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 clear-cut 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 Catechol-O-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 Mono-Amino-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 non-motor 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 sensori-motor 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).

Impaired performance at cerebellar tests during suggestions of anesthesia had suggested a cerebellar involvement in the motor response to sensory suggestions, but hypnotic relaxation may have influenced the performance (Wallace and Hoyenga, 1981). It has been also shown that overactivity in the cerebellum and in the parietal cortex is associated with the misattribution of actions to an external source (Blakemore et al., 2003), but this report did not consider a possible real, physiologically based involuntariness.

In our study of the imagery of rotated head (Santarcangelo et al., 2010) the embodiment of the mental image of rotated head involuntarily produced is likely to enable the cerebellum to act routinely on the imaginatively constructed sensory information. Alternatively, imagery-triggered top down signals may change the cerebellar internal model according to the mental images' content. In both instances, the perceived involuntariness would be physiologically sustained. In contrast, in the studies on imagery of leg pain (Scattina et al., 2012) and of inertial backward falling (Carli et al., 2006), in which the response to suggestions could be expected and voluntarily controlled (despite the participants' subjective reports), real involuntariness could be sustained by reduced cerebellar inhibition (Groiss and Ugawa, 2013) of motor cortices (Figure 1).

Together with parietal associative areal, the cerebellum is likely to be responsible also for the different internal models (Ito, 2006) for postural control observed in highs and lows in the absence of suggestions (Santarcangelo et al., 2008). It should be also studied whether cerebellar peculiarities may contribute to the cognitive aspects of hypnotic responding through vestibulo-coeruleus and rubro-thalamo-prefrontal pathways (Groiss and Ugawa, 2013).

Studies of the post-occlusion flow-mediated dilation (FMD) of the brachial artery (Green et al., 2014) have indicated greater availability of endothelial nitric oxide (NO) in highs during mental stress and nociceptive stimulation (Jambrik et al., 2005). A quite important question is whether, in these individuals, larger NO availability characterizes also the brain vessels. In fact, the brain endothelial NO is responsible for basal vascular tone, interacts with other mediators, and acts as a neurotransmitter after diffusion to the extracellular compartment. Thus, (Figure 1), in the whole brain, endothelial NO could prime cascade processes modulating the neuronal activity in highs and lows differentially. In addition, in animals NO increases the release of acetylcholine, decreases the dopamine release and increases its metabolism; in humans, larger endothelial NO availability in hypnotizability-related circuits (Hoeft et al., 2012) could account for the highs' attentional characteristics (see Presciuttini et al., 2014) and could be involved in the cognitive aspects of hypnotizability.

In the cerebellum NO acts also as a negative regulator of granule cell precursor proliferation, promotes survival and differentiation of these neurons and regulates bidirectional plasticity at parallel fiber-Purkinje neuron synapses (Contestabile, 2012). Thus, in the cerebellum, NO could be responsible for different functional properties. On the whole, highs and lows may exhibit NO-dependent different activation of sensory and associative regions such as those belonging to the salience and executive circuits, cerebellar inhibition of the motor cortex, internal model for postural control.

In conclusion, our findings allow to include hypnotizability among the individual traits responsible for part of the variability in postural control and indicate that it may be involved in the construction of individual sensorimotor selves. They suggest a key role of the cerebellum in hypnotic responding and go beyond psychological theories by suggesting that the involuntariness in action reported after a few sensori-motor suggestions can be real rather than merely experienced.