower after hypnotic
induction, it is not possible to disentangle whether the absence of the effect following an hypnotic suggestion is due to the
specific suggestion or rather is a consequence of the fact that responses are selected and emitted when the code generated on
the basis of the irrelevant spatial information is no longer available. Such a confound is not present for centrally-presented
arrows. It has indeed been shown that the response code generated on the basis of the spatial information conveyed by ar-
rows does not decrease with increasing response times, but either remains constant (Pellicano et al., 2009) or slightly in-
creases (Ricciardelli et al., 2007). Hence, the disappearance of the interference effect following a posthypnotic suggestion
can be safely attributed to the suggestion itself and not to response slowing. To further verify that the differences in the size
of the Simon effect evident between conditions and groups were not due to differences in response times, we investigated
the changes over time in the effect magnitude using the distributional analysis technique (e.g., De Jong et al., 1994; Ratcliff,
1979; Rubichi et al., 1997).
Twenty-eight undergraduate students from the University of Modena and Reggio Emilia (age range 19–38 years, 10 males
and 18 females) participated in the study. They were selected from a larger group of volunteers on the basis of their hypnotic
susceptibility. Hypnotic susceptibility was determined through pre-screening in an hypnotic context using the Italian ver-
sion of the Harvard Group Scale of Hypnotic Susceptibility, Form A (herein, HGSHS-A; Shore & Orne, 1962, 1976) and then
individually using the Italian version of the Stanford Hypnotic Susceptibility Scale, Form C (herein, SHSS-C, Weitzenhoffer &
Hilgard, 1959, 1962) without the taste hallucination, age regression and anosmia to ammonia items. To be included in the
study, participants had to score either in the higher susceptible range (HGSHS-A and SHSS-C P8) or in the lower susceptible
range (HGSHS-A and SHSS-C 63). Fourteen participants (3 males and 11 females) scored in the higher susceptible range
(scores 10–12 out of 12 on the HGSHS-A; 8–9 out of 9 on the SHSS-C) and fourteen (8 males and 6 females) in the less sus-
ceptibility range (0–3 out of 12 on the HGSHS-A and 0–3 out of 9 on the SHSS-C).
C. Iani et al./Consciousness and Cognition 18 (2009) 856–863
All participants were right-handed, reported normal or corrected-to-normal vision and received extra credit for their par-
ticipation in the study.
2.2. Materials and apparatus
Stimuli appeared on a computer monitor and consisted of two filled arrows (i.e. blue or green) pointing both either to the
subject’s left or to the right. The overall stimulus configuration was presented in the center of the display and measured
12? ? 5?. Viewing distance was 60 cm.
Participants were to respond to the color of the arrows, by pressing either the ‘‘L” or ‘‘D” button on the computer key-
board. Half of the participants responded to the blue color with the left key and to the green color with the right key, while
the remaining half experienced the opposite mapping rule. The direction indicated by the stimuli, which was irrelevant for
performing the task, could be corresponding or non-corresponding with the response key.
2.3. Design and procedure
At the beginning of the experimental session, participants received an explanation of the procedures. They were told that
the purpose of the experiment was to investigate the effect of hypnosis on cognitive performance and that, to this aim, they
would be asked to perform a task on the computer with the experimenter present in the room. They were also informed that
hypnotic suggestions would be administered during the experiment. All procedures were in accordance with the ethical code
of the Italian Psychological Association and the Declaration of Human Rights, Helsinki 1975 and written informed consent
was obtained from all participants prior to the beginning of the experimental session.
Participants performed the Simon task after having undergone an hypnotic induction and with no hypnotic induction.
One-half of the participants were first run on the Simon task following the hypnotic induction and then when naturally alert,
while the remaining participants experienced these two conditions in the reverse order.
In the posthypnotic suggestion condition, before performing the task and after a standard hypnotic induction (Weitzenh-
offer & Hilgard, 1959, 1962), the following suggestion (translated here from Italian) was verbally presented to all
Soon you will be in front of the computer screen, ready to play a computer game. Your gaze will be captured, like a mag-
net, by the computer screen. Your attention will be completely absorbed by the cross appearing at the center of the
screen. At a certain point two strange identical patches of color will appear on the screen. These two patches of color
are so strange and have an unknown shape that you will pay attention only to their color. Your attention will be com-
pletely absorbed by their color. Any other information will appear as irrelevant and meaningless. You will be able to
attend to the color of the patches only and to respond to it in an easy and automatic way. Each time you see a green
splash, your right index finger will press the computer key in a fast and automatic way. Each time you see a blue splash,
your left index finger will press the computer key in a fast and automatic way. Your index fingers will respond to the color
of the splashes that will completely capture your attention. Nothing will disturb you, and you will be able to play this
game easily and effortlessly.
