2.1 Participants
Participants were recruited at the University Hospital of Tübingen and via ambulant psychotherapists. All procedures were approved by the ethics committee at the University Hospital and University of Tübingen and in line with the Declaration of Helsinki in its latest version. All participants gave their written informed consent prior to data collection. Exclusion criteria comprised: diabetes mellitus, kidney insufficiency, hypertension, dysrhythmia, cushing syndrome, substance abuse, adrenal insufficiency, cortisone medication, pacemaker, craniocerebral trauma as well as any medication except for oral contraceptives. In the first study, only healthy controls without a history of mental or neurological disorder were assessed. In the second study, healthy controls meeting the same criteria were compared to a group of patients with major depressive disorder (MDD), from which any other primary mental disorder except ICD-10 diagnosis F32.x, F34.1 and F33.x, as well as suicidality, extraordinarily severe depressive symptoms (BDI-II > 50), deficient emotional stability according to the currently treating psychologist and decompensation under social stress in the past led to exclusion.
The combined sample consisted of 45 healthy controls, 23 subclinical high trait ruminators and 22 depressed patients (see table 1). The diagnosis in the patient sample included recurrent Major Depressive Disorder (MDD) (n = 15) and first episode MDD (n = 7). The comorbid diagnoses included somatic symptom disorder (n = 2), anxiety disorders (n = 2) and personality disorders (n = 2). 60% of the patients were currently receiving psychotherapy and 58% antidepressant medication. The mean age of the total sample was 24.11 years (SD = 5.24 years) and 80% of all participants were female.
Differences between the study samples (e.g. the BDI-II-score) only concerned outcome measures irrelevant for our analysis. We did not differentiate between healthy and depressed participants, as we could observe a high correlation of social anxiety and rumination in both subsamples. Therefore, we prioritized a larger rather than a homogenous sample. In addition, as social anxiety and rumination are dimensional rather than categorical constructs and seem to play a role not just in depressed patients but also in healthy controls 56,57, we combined the different subsamples from our previous studies for a dimensional approach.
Table 1 gives an overview of the main outcome measures of our analysis, subdivided into the different study samples.
Table 1 Demographic variables of the samples
|
Study 1
|
Study 2
|
|
Low trait
ruminators
(n = 22)
|
High trait
ruminators
(n = 23)
|
Depressed patients
(n = 22)
|
Healthy
controls
(n = 23)
|
Variable
|
M
|
SD
|
M
|
SD
|
M
|
SD
|
M
|
SD
|
Age
|
22.32
|
3.88
|
21.70
|
2.69
|
27.14
|
6.15
|
25.35
|
5.75
|
Percent of
female participants
|
86.4
|
|
78.3
|
|
77.3
|
|
78.3
|
|
BDI-II
total score
|
1.95
|
2.26
|
8.57
|
5.80
|
24.14
|
11.85
|
2.13
|
1.96
|
RRS mean
|
1.54
|
0.22
|
2.67
|
0.17
|
2.59
|
0.50
|
1.73
|
0.39
|
LSAS mean
|
0.47
|
0.19
|
0.89
|
0.50
|
1.32
|
0.55
|
0.51
|
0.33
|
Note. BDI-II = Beck Depression Inventory II 58, RRS = Rumination Response Scale 9, LSAS = Liebowitz Social Anxiety Scale 59. |
2.2 Procedures
In the following section we will briefly describe the paradigm and the used questionnaires as well as the physiological measures we did in the experiment. Figure 1 gives an overview of the procedure and when which measurement and which questionnaire was done.
TSST. The Trier Social Stress Test (TSST) 17 is a highly reliable paradigm to induce psychological stress consisting of a free speech and a mental arithmetic task. We adapted the TSST according to the fNIRS-setting by performing a 7-min resting-state previous to two non-stressful control tasks and another 7-min resting-state after the TSST. Through this adaptation, the TSST delivered an ecologically valid and fNIRS-conform stress situation, that leads to post-event processing in rumination-vulnerable and/or anxious subjects 22,45. Due to saliva sampling, participants rested for another 45 min, however fNIRS was not recorded during this part of the post-stress phase. After arriving at the laboratory, participants completed several questionnaires assessing depression symptom severity (BDI-II), trait rumination (RRS), as well as social anxiety symptoms (LSAS). Previous to as well as after the stress induction, we assessed momentary affect (PANAS) and state rumination. Further, at several time points, subjective stress was measured on a Visual Analogue Scale (VAS) ranging from 0–100% as well as samples of salivary cortisol were taken using Salivettes. As this investigation focused on behavioral data assessed by the aforementioned questionnaires and fNIRS data during the post-stress resting-state, in the following we will briefly describe these measures, however more detailed information regarding the stress induction and general procedure is to find in 22,45.
