Oxytocin and post-hypnotic suggestion 1 Oxytocin impedes the effect of the word blindness posthypnotic suggestion on Stroop task performance

The ability to enhance sensitivity to relevant (post)hypnotic suggestions has implications for creating clinically informed analogues of psychological and neuropsychological conditions and for the use of hypnotic interventions in psychological and medical conditions. The aim of the present study was to test the effect of oxytocin inhalation on a post-hypnotic suggestion that previously has been shown to improve the selectivity of attention in the Stroop task. In a double-blind placebo-controlled between-subjects study medium hypnotisable individuals performed the Stroop task under normal conditions and when they had been given a post-hypnotic suggestion that they would perceive words as meaningless symbols. In line with previous research, Stroop interference was substantially reduced by the suggestion in the placebo condition. However, contrary to expectations, oxytocin impeded the effect of the word blindness suggestion on performance. The results are explained in terms of the requirement for the re-implementation of the word blindness suggestion on a trial-by-trial basis and the need to sustain activation of the suggestion between trials. The findings contrast with a recent study showing a beneficial effect of oxytocin on sensitivity to (post)hypnotic suggestions but are consistent with findings showing a detrimental effect of oxytocin on memory processes. Oxytocin and post-hypnotic suggestion 3 Introduction The neuropeptide oxytocin is thought to be important for learning, memory and behavioural regulation in humans and non-humans and plays an important role in social affiliation. Central oxytocin receptors are found throughout the brain in many structures important for information processing, memory, and emotion/reward including the hippocampus, amygdala, striatum, hypothalamus, nucleus accumbens, and midbrain (Gimpl & Fahrenholz, 2001). In terms of its role in social affiliation, oxytocin administration has been shown to enhance trust as well as increase attention to social stimuli (Kosfeld et al., 2005; Bartz & Hollander, 2006). Oxytocin’s role in increasing social affiliation has recently been investigated in the hypnotic context in which the relationship between the hypnotist and subject is pivotal (Sheehan & McConkey, 1982). Bryant et al. (2012) wanted to explore whether hypnotizability could be increased given the implications such a finding would have for hypnotic interventions for psychological and medical conditions. After an initial screening session establishing participants as low hypnotizable individuals (LHIs), Bryant et al. had participants inhale either oxytocin or a placebo and then re-screened them in a randomized, double-blind, between-subjects experiment, and compared their hypnotizability score to the previous screening session. Bryant et al. showed that for the participants that inhaled oxytocin, susceptibility to hypnotic suggestions increased significantly. Indeed some of their participants (42%) went from scoring as LHIs to scoring as medium hypnotisable individuals (MHIs), although none went from LHIs to highly hypnotisable individuals (HHIs). Interestingly, only cognitive suggestions (e.g. swatting a hallucinated mosquito, hallucinating a taste, anosmia to ammonia, posthypnotic amnesia) benefitted from oxytocin inhalation. No effects of oxytocin were detected for motor (moving hands apart) nor challenge (difficulty Oxytocin and post-hypnotic suggestion 4 bending extended arm, difficulty lifting arm) suggestions. Nevertheless, the finding of an effect on the more difficult suggestions is important, especially given the predominance of these suggestions in clinical settings. The authors interpreted their effects by suggesting that oxytocin leads to greater motivation to initiate appropriate cognitive strategies to respond to the suggestions as a result of increased attention to the hypnotist’s social cues. In the present study, we aimed to test the effect of oxytocin on a post-hypnotic suggestion whose remarkable effect has implications for creating clinically informed analogues of psychological and neuropsychological conditions (Oakley & Halligan, 2011). Raz et al. (2002) showed that the Stroop effect (Stroop, 1935), one of the most robust effects in cognitive psychological research, can be virtually eliminated following a post-hypnotic suggestion. The Stroop task requires participants to identify the colour of the font in which a word is presented, whilst ignoring the meaning of the word itself. When the written word is incongruent with the ink colour (red written in blue), the time it takes to identify the colour is increased relative to neutral stimuli (i.e. when the word is not colour-related e.g. top written in red). When the word is congruent with the colour (red in red), colour identification time is reduced compared to neutral stimuli. The effect of word congruency on colour classification times is known as the Stroop effect. The Stroop effect has been referred to as the “gold standard” of attentional processes and has long been held as strong evidence for the obligatory nature of word processing (MacLeod, 1992). However, a post-hypnotic suggestion describing the word dimension of the Stroop stimulus as being made up of 'meaningless symbols' and 'characters of a foreign language' (to be referred to as the word blindness suggestion), resulted in the virtual elimination of not only Stroop interference (Incongruent-Neutral Stroop trials), but also Stroop Oxytocin and post-hypnotic suggestion 5 facilitation (Neutral-Congruent trials) in the reaction time data, and Stroop effects typically observed in error data. The authors