Participants were brought back to the wake state before performing the task. Under the non-hypnotic suggestion condi-
tion, the suggestion was not preceded by an hypnotic induction.
Each trial began with the exposure of a central fixation cross (0.8? ? 0.8?). After 450 ms the fixation cross disappeared and
was followed after 50 ms by the imperative stimuli, which remained on the screen for 1500 ms, or until a response was
made. The inter-stimulus interval was 1 s. No feedback was provided.
There were 140 trials in each condition (non-hypnotic suggestion vs. posthypnotic suggestion), with corresponding and
non-corresponding trials being equiprobable.
Mean correct RTs and error percentages as a function of condition (non-hypnotic suggestion and posthypnotic sugges-
tion) and correspondence between arrows’ direction and response for the two susceptibility groups are presented in Table
For both experimental conditions, the first 20 trials were considered as practice and discarded from the analyses. Incor-
rect responses were excluded from the RT analysis, as were responses faster than 200 ms and slower than 1500 ms. Deviant
RTs were less than 0.1% of the total trials.
3.1. RT data
In order to evaluate the time course of the Simon effect, mean RTs were computed from the first to the fifth bin of
the individual rank-ordered raw data separately for each group (high- vs. low-susceptible groups), condition (posthypnotic
C. Iani et al./Consciousness and Cognition 18 (2009) 856–863
suggestion and non-hypnotic suggestion) and correspondence between arrows direction and response location (e.g., De Jong
et al., 1994; Rubichi et al., 1997). Since the order of presentation of the two experimental conditions did not reach statistical
significance, data were accordingly collapsed and entered into a repeated-measures ANOVA with susceptibility group as be-
tween-subjects factor, and condition, correspondence and bin as within-subject factors. When necessary, comparisons were
performed using Bonferroni’s test for multiple comparisons.
The analysis revealed a significant main effect of correspondence, F(1, 26) = 33.99, p < .001, MSe = 3188, with correspond-
ing responses being 28 ms faster than non-corresponding responses (458 vs. 486 ms). The interaction between correspon-
dence and condition was significant, F(1, 26) = 6.08, p < . 03, MSe = 894, and was further modulated by susceptibility
group, F(1, 26) = 5.42, p < .03, MSe = 894. Post-hoc comparisons showed that for low-susceptible participants the Simon ef-
fect was significant under both experimental conditions (27 and 26 ms in the non-hypnotic and posthypnotic suggestion
conditions, respectively; ps < .001). Differently, for high-susceptible participants the Simon effect was significant in the
non-hypnotic suggestion condition (41 ms, p < .001) and tended to significance in the posthypnotic suggestion condition
(17 ms, p = .06). A follow-up analysis with the magnitude of the Simon effect evident in the non-hypnotic and posthypnotic
suggestion conditions as dependent variable showed that for high-susceptible participants the 24-ms reduction of the effect
after the administration of the posthypnotic suggestion was significant (p < .02). No other difference reached statistical
We found significant interactions between correspondence and bin, F(4, 104) = 5.94, p < .001, MSe = 345; condition, cor-
respondence and bin, F(4, 104) = 4.70, p < .01, MSe = 232; susceptibility group, condition, correspondence and bin,
F(4, 104) = 4.33, p < . 001, MSe = 232. To further assess the four-way interaction, we ran separate analyses for the two sus-
Mean RT (ms) and error (%) for the two experimental conditions (i.e., non-hypnotic suggestion vs. posthypnotic suggestion). Standard deviations are shown in
parentheses. Asterisks indicate significant differences between corresponding and non-corresponding trials (p < .05). The Simon effect (SE) was calculated by
subtracting RTs on corresponding trials from RTs on non-corresponding trials. (HS = high-susceptible; LS = low-susceptible).
Group ConditionCorrespondingNon-corresponding SE
RT Error RTError
HS (n = 14)Non-hypnotic suggestion
LS (n = 14) Non-hypnotic suggestion
Non-hypnotic suggestion Posthypnotic suggestion
Non-hypnotic suggestionPosthynotic suggestion
Fig. 1. Mean reaction times (RTs) for high-susceptible (HS, upper panel) and low-susceptible (LS, lower panel) participants as a function of correspondence
and bin (C = corresponding trial; NC = non-corresponding trial).
