The actual administration of the HRV-Biofeedback protocol: 5 HRV biofeedback training sessions of 45 minutes each, conducted biweekly.

Placebo condition: 5 control sessions of 45 minutes each, conducted biweekly.

The intervention involves 5 training sessions lasting 45 minutes (specifically, 5-minute baseline and 5 HRV biofeedback trials lasting 5 minutes each) according to the protocol published by Lehrer et al. (2013). Physiological signals (ECG and respiratory rate) will be recorded during all sessions. During the training, participants will see on the screen a graph representing heart rate superimposed on a graph representing abdominal breathing. They will be asked to synchronize the two signals so that the changes in heart rate are in phase with the respiratory cycle in order to maximize the difference between the maximum and minimum heart rate within each respiratory cycle [i.e., respiratory sinus arrhythmia (RSA), an index of vagal modulation on the heart] (Lehrer et al., 2003; Lehrer et al., 2000).

The Depression Anxiety Stress Scales - 21 (DASS-21) is 21-item self-report measure designed to assess the severity of general psychological distress and symptoms related to depression, anxiety, and stress in adults older adolescents (17 years +).

The Depression Anxiety Stress Scales - 21 (DASS-21) is 21-item self-report measure designed to assess the severity of general psychological distress and symptoms related to depression, anxiety, and stress in adults older adolescents (17 years +).

The Emotion Regulation Questionnaire (ERQ) is composed of ten items divided into two factors: cognitive reappraisal with six items and expressive suppression with four items. It is answered on a Likert scale ranging from 1 (total disagreement) to 7 (total agreement).

This scale evaluates primarily the intensity of IBS symptoms during a 10-day period: abdominal pain, distension, stool frequency and consist- ency, and interference with life in general. The IBS-SSS calculates the sum of these 5 items each scored on a visual analog scale from 0 to 100. Although the IBS-SSS uses patient-rated intensity of IBS symptoms, the determination of severity by the scoring system was originally anchored to a physician's assessment of patient severity.

Electrocardiogram (ECG) and Heart Rate Variability in particular, recorded through three Ag/AgCl electrodes placed with a proximal type fitting, using the second Einthoven lead. Heart rate and HRV parameters in time and frequency and nonlinear indices will be extracted from the ECG. They will also be monitored during the proposed training (experimental and control).

The Body Perception Questionnaire (BPQ) is a self-report measure of body awareness and autonomic symptoms (i.e. symptoms experienced in organs innervated by the ANS). Its items are based on the organization of the autonomic nervous system (ANS), a set of neural pathways connecting the brain and body.