Participants
Twenty-two healthy volunteers (15 females, age 19.9 ± 2.0 years) participated, including graduate and undergraduate students. Fourteen of them were also included in our previous investigation about the effects of interoceptive training on decision-making 21. All participants have no history of psychiatric disorders. When conducting regression analysis, the strategy is that a sample size 10 times larger than the number of variables being treated is sufficient 29. In this case, since we have only one IA score as a dependent variable, the idea is that 10 or more cases should be sufficient. Given this, we suppose that the sample size is sufficient for our study.
Written informed consent was obtained from each subject. The experiment protocol was approved by the ethical committee at the National Center of Neurology and Psychiatry (A2018-013). This protocol has been registered in the University Hospital Medical Information Network (UMIN) Clinical Trials Registry (URL: http://www.umin.ac.jp), No. UMIN000037548.
Procedure
The interoceptive training programs were similar to our previous investigation 21, developed in-house using matlab2012a. The interoceptive training program installed on a personal computer was provided to each participant. Participants were asked to undergo at least four time training in one week because four time training enhanced interoceptive accuracy in the past study 20, 22. Training on each day lasted about 40 min, but the total time depended on the self-pacing intervals between trials. All participants underwent psychological and behavioral assessments before (Pre) and after (Post) one-week period.
Interoceptive training task
The interoceptive training task was also similar to our previous investigation 21, which was developed by modifying a heartbeat discrimination task 23–25. In this task, subjects were presented with a series of tones generated either corresponding to their heartbeat (synchronous condition) or a delay (asynchronous condition). Each trial consisted of 10 tones presented at 440 Hz with 100 ms duration, triggered by the participant's heartbeat, which was monitored by a pulse meter attached to an index finger. Under the synchronous condition, tones were generated at the beginning of the rising edge of the pressure wave. Under the asynchronous condition, a delay of 300 ms was inserted. Following Garfinkel et al.'s study 20, 30, we added immediate correct or incorrect feedback for participants at the end of each trial to be able to update their heartbeat perception. The task consisted of 80 trials in a one-day session.
Psychological assessments
Interoceptive accuracy
Interoceptive accuracy was estimated using the heartbeat perception task 31. Subjects were asked to count their heartbeat three times in certain periods without taking a pulse, while their actual heartbeat was recorded by a pulse meter. Using both heartbeat data, we can calculate Interoceptive accuracy (IA) scores in the following formula.
IA score =1/3 ∑ཛ1-(|Recorded count-Counted count|/ Recorded count)ཝ.
State-Trait Anxiety
Anxiety symptom was evaluated by using State-Trait Anxiety Inventory (STAI) 32, 33. This self-reported questionnaire consists of 20 items to measure anxiety state and traits using normal and reversed four-point Linkert scales. Greater scores indicate greater anxiety.
Social anxiety
The anxiety traits of the participants were assessed in the following questionnaires: the Social Anxiety Disorder Scale (SADS) 34. SADS is a Japanese questionnaire that assesses social anxiety traits on four subscales: social fear, avoidance, somatic symptoms and daily life interference.
Neuroticism
To assess the neuroticism of the subjects, all subjects were asked to complete a 60-item Japanese version (5-point scale) of the NEO-FFI 35 36. The neuroticism traits were previously described as follows37: neuroticism, the tendency to experience negative emotions and psychological distress in response to stressors.
Somatic symptoms
A modified somatic perception questionnaire (MSPQ) 38 assessed somatic symptoms in daily life. This questionnaire consists of 22 items to evaluate how subjects feel during the past week about somatic symptoms, including heart rate increase, pulse in the neck, butterflies in the stomach, pain or ache in the stomach, difficulty in swallowing, mouth becoming dry, and so on. Subjects were asked to check four-point Linkert scales. Greater scores indicate greater sensitivity to somatic perception.
MRI data acquisition
MRI images were acquired using a 3-T MR scanner with a 32-channel phased array head coil (MAGNETOM Verio Dot, Siemens Medical Systems, Erlangen, Germany). Resting-state functional MRI (rsfMRI) and structural MRI data were obtained from each participant using the following MRI acquisition protocol. rsfMRI data were acquired with gradient-echo echo planar imaging (GE-EPI) for 10 min, during which participants were asked to clear their minds and to focus on a central fixation cross. We administered the Stanford sleepiness scale 39 to guarantee the wakefulness of participants during scanning. Acquisition parameters of GE-EPI were repetition time (TR) = 2500 ms, echo time (TE) = 30 ms, flip angle (FA) = 80°, voxel size = 3.3 × 3.3 × 3.2 mm3 (with a 0.8-mm gap), 40 axial slices, and 240 volumes. Structural MRI was acquired using a three-dimensional T1-weighted magnetization-prepared rapid gradient-echo (MPRAGE) sequence: TR = 1900 ms, TE = 2.52 ms, inversion time (TI) = 900 ms, FA = 90°, 192 sagittal slices, and voxel size = 0.98 × 0.98 × 1 mm3.
Data analyses
Data from one participant who did not perform intervention tasks because of a personal reason was omitted. Behavioral data from another participant were not available because of an unexpected technical error in the tablet PC. Post-intervention MRI data from 2 participants were not obtained because of an unexpected MRI machine failure. Also, scores of interoceptive accuracy, an independent value for the MRI analyses in the present study, from one participant were omitted because of an outlier that was more or less than two standard deviations. Thus, the analysis included longitudinal MRI and psychological data from 17 participants.
Psychological data analyses
Psychological and behavioral data were analyzed using statistical software, IBM SPSS Statistics v.25. To detect significant longitudinal changes from before and after the training, paired t-tests were conducted on indices of interoceptive accuracy and scores of anxiety symptoms and somatic symptoms. Statistical thresholds were set at p = 0.05 one-tailed, based on our hypotheses that indices of interoceptive accuracy and anxiety and somatic symptoms would decrease after training. Statistical thresholds were set at p = 0.05 one-tailed, based on our hypotheses that according to the improvement of interoceptive accuracy.
MRI data analyses
T1w anatomical data were preprocessed using N4 inhomogeneity correction 40. rsfMRI data were analyzed using CONN 41 with SPM12. Preprocessing includes slice-timing correction, realignment, coregistration, segmentation, normalization, and smoothing with an 8 mm full width at half maximum (FWHM) isotropic Gaussian kernel, denoising (using white matter, CSF, and realignment parameters), and motion scrubbing. Then, the time-series data were band-pass filtered (0.01-0.1Hz).
Seed to Voxel maps was created as resting state functional connectivity (RSFC) from rsfMRT data. Right (47, 14, 9) and left (-44,13,1) anterior insula were set as seed ROIs. These coordinates of bilateral insula were defined by default in the CONN software. Delta images of the seed-to-voxel maps (Post – Pre) were calculated for each subject. Multiple regression analyses for the delta images using change ratio of IA scores. The statistical threshold was set at p < 0.001 and corrected to p < 0.05 for multiple comparisons using cluster size.