Introduction
Hypnosis is a state of consciousness which can be self-induced or
promoted through various procedures (“induction”) enacted by other
persons (Elkins et al., 2015). It is described as different from the
ordinary state of consciousness (Pekala et al., 2006) and cannot be
defined independently from self-reports, although many cortical
correlates have been observed by imaging (Landry et al., 2017; Wolf et
al., 2022) and EEG studies (Baghdadi & Nasrabadi, 2012; Hiltunen et
al., 2021; Rho et al., 2021; Yargholi & Nasrabadi, 2015). Neutral
hypnosis (NH) is a state following hypnotic induction without specific
suggestions, i.e., requests to imagine perception, memory and behavior
different from the actual ones (for instance, analgesia, hallucination,
movement) and to experience them as real and involuntary. According to
the bio-psycho-social model of hypnosis (Jensen et al., 2015), the
proneness to enter the hypnotic state and accept suggestions is
influenced by contextual and individual factors, one of them being the
psychophysiological trait of hypnotizability. It is substantially stable
through life (Piccione et al., 1989), is measured by scales (Elkins et
al., 2015) classifying high (highs, 15% of the general population),
medium (mediums, 70%) and low (lows, 15%) hypnotizable individuals,
and displays physiological correlates observable in the ordinary state
of consciousness even in the absence of suggestions (Bocci et al., 2017;
Ibanez-Marcelo et al., 2019; Rashid et al., 2022; Santarcangelo &
Scattina, 2016; Spina et al., 2020).
Hypnotizability and hypnosis could be relevant to the integration of
bodily signals with ongoing conscious and unconscious mental processes
at high levels of the central nervous system (Quadt et al., 2018). Their
integration, in fact, differs according to cognitive-emotional states
(Gentsch et al., 2019; Kritzman et al., 2022) and traits (Judah et al.,
2018; Zhou et al., 2022), and interoceptive information is conveyed to a
few brain structures displaying hypnotizability-related
morpho-functional differences, i.e., insula, cingulate cortex and
cerebellum (Landry et al., 2017; Picerni et al., 2019). Moreover, the
highs’ sensitivity to interoceptive signals - the self-reported mode of
interpretation of bodily signals - is different and more “adaptive”
than in lows and mediums, indicating a good relationship with the body
and a tendency to positively interpret bodily signals (Diolaiuti et al.,
2020). In contrast, the interoceptive accuracy - the ability to detect
bodily signals -, which can be tested using the heartbeats count, is
lower in highs than in lows, although only during the first of three
heartbeats count trials (Rosati et al, 2021).
The results of heartbeat count have been associated with the amplitude
of the cortical potential evoked by heartbeat (HEP) which has been
related to cardiac afferents, despite the presence of some
non-interoceptive information (Desmedt et al., 2018). It is obtained by
averaging electrophysiological signals (such as the EEG and MEG)
synchronized to R-peaks or T-peaks of a simultaneously recorded ECG
signal (Park & Blanke, 2019b). In contrast to exteroceptive cortical
potentials, the HEP amplitude increases during deep sleep (N3) with
respect to light sleep, and is like N2 during REM sleep (Lechinger et
al., 2015), thus overcoming the effect of the thalamic gate during N3
and of the disrupted brain activity during REM. The amplitude of the HEP
earlier component, which reflects cardiac interoceptive accuracy
(200-350 ms), has been associated with heartbeat counting scores,
although not unanimously (Park & Blanke, 2019b), and is more pronounced
at the medial-right fronto-central sites. The amplitude of the later
component (400-600 msec) is related to the proneness to not worry about
body sensations, to stress-induced changes in cardiac output,
emotional
arousal, dysregulation of emotions and some clinical disorders
(Baranauskas et al., 2021; Luft & Bhattacharya, 2015).
Since the heartbeat count indicated lower interoceptive accuracy in
highs than in lows (Rosati et al., 2021), the aim of the present study
was to assess whether this finding is supported by differences in the
HEP amplitude, and whether the HEP amplitude changes during neutral
hypnosis in highs and lows differentially.