C. Iani et al./Consciousness and Cognition 18 (2009) 856–863
ceptibility groups. The interaction between condition, correspondence and bin was significant only for high-susceptible par-
ticipants, F(4, 52) = 6.07, p < .001, MSe = 336. For this group, the Simon effect showed a different time course in the two sug-
gestion conditions. Post-hoc test indicated that in the non-hypnotic suggestion condition the effect was significant from the
second bin and largest in the last bin (22, 29, 38, 45 and 74 ms, from the first to the fifth bin, respectively), while it never
reached statistical significance in the posthypnotic suggestion condition (17, 19, 22, 18 and 9 ms, from the first to the fifth
bin, respectively), see Fig. 1.
For low-susceptible participants, the condition ? correspondence ? bin interaction did not reach significance (F < 1). The
Simon effect was significant starting from the third bin in both non-hypnotic (14, 19, 27, 37 and 36 ms from the first to the
fifth bin, respectively) and posthypnotic suggestion conditions (17, 20, 25, 30, and 38 ms from the first to the fifth bin,
To further examine the relation between cognitive performance and hypnotic susceptibility, we calculated the Pearson
correlation coefficient between the SHSS-C score and the magnitude of the Simon effect for the two experimental conditions.
No significant correlation was found (non-hypnotic suggestion condition: r = .17, p = .19; posthypnotic suggestion condition:
r = ?.21, p = .14).
3.2. Error data
Since errors were very few, to approach normal distribution the analysis was performed on arc-sin transformed error
rates. Data were entered into a repeated-measures ANOVA with susceptibility group as between-subjects factor, and condi-
tion and correspondence as within-subject factors.
Errors were significantly higher for non-corresponding (4.5%) than for corresponding trials (1.9%), as indicated by the
main effect of correspondence, F(1, 26) = 17.79, p < .001, MSe = .001. No other main effect or interaction reached significance.
The present study was motivated by two questions. The first question was whether outside from an hypnotic context
high- and low-susceptible participants are affected to the same extent by the irrelevant spatial information conveyed by
directional stimuli. Our results showed that, outside from the hypnotic context, irrelevant information negatively affected
performance in a similar way for the two groups. Consistent with previous studies, participants could not ignore the spatial
information conveyed by arrow stimuli. For both groups, the interference effect increased with RTs, thus supporting the idea
that in the case of central directional stimuli spatial information needs to be extracted from the stimulus in order to auto-
matically activate a response (e.g., Ansorge, 2003; Ricciardelli et al., 2007). To note, the numerically larger effect shown by
high-susceptible participants compared to low-susceptible ones (41 vs. 27 ms) could suggest the existence of susceptibility-
related differences in attention control. This difference, however, was far from being significant. Even though these results do
not allow us to reach a conclusion, they may suggest that if high- and low-susceptible individuals differ in attentional control
abilities, these differences are not as marked as predicted on the basis of the neuropsychological model of hypnosis (Craw-
ford, 1994; Crawford & Gruzelier, 1992; Gruzelier 1988, 1998).
The second question was whether in high-susceptible participants the Simon effect evident outside of hypnosis can be
eliminated by means of a suggestion aimed at blocking the attribution of a directional meaning to the stimuli. Our results
indicated that in high-susceptible participants, the Simon effect was significantly reduced after the administration of the
posthypnotic suggestion and this reduction was evident irrespective of response latencies. Importantly, the reduction of
the interference effect was obtained only when the suggestion followed the hypnotic induction and not when it was pro-
vided out of hypnosis. These results are ad odds with those by Raz et al. (2006) who found a significant reduction of Stroop
interference under both hypnotic and non-hypnotic suggestions, and do not support the proposal that susceptibility to sug-
gestion and not the specific hypnotic context is responsible for the reduction of the interference effect. Rather, it seems that
the hypnotic context is necessary for the suggestion to work.
It should be noted that even though following a posthypnotic suggestion the Simon effect for high-susceptible partici-
pants did not reach statistical significance, corresponding responses tended to be in average 17 ms faster than non-corre-
sponding responses and this effect did not significantly differ from the effect evident for low-susceptible participants.
