Decoding Mechanisms of Pain Modulation Through Hypnosis and Vagus Nerve Stimulation

Pain is a subjective experience, influenced by biological, psychological and social factors. This multidimensional view of pain has led to various efforts to affect people's pain experience. Nonpharmacological interventions, such as hypnosis, have proven successful in reducing pain whilst providing few, if any, negative side effects. Hypnosis involves a state of highly focused attention, with a constriction in peripheral awareness and a heightened responsiveness to social cues. This particular state can exert a powerful influence on the mind and body, yet the mechanisms responsible for this effect remains to a large degree unknown. The aim of this study is to investigate the effect of hypnosis given prior to a painful procedure (cold pressor test, CPT), to investigate the effect on pain perception and tolerance, but most importantly, to investigate if the effect is mediated by a change in expectations of coping. Previous studies have provided support for the the effect of hypnosis on expectations, but they have focused exclusively on stimulus expectancies (expectations of pain intensity), while the current study will focus on response outcome expectancies (expectations of coping) in line with the Cognitive Activation Theory of Stress (CATS).

Whilst hypnosis may dampen the stress response through expectancies (top-down), another way of dampening the stress response is through transcutaneous vagus nerve stimulation (tVNS) (bottom-up). Vagus nerve stimulation is proposed as another nonopioid pain treatment with minimal side effects. The vagus nerve is the 10th cranial nerve, connecting the viscera and the brain and influencing multiple systems of the body including the cardiac, immunologic, and endocrine system, and the activity of many visceral organs. This makes the vagus nerve a possibly important mediating (transmitting) and modulating nerve of pain signals . Stimulation of the vagus nerve is believed to modulate pain by inhibiting inflammation, oxidative stress, and sympathetic activity, and possibly also by inducing a brain activation pattern that may be incongruent with the pain matrix (i.e. brain regions commonly active during pain). VNS might also mediate the effects of the opioid system in pain modulation. These mechanisms have in common that they are hypothesized to affect neuronal hyperexcitability, resulting in a reduced pain perception, which is supported by experimental animal studies.