New views of hypnotizability

INTRODUCTION Hypnotizability, or hypnotic susceptibility (Green et al., 2005), is a good predictor of the response to suggestions in and out of hypnosis (Meyer and Lynn, 2011). It can be measured by scales (Sheehan and McConkey, 1982) allowing to classify subjects as high (highs), medium (mediums), and low (lows) hypnotizable by indicating the individual ability to modify experience and behavior according to the suggestions’ content and to feel that this occurs independently of will. In the ordinary state of consciousness, highs and lows can be discriminated by the predictability of their electroencephalographic (EEG) dynamics, while the several studies performed through spectral analysis of EEG failed to indicate clearcut discrimination criteria (Madeo et al., 2013). Imaging studies (Hoeft et al., 2012) have associated high hypnotizability with greater functional connectivity between the left dorsolateral prefrontal cortex, an executive-control region, and the salience network (dorsal anterior cingulate cortex, anterior insula, amygdala, ventral striatum) involved in detection and processing of relevant information (Figure 1). This is consistent with the evidence that highs tend to become deeply absorbed in any task of everyday life (Tellegen and Atkinson, 1974; Kihlstrom et al., 1989); nonetheless, the theory attributing the highs’ peculiar ability of focusing attention on selected internal or external objects as the basis of hypnotic responding (Raz, 2005; Szekely et al., 2010) has been challenged by neuropsychological and genetic studies. The former have denied any association between various attentional abilities and hypnotisability (Varga et al., 2011), have shown only higher arousal in highs (Castellani et al., 2007) and have suggested that the highs’ attention is more stable (less distractible) rather than more flexible than the lows’ one (Jamieson and Sheehan, 2004; Egner et al., 2005); genetic studies (Presciuttini et al., 2014) have refuted the hypothesis that reduced dopamine catabolism associated with polymorphism of the brain CatecholO-Methil-Transferase (COMT) may be responsible for the highs’ attentional abilities, as no difference between highs and lows has been found in COMT polymorphism (Szekely et al., 2010). Nonetheless, theoretically, higher dopaminergic tone could be sustained in highs by other mechanisms such as different receptors density/distribution/sensitivity, dopamine production, and catabolism by the MonoAmino-Oxidase system. Since the very beginning of my research activity I have focused my interest on the physiological correlates of hypnotisability rather than on its psychological factors possibly accounting for hypnotic responding (Killeen and Nash, 2003; Dienes et al., 2009; Lynn and Green, 2011). In fact, I considered that the existence of a trait influencing only one aspect of behavior—the proneness to accept suggestions—would be a serious challenge to common sense, all the more that hypnotic performance was considered a consequence of peculiar attentional abilities (Raz, 2005), attention modulates several sensorimotor processes (Woollacott and Shumway-Cook, 2002; Ruff, 2013), cognitive-emotional traits have been associated with peculiar morphological characteristics of cerebellar structures (Picerni et al., 2013), and both the cerebellum and basal ganglia are often involved in nonmotor functions (Stoodley, 2012; Leisman and Melillo, 2013; Keren-Happuch et al., 2014). My intuition was correct. Indeed, I have found hypnotizability-related differences (Carli et al., 2006, 2008; Santarcangelo et al., 2008, 2010; Menzocchi et al., 2010, 2012; Castellani et al., 2011; Scattina et al., 2012) in many aspects of sensorimotor integration in both the absence (Table 1) (Collins and De Luca, 1993; Caratelli et al., 2010; Mecacci et al., 2013) and the presence of suggestions. I have chosen the differences in postural control induced by imagined sensory alteration (Carli et al., 2006; Santarcangelo et al., 2010; Scattina et al., 2012) as the object of this article. I will also show that my physiological approach to the field of hypnotizability allows to suggest that the involuntariness reported by highs in their response to sensory suggestions is physiologically sustained and, thus, “real” rather than only subjectively experienced (Santarcangelo et al., 2010).