This experimental proof-of-concept study in healthy volunteers investigates whether a specific form of self-relating, namely, self-compassion, can be controlled through non-invasive stimulation of the vagus nerve. The vagus nerve is part of the parasympathetic nervous system (the 'rest and digest' system) and its fibers are spread throughout the upper body, face, and neck, including a branch that innervates parts of the external ear (e.g. the tragus), close to the surface of the skin. This allows the 'auricular branch' of the vagus nerve to be readily stimulated electrically. Importantly, this can be achieved non-invasively using a transcutaneous vagus nerve stimulation (tVNS) device. The investigators will test the effects of tVNS alone and in combination with a meditation-like technique that is intended to increase participants' feelings of self-compassion (self-compassion imagery). In particular, the investigators will assess participants' experience of self-compassion and self-criticism, as well as other outcome measures, before and during stimulation, and again after they perform the self-compassionate imagery exercise. These effects will be compared to three other groups: one that receives tVNS along with a control (sham) imagery procedure, another that receives sham tVNS and self-compassion imagery, and a final group that receives sham tVNS and sham imagery. It is expected that the group that receives active tVNS and the self-compassion imagery exercise will experience the largest average increases in self-compassion and decreases in self-criticism. The findings of this study will help us better understand the role of the vagus nerve in complex cognitive-emotional experiences - like compassion - and may also motivate the development of bioelectronic stimulation methods that can be used alongside psychological techniques for improving self-compassion and reducing self-criticism, particularly among people with certain psychological disorders.

The role of the vagus nerve in regulating cardiac, respiratory, and gut functioning via brainstem nuclei is well-established. More recently, its role has been examined in relation to the modulation of higher cognitive functions. Bodily signals (e.g. from the heart or gut) conveyed by vagal afferents could affect cognitive functioning via projections from the nucleus of the solitary tract (NTS) to the locus coeruleus and thence to widespread cortical areas through noradrenergic outputs. In addition, the NTS transmits sensory-visceral information to the central autonomic network, which includes higher brain areas (e.g. prefrontal cortex). Recent research suggests that vagus nerve activity is associated with a range of psychological functions including memory, executive function, and threat/fear processing. In addition, biobehavioral theories of vagal functioning, and emerging empirical evidence suggest a role for the vagus nerve in modulating social behavior as well as complex emotional states.

A number of researchers have proposed a role for vagal nerve functioning in well-being, and particularly in generating or modulating feelings of 'safeness', contentedness, and relaxation that underlie complex social-affective-motivational states, such as compassion. Previous research has examined the effects of inducing compassionate feelings while measuring vagus nerve activity (via heart rate variability). However, to our knowledge, no published study has demonstrated an association between vagal activity and compassion by directly modulating the vagus nerve itself. Such studies have important theoretical implications but are also potentially valuable in the future development of interventions that employ tVNS or similar technology to augment psychological treatments that employ compassion-focused techniques (e.g. compassionate mind training; compassion-focused cognitive therapy; loving-kindness meditation-based treatments, etc.)

The current experiment aims to determine if vagus nerve activation using tVNS is sufficient to generate positive affective states, especially compassion (specifically, self-reported self-compassion). Alternatively, tVNS may produce a permissive physiological context that facilitates compassionate responding. For example, tVNS may synergize the effects of a compassion-inducing behavioral intervention. To examine these possibilities, the investigators will test the separate and combined (interacting) effects of tVNS and a compassion-induction procedure (self-compassion mental imagery) compared to sham conditions, in a four-group factorial (2 x 2) design. Participants will be randomly assigned to (i) active tVNS + sham imagery, (ii) active tVNS + active self-compassion imagery, (iii) sham-tVNS + active self-compassion and (iv) sham-tVNS + sham imagery.

In addition to examining the effects of stimulation and imagery conditions on self-compassion, the investigators will also test their effects on other types of positive affect, as well as self-criticism, which is an opposing form of self-representation to self-compassion. Self-criticism is a transdiagnostic psychological marker of psychopathology, and measures of self-criticism are sensitive to the effects of compassion-focused interventions , including brief self-compassion imagery, as will be used here. Vagal activity will be indexed using heart rate variability, and effects of stimulation and imagery will additionally be tested on attentional bias (e.g. first fixation, increased dwell time) to compassionate facial expressions.

Participants attend two lab sessions (on Day 1 and 8) in which they will undergo either supervised active or sham tVNS and either the self-compassion or sham imagery exercise (delivered as standardized audio instructions), per their randomized condition. Assessments of heart rate variability, self-compassion, self-criticism, positive state affect, and state mindfulness will occur before stimulation, during stimulation, and after the imagery exercise. In addition, participants will complete the Day 1 stimulation procedure and imagery task on Days 2-7, guided via online audio and written instructions. This will allow us to test the effects of extended (daily) stimulation and imagery practice. Additional study details are available on the Open Science Framework (osf.io/k2dje).

Inclusion Criteria:

• Fluency in English
• Good (including corrected) vision and hearing.

Exclusion Criteria:

• Current use of any medication for a psychiatric condition
• Regular use of any medication used to treat a cardiovascular condition or inflammation
• Use of any illicit recreational drug >2/week
• Regularly consuming >14 standard UK 'units' of alcohol
• Currently receiving treatment for any mental health condition
• Scores on screening measures of depression (PHQ-2) and/or anxiety (GAD-2) that indicate significant levels of current/recent anxiety or depression (scores on either >4)
• History of serious mental health problems (schizophrenia, bipolar disorder)
• Past or current cardiovascular or neurological problems
• Current/past problems related to chronic/recurring facial or ear pain
• Skin irritation/ broken skin at stimulation site
• Pregnancy or likelihood of becoming pregnant during study
• Previous adverse response to meditation.

Data will be shared upon reasonable request to PI

Study design and aims will be concealed from participants. They will not be made aware of the presence of active versus sham conditions (all participants are given the expectation of active stimulation and imagery).

Participants are informed of their imagery condition before consent is obtained on the first lab session.

Study aims will be disclosed at debriefing, after all experimental procedures have been completed.