Furthermore, accuracy was not affected by the suggestion, with corresponding responses being more accurate than non-cor-
responding ones. Hence, its seems that our suggestion was not strong enough to completely eliminate the Simon effect. Such
an interpretation is supported by the post-experiment interviews conducted before the hypnotic suggestion was cancelled.
Indeed, most of the high-susceptible participants reported perceiving the central stimuli as arrows. However, they also re-
ported that they did not pay attention to the direction arrows were pointing to but only to their color. Hence, the reduction of
the effect following the hypnotic suggestion was not specifically due to blocked assess to stimulus meaning.
A number of studies have shown that seeing an arrow automatically directs one’s visual attention to the location it des-
ignates (e.g., Hommel, Pratt, Colzato, & Godijn, 2001; Ristic & Kingstone, 2006; Ristic, Wright, & Kingstone, 2007; Tipples,
2002, 2008). This is thought to occur because arrows are overlearned symbols, conventionally used by humans to commu-
nicate spatial information which may be useful for ongoing behavior. These shifts of attention seem to be reflexive and invol-
untary and may be responsible for the occurrence of the Simon effect. Indeed, according to the Premotor Theory of Attention
C. Iani et al./Consciousness and Cognition 18 (2009) 856–863
(Rizzolatti, Riggio, Dascola, & Umiltà, 1987), the orienting of attention in space requires the programming of a motor act, spe-
cifically an eye movement. This program contains spatial parameters and it is supposed to bring to the creation of a spatial
code which, because of its similarity to the spatial representation of the response, determines the automatic activation of the
corresponding response (e.g., Rubichi et al., 1997). This code is thought to increase in strength as more spatial information is
gradually derived from the stimulus, as indicated by the time course of the effect evident in the non-hypnotic condition of
our study and in previous studies (e.g., Pellicano et al., 2009; Ricciardelli et al., 2007). What our data seem to suggest is that
the suggestion may not completely prevent the initial processing of the stimulus meaning but it can prevent the spatial code
from reaching a higher activation level and, as a consequence, from exerting a stronger influence on performance. This con-
clusion is supported by the effect functions evident for high-susceptible participants: while the Simon effect increased with
RTs in the non-hypnotic suggestion condition, it never reached statistical significance in the posthypnotic suggestion
To conclude, the results of the present study contribute to a growing body of evidence showing that specific hypnotic sug-
gestions can modulate information processing and reduce conflict between competing information (e.g., Iani et al., 2006; Raz
et al., 2002, 2003, 2005, 2006, 2007). In addition to previous studies, they provide some insight on which kind of processes
can be ‘‘controlled” by means of a specific suggestion.
We would like to thank Bernhard Hommel and two anonymous reviewers for helpful comments on an earlier version of
this manuscript. We also thank Elena Gherri for helping with programming the experiment and with data collection.
Ansorge, U. (2003). Influences of response-activating stimuli and passage of time on the Simon effect. Psychological Research, 67, 174–183.
Blum, G. S., & Graef, J. R. (1971). The detection over time of subjects simulating hypnosis. International Journal of Clinical and Experimental Hypnosis, 19,
Castellani, E., D’Alessandro, L., & Sebastiani, L. (2007). Hypnotizability and spatial attention functions. Archives Italiennes de Biologie, 145, 23–37.
Crawford, H. J. (1994). Brain dynamics and hypnosis: Attentional and disattentional processes. International Journal of Clinical and Experimental Hypnosis, 52,
Crawford, H. J., & Gruzelier, J. H. (1992). A midstream view of the neuropsychophysiology of hypnosis: Recent research and future directions. In E. Fromms &
M. R. Nash (Eds.), Contemporary hypnosis research (pp. 227–266). London: Guilford Press.
De Jong, R., Liang, C.-C., & Lauber, E. (1994). Conditional and unconditional automaticity: A dual-process model of effects of spatial stimulus–response
correspondence. Journal of Experimental Psychology: Human Perception and Performance, 20, 731–750.
Dixon, M., Brunet, A., & Laurence, J.-R. (1990). Hypnotizability and automaticity: Toward a parallel distributed processing model of hypnotic responding.
Journal of Abnormal Psychology, 99, 336–343.
Dixon, M., & Laurence, J. R. (1992). Hypnotic susceptibility and verbal automaticity: Automatic and strategic processing differences in the Stroop color-
naming task. Journal of Abnormal Psychology, 101, 344–347.
Egner, T., Delano, M., & Hirsch, J. (2007). Separate conflict-specific cognitive control mechanisms in the human brain. NeuroImage, 35, 940–948.