Visual Analogue Scale (VAS). Throughout the experiment, participants rated their momentary stress levels on a scale ranging from 0–100%, where steps of 10% were marked at steps of one centimeter. The questionnaire comprised all ratings of the current day of measurement on one page, so participants were able to allow for their last ratings.
Beck Depression Inventory (BDI-II; Hautzinger et al., 2009). In order to screen depression symptom severity, we used the German version of the self-report questionnaire Beck Depression Inventory II. Regarding the previous two weeks, the occurrence of 21 symptoms is rated and symptom severity is assessed as a total score ranging from 0–63. Investigating psychometric properties across different populations and languages, respectively, Wang and Gorenstein 60 could observe overall high internal consistencies (Cronbach’s α around 0.9) as well as high retest reliability (mean interval of 2 weeks; r around 0.7–0.9).
Liebowitz Social Anxiety Scale (LSAS; Liebowitz, 1987). The LSAS is a screening measure for social anxiety disorder, comprising 24 social situations that are rated on a 4-point Likert-scale for level of fear (none, mild, moderate, severe) and avoidance (never, occasionally, often, usually) regarding the previous week. The resulting total score (range from 0 to 144) has been shown to have excellent psychometric properties (Cronbach’s α = .95) 61. Also, the LSAS was found to be capable of differentiating clinical as well as non-clinical samples 62,63 which is the reason why in the following we will consider LSAS-scores as a measure of levels of social anxiety. Note that we will report LSAS mean scores in order to account for potential missing values.
Ruminative Response Scale (RRS; Noelen-Hoeksema, 1991). Trait rumination was assessed using 22 items which are rated on a 4-point Likert scale ranging from "hardly ever" to "almost always". The total score (range from 22 to 88) has been shown to have high internal consistencies (Cronbach’s α = .88–92) 64–66. Note that we will report RRS mean scores in order to account for potential missing values.
Positive and Negative Affect Schedule (PANAS; 67. Using the PANAS we assessed momentary positive and negative affect. 20 items are rated using 5-point Likert scales ranging from 1 (“very slightly”) to 5 (“extremely”). Both subscales, positive (PA) and negative affecrt (NA) have acceptable internal consistencies in clinical and non-clinical samples 68,69 of ɑ = .85-.86 for NA and ɑ = .84-.89 for PA.
State rumination
We assessed stress-reactive rumination pre and post stress using adapted items from the RRS 9, ARSQ 70, PTQ 71,72. The 18 items were answered using a 5-point Likert scale ranging from 1 (“not at all”) to 5 (“very often”), totaling to a score between 18 and 90. Note that we will report mean scores in order to account for potential missing values. Subjects were instructed to rate whether the items were in line with their mental state during the last 10 minutes (see supplemental table 1).
fNIRS. During the stress induction as well as both resting-states, an fNIRS measurement was performed to assess cortical activation using a 46-channel continuous wave multichannel fNIRS system (ETG-4000 Optical Topography System; Hitachi Medical Co., Japan) with a sampling rate of 10 Hz. According to our regions of interest, we placed two frontal probesets and one parietal probeset with reference position to Fpz and Cz according to the 10–20 system 73 using an Easycap with sponge rings for additional fixation of the optodes (see table 2 and figure S3). Like this, a fixed inter-optode distance of 3 cm was set for all optodes. Changes in levels of oxygenated (O2Hb) and deoxygenated (HHb) hemoglobin were computed by means of the modified Beer-Lambert Law. Data preprocessing was done in MATLAB 74, using customized scripts, including interpolation of single noisy channels, correction of motion artifacts using Temporal Derivative Distribution Repair (TDDR) in order to remove spikes primarily caused by head movements in our case 75 and Correlation-based signal improvement (CBSI) 76 to combine both signals (O2Hb and HHb) in their merits: High sensibility (O2Hb) and high resilience to arousal artifacts (HHb). We further used bandpass-filtering to remove low-frequency baseline-drifts (below 0.01 Hz) and high-frequency noise (above 0.1 Hz). Then, a second step of channel interpolation followed in case of artifacts due to data correction. Afterwards, a global signal reduction was performed with a spatial gaussian kernel filter with a standard deviation of σ = 40 77. For data analysis, we calculated event-related averages for each trial including a 5 seconds baseline-correction for every region of interest (ROI).
Table 2 Regions of interest and corresponding probesets and channels, extrapolated based on the Colin 27 template 78. For probeset placement see also Figure S3
region of interest
|
probeset
|
corresponding channels
|
left inferior frontal gyrus (lIFG)
|
left frontal
|
6 7 9
|
left dorsolateral prefrontal cortex (lDLPFC)
|
left frontal
|
10 11 12
|
right inferior frontal gyrus (rIFG)
|
right frontal
|
18 19 21
|
right dorsolateral prefrontal cortex (rDLPFC)
|
right frontal
|
20 23 24
|
somatosensory association cortex (SAC)
|
parietal
|
25 26 27 28
30 31 32
35 36
|
Heart rate. We assessed heart rate during both resting-states as well as the stress induction using a 1-channel electrocardiogram in order to monitor heart rate variability as an index of the stress response. After disinfection of the corresponding skin areas, three standard Ag/AgCl ring electrodes were attached using Ten20 conductive paste. Electrodes were placed above the right clavicle, below the left costal arch and on the neck (reference). The signal was recorded using the BrainAmp ExG amplifier with a sampling rate of 1000 Hz and BrainVision Recorder Software (Brain Products, Munich, Germany).