argued that their results were inconsistent with the notion that the processes of visual word recognition are obligatory and that the posthypnotic suggestion works via a top-down mechanism that modifies the processing of input words through a means not voluntarily available. As noted above, the effect of the word blindness suggestion highlights the potential of (post)hypnotic suggestions in exploring the cognitive and biological substrates underlying normal and impaired psychological functions (see Oakley & Halligan, 2009; Oakley & Halligan, 2011). For example, Raz and colleagues have shown that the word blindness post-hypnotic suggestion reduces activity in visual processing areas of the brain and areas known to be involved in conflict processing (Raz et al., 2005), indicating that the word blindness suggestion might actually disable word reading, inducing a form of alexia that could be studied in the same way as any neuropsychological impairment (Oakley & Halligan, 2011; although see Augustinova & Ferrand, 2012, for a contrasting viewpoint). Indeed so remarkable is the effect of the word blindness suggestion on Stroop task performance that it stands as a marker in the literature for the potency of (post)hypnotic suggestions and provides an objective measure (one that does not rely on self-reporting) of the effect of a suggestion on performance. In the original study, the word blindness suggestion effect was observed in highly hypnotisable individuals (henceforth HHIs) only, and was remarkable in its allencompassing effect on indices of Stroop task performance. In numerous subsequent studies Raz and colleagues have shown that the suggestion effect on interference is replicable, although the effect on facilitation less so (Raz et al., 2003; Raz et al., 2007; Raz & Campbell, 2011) and that the effect is observable in low hypnotisable Oxytocin and post-hypnotic suggestion 6 individuals (LHIs) indicating a greater utility for (post)hypnotic suggestions in research and in psychological and medical interventions, since HHIs represent only a small proportion of the population (Raz & Campbell, 2011). However, the effect of the suggestion was much reduced in LHIs, being about half that observed in HHIs. Here we explore the influence of oxytocin on the word blindness post-hypnotic suggestion in medium hypnotizable individuals (MHIs). Showing an enhanced effect of oxytocin inhalation on this particular suggestion would be of use not only in psychological and medical interventions, but also for gaining a better understanding of the cognitive and biological substrates underlying normal and impaired psychological functions. Other authors have also shown an effect on the word blindness suggestion on Stroop task performance. In a recent study, Parris et al. (2012) have shown that the effect of the word blindness suggestion is more likely when response-stimulus interval is short (500ms) compared to the interval used by Raz and colleagues (3500ms). The implication here is that the suggestion is re-implemented on every trial. When time between re-implementations is too long, activation of the suggestion cannot be sustained. Hence, the ability to sustain the representation of the suggestion in memory is key to the successful application of the suggestion. In the present study we utilized the short RSI only to increase the likelihood of observing an effect on performance in both conditions. It was expected that there would be a word blindness suggestion effect in the placebo condition and that this effect would be enhanced by the inhalation of oxytocin. However, to foreshadow the results, despite the use of the shorter RSI, the reduction in interference observed in the placebo condition was not observed after oxytocin inhalation. Methods and Materials Oxytocin and post-hypnotic suggestion 7 Participants 137 students from the University of Bournemouth were screened for suggestibility using the Waterloo-Stanford Group Scale of Hypnotic Susceptibility, Form C (WSGC) (Bowers, 1993). The scale gives possible ratings from 0 to 12. However, we did not include the age regression suggestion, which means that the maximum possible score was 11. Participants were excluded from the experiment if they did not score in the medium suggestible range (4 7) on this scale and if they were pregnant, on medication, had a history of significant medical or psychiatric illness, had a history of substance abuse, or had epilepsy. The selected 36 proficient English speakers who took part in the study had an average age of 19.94 years (SD = 1.56). These participants were randomly assigned to one of the groups that would receive either the oxytocin or placebo nasal sprays. Neither the participants nor the hypnotist/experimenter were aware of group allocation at the time of testing. The two groups were matched for age: Oxytocin (average age: 19.8 years (SD = 1.09)) and Placebo (average age: 20.1 years (SD = 1.95)); Gender: 13 females and 5 males in both groups; and Hypnotisability: Both groups scored an average of 5.2 on the Waterloo-Stanford Group Scale with standard deviations of 0.94 (oxytocin) and 0.88 (placebo: p > .9). Participants were paid £12 for their participation. Participants were asked to abstain from food and drink other than water for two hours before the experiment, and from alcohol, smoking and caffeine for 24 hours before the experiment. Ethical considerations: The Ethics Committee at Bournemouth University approved the study and the research was carried out in line with institutional guidelines and regulations. All participants were informed about the risks of oxytocin inhalation prior to taking part and provided written consent. All participants were adult-age university students and therefore capable of giving informed consent.