Egner, T., Jamieson, G., & Gruzelier, J. (2005). Hypnosis decouples cognitive control from conflict monitoring processes of the frontal lobe. NeuroImage, 27,
Eriksen, B. A., & Eriksen, C. W. (1974). Effects of noise letters upon the identification of a target letter in a non search task. Perception and Psychophysics, 16,
Gruzelier, J. H. (1988). The neuropsychology of hypnosis. In M. Heap (Ed.), Hypnosis: Current clinical, experimental and forensic practices (pp. 68–76). London:
Gruzelier, J. H. (1998). A working model of the neurophysiology of hypnosis: A review of evidence. Contemporary Hypnosis, 15, 3–21.
Hommel, B. (1994). Spontaneous decay of response-code activation. Psychological Research, 56, 261–268.
Hommel, B., Pratt, J., Colzato, L., & Godijn, R. (2001). Symbolic control of visual attention. Psychological Science, 12, 360–365.
Iani, C., Ricci, F., Gherri, E., & Rubichi, S. (2006). Hypnotic suggestion modulates cognitive conflict. Psychological Science, 17, 721–727.
Jamieson, G. A., & Sheehan, P. W. (2004). An empirical test of Woody and Bower’s dissociated control theory of hypnosis. International Journal of Clinical and
Experimental Hypnosis, 52, 232–249.
Kaiser, J., Barker, R., Haenschel, C., Baldeweg, T., & Gruzelier, J. H. (1997). Hypnosis and event-related potential correlates of error processing in a Stroop-like
paradigm: A test of the frontal hypothesis. International Journal of Psychophysiology, 27, 215–222.
Kornblum, S. (1992). Dimensional overlap and dimensional relevance in stimulus–response and stimulus–stimulus compatibility. In G. Stelmach & J. Requin
(Eds.). Tutorials in motor behavior (Vol. II, pp. 743–777). Amsterdam: North-Holland.
Kornblum, S., Hasbroucq, T., & Osman, A. (1990). Dimensional overlap: Cognitive basis for stimulus–response compatibility—A model and taxonomy.
Psychological Review, 97, 253–270.
Lichtenberg, P., Bachner-Melman, R., Ebstein, R. P., & Crawford, H. J. (2004). Hypnotic susceptibility: Multidimensional relationships with Cloninger’s
tridimensional personality Questionnaire, COMT polymorphisms, absorption, and attentional characteristics. International Journal of Clinical and
Experimental Hypnosis, 52, 47–72.
Lichtenberg, P., Bachner-Melman, R., Gritsenko, I., & Ebstein, R. P. (2000). Exploratory association study between catechol-O-methyltransferase (COMT)
high/low enzyme activity polymorphism and hypnotizability. American Journal of Medical Genetics, 96, 771–774.
MacLeod, C. M. (1991). Half a century of research on the Stroop effect: An integrative review. Psychological Bulletin, 109, 163–203.
Masaki, H., Takasawa, N., & Yamazaki, K. (2000). An electrophysiological study of the locus of the interference effect in a stimulus–response compatibility
paradigm. Psychophysiology, 37, 464–472.
Nordby, H., Hugdahl, K., Jasiukaitis, P., & Spiegel, D. (1999). Effects of hypnotizability on performance of a Stroop task and event-related potentials.
Perception and Motor Skills, 88, 819–830.
Pellicano, A., Lugli, L., Baroni, G., & Nicoletti, R. (2009). The Simon effect with conventional signals: A time course analysis. Experimental Psychology, 56,
Proctor, R. W., & Vu, K.-P. L. (2006). Stimulus–response compatibility principles: Data, theory, and application. Boca Raton, FL: Taylor & Francis.
Ratcliff, R. (1979). Group reaction time distributions and an analysis of distribution statistics. Psychological Bulletin, 86, 446–461.
Raz, A., Fan, J., & Posner, M. I. (2005). Hypnotic suggestion reduces conflict in the human brain. Proceedings of the National Academy of Sciences of the United
States of America, 102, 9978–9983.
C. Iani et al./Consciousness and Cognition 18 (2009) 856–863
Raz, A., Fan, J., & Posner, M. I. (2006). Neuroimaging and genetic associations of attentional and hypnotic processes. Journal of Physiology – Paris, 99, 483–491. Download full-text
Raz, A., Kirsch, I., Pollard, J., & Nitkin-Kaner, Y. (2006). Suggestion reduces the Stroop effect. Psychological Science, 17, 91–95.