Salivary cortisol. Saliva was collected in salivettes (Sarstedt AG & Co., REF 51.1534.500) and was stored at − 20°C and later thawed and centrifuged for 2 min at 1000g to collect saliva. Further analysis was performed with enzyme immunoassay (IBL International, Cortisol ELISA, REF RE52611) according to the manufacturer's instructions. Average cortisol levels were taken from duplicate runs if intra-assay variation was below 10%. Finally, daytime was regressed out of cortisol coefficients to account for circadian rhythm fluctuations that are not related to the TSST and values were log-transformed. Participants were instructed not to drink alcohol the day before the measurement, to sleep as long as they usually do and to perform no physical activities on the day of the measurement. Also, subjects were told not to drink or eat 30 min before the measurement started.
2.3 Data analysis
After data preprocessing, data analysis was done using R 79 and SPSS 80. Mixed models were fitted using the R packages lme4 81 and lmerTest 82 in order to obtain p-values using the Satterthwaite approximation. Graphics were plotted using the R package ggplot2 83 and MATLAB 74. Using the R-package MuMIn 84, marginal R² was computed as a measure of variance explained by the fixed effects in the mixed models. By reporting changes in R2 we are comparing the more complex with the corresponding less complex model.
In the following, we will analyze subjective stress, negative affect and state rumination. Further, we will analyze heart rate and salivary cortisol, but these analyses are to be found in the supplemental material as they were not related to our primary hypotheses. We investigated in how far social anxiety (LSAS) and trait rumination (RRS) play a role in each measure and set up mixed models with an increasing number of parameters. Note that we did not include BDI-II as a predictor due to its high correlations with social anxiety and trait rumination (see table 4) and issues of multicollinearity. Fitting mixed models offers the chance to include random effects accounting for non-independence 85, as well as to handle unbalanced data and include continuous predictors 86. First, we conducted a basic model consisting of the parameters set by the experimental design, such as time (model 1). For the two more complex models, we added z-standardized LSAS (model 2) and z-standardized RRS total scores (model 3) as main effects and interaction effects with time, accounting for social anxiety and trait rumination, respectively. As most complex models, we added the z-standardized RRS (model 4) and LSAS scores (model 5) only as main effects to the previous models. Like that, we controlled for trait rumination and social anxiety and investigated whether there is unique variance explained by the corresponding measure. Note that all predictors were included as fixed effects whereas due to repeated measurements on each subject, intercepts for every participant were modeled as random effects. Table 3 gives an overview over the models conducted for each measure.
For the analysis of cortical oxygenation, we fitted a MANOVA in order to increase statistical power and assess multiple dependent variables simultaneously. For this, we added the same parameters as predictors as in our previous mixed models.
Prior to our analyses, we checked assumptions of the used methods and corrected for potential violations of assumptions. Note that corrections were not applied unless stated otherwise. In case of subjective stress ratings and state rumination, normality assumption was not met but due to insufficient success of common transformations and the robustness of the mixed models 87, we decided to perform the analyses as planned. Furthermore, in case of a detection of outliers, we will report the corrected analyses in detail and summarize results of the uncorrected analyses.
Note that physiological data of heart rate was missing in five subjects, leaving 85 participants, and one participant had to be excluded in the analyses of negative affect for study 1, due to missing data.
Table 3 Parameters included in the mixed models for each outcome measure
Name
|
parameters
|
Model 1: Basic model
|
Time
|
Model 2: Basic model including LSAS
|
Time + LSAS + Time:LSAS
|
Model 3: Basic model including RRS
|
Time + RRS + Time:RRS
|
Model 4: Basic model including LSAS and correcting for RRS
|
Time + RRS + LSAS + Time:LSAS
|
Model 5: Basic model including RRS and correcting for LSAS
|
Time + LSAS + RRS + Time:RRS
|
Note. In case of subjective stress and heart rate, time was modelled as linear and quadratic term. Colons symbolize interaction effects. All included parameters except for time were z-standardized. |
Table 4 Correlation matrix of variables of interest
|
BDI-II
|
RRS
|
RRS
|
.603***
|
-
|
LSAS
|
.600***
|
.519***
|
Note. BDI-II = Beck Depression Inventory II 58, RRS = Rumination Response Scale 9, LSAS = Liebowitz Social Anxiety Scale 59. ***p < .001. |