Raz, A., Landzberg, K. S., Schweizer, H. R., Zephrani, Z. R., Shapiro, T., Fan, J., et al (2003). Posthypnotic suggestion and the modulation of Stroop interference
under cycloplegia. Consciousness and Cognition, 12, 332–346.
Raz, A., Moreno-Íñiguez, M., Martin, L., & Zhu, H. (2007). Suggestion overrides the Stroop effect in highly susceptible individuals. Consciousness and Cognition,
Raz, A., Shapiro, T., Fan, J., & Posner, M. I. (2002). Hypnotic suggestion and the modulation of Stroop interference. Archives of General Psychiatry, 59,
Ricciardelli, P., Bonfiglioli, C., Iani, C., Rubichi, S., & Nicoletti, R. (2007). Spatial coding and central patterns: Is there something special about the eyes?
Canadian Journal of Experimental Psychology, 61, 79–90.
Ristic, J., & Kingstone, A. (2006). Attention to arrows: Pointing to a new direction. The Quarterly Journal of Experimental Psychology, 59, 1921–1930.
Ristic, J., Wright, A., & Kingstone, A. (2007). Attentional control and reflexive orienting to gaze and arrow cues. Psychonomic Bulletin and Review, 14, 964–969.
Rizzolatti, G., Riggio, L., Dascola, I., & Umiltà, C. (1987). Reorienting of attention across the horizontal and vertical meridians: Evidence in favor of a premotor
theory of attention. Neuropsychologia, 25, 31–40.
Rubichi, S., Nicoletti, R., Iani, C., & Umiltà, C. A. (1997). The Simon effect occurs relative to the direction of an attention shift. Journal of Experimental
Psychology: Human Perception and Performance, 23, 1353–1364.
Rubichi, S., Ricci, F., Padovani, R., & Scaglietti, L. (2005). Hypnotic susceptibility, baseline attentional functioning and the Stroop task. Consciousness and
Cognition, 14, 296–303.
Sheehan, P. W., Donovan, P., & MacLeod, C. M. (1988). Strategy manipulation and the Stroop effect in hypnosis. Journal of Abnormal Psychology, 97, 455–460.
Shore, R. E., & Orne, E. C. (1962). Harvard group scale of hypnotic susceptibility, Form A. Palo Alto, CA: Consulting Psychologists Press.
Shore, R. E., & Orne, E. C. (1976). Scala Harvard per la suscettibilità ipnotica di gruppo: Forma A. Firenze, Italy: O.S. Organizzazioni Speciali.
Simon, J. R., Acosta, E., Jr., Medwaldt, S. P., & Speidel, C. (1976). Effect of compatibility of S–R mapping on reaction toward the stimulus source. Acta
Psychologica, 47, 63–81.
Simon, J. R., & Rudell, A. P. (1967). Auditory S–R compatibility: The effect of an irrelevant cue on information processing. Journal of Applied Psychology, 51,
Sommer, T., Fossella, J. A., Fan, J., & Posner, M. I. (2003). Inhibitory control: Cognitive subfunctions, individual differences and variation in dopaminergic
genes. In J. Reinvang, M. W. Greenlee, & M. Hermann (Eds.), The cognitive neuroscience of individual differences – New perspectives (pp. 27–44). Oldenburg:
Stroop, J. R. (1935). Studies of interference in serial verbal reactions. Journal of Experimental Psychology, 18, 643–662.
Stürmer, B., Redlich, M., Irlbacher, K., & Brandt, S. (2007). Executive control over response priming and conflict: A transcranial magnetic stimulation study.
Experimental Brain Research, 183, 329–339.
Tipples, J. (2002). Eye gaze is not unique: Automatic orienting in response to uninformative arrows. Psychonomic Bulletin and Review, 9, 314–318.
Tipples, J. (2008). Orienting to counterpredictive gaze and arrow cues. Perception and Psychophysics, 70, 77–87.
Weitzenhoffer, A. M., & Hilgard, E. R. (1959). Stanford hypnotic susceptibility scale: Form C. Palo Alto, CA: Consulting Psychologists Press.
Weitzenhoffer, A. M., & Hilgard, E. R. (1962). Scala per il profilo stanford di suscettibilità ipnotica: Forma C. Firenze, Italy: O.S. Organizzazioni Speciali.
C. Iani et al./Consciousness and Cognition 18 (2009) 856–863