Participants
Sixty-four healthy subjects between 20 and 73 years old (M = 36.88; SD = 13.62; female (n = 38) were enrolled in this study. They have an average of 11 years of education (M = 11.1, SD = 3.09, range 2 – 20 years of education) and reported no history of psychiatric or neurological conditions. Participants were recruited by accessibility from the general population.
To control for individual differences in executive function, all participants were screened for executive functioning using the INECO frontal screening 36. This is a sensitive instrument for assessing executive dysfunction (see supplementary instruments). The mean score for our sample was 20.92 points, just above the 18-point cut-off for the Chilean population 37.
Every procedure of this research was approved by the Universidad Diego Portales ethics committee. All participants signed an informed consent according to the principles of the Declaration of Helsinki and received a payment as retribution for their collaboration and time.
Procedure
Participants were contacted via telephone and/or social network media and invited to attend the laboratory to complete various scales tapping on social adaptation, loneliness, and social network. fMRI scanning session was carried out on a separate day using a Siemens Avanto 1.5 T scanner equipped with a standard head coil.
Self-report assessment
Social Adaptation Self-administered Scale (SASS)
The SASS 38 is a 21-item scale that explores social adjustment and motivation. The items tap levels of engagement with the environment (e.g., “Do you feel able to organize your environment according to your wishes and needs?”), family relationships (e.g., “How frequently do you seek contact with your family members?”), friendships (e.g., “What value do you attach to the relationship with others?”), and engagement to work (“How interested are you in your occupation?”), among others. Responses are recorded via a 4-point Likert scale (from 0 to 3). The total score ranges from 0 to 60, corresponding to minimal and maximal social adjustment. Scores within 35 and 52 are considered normal 38. The instrument showed good reliability in our sample (α = .73).
University of California Loneliness Scale (UCLA)
The UCLA 4 is a widely used measure of the subject’s feelings of loneliness and levels of satisfaction with social experiences. For the present study, an abbreviated version of the UCLA was used 39, comprising the following 8 items: (1) “I can find companionship when I want it”; (2) “I feel left out”; (3) “I feel isolated from others”; (4) “I lack companionship”; (5) “There is no one I can turn to”; (6) “I’m unhappy with being so withdrawn”; (7) “People are around me but not with me”; (8)“I am an outgoing person”. Responses were recorded via a 4-point Likert scale, ranging from 1 (never) to 4 (always). The total score is obtained by inverting positive items and summarizing the score of all items. Thus, larger scores indicate a more pronounced experience of loneliness. The UCLA has shown good levels of reliability as evidenced by an α coefficient of .89 4. We used a shorter 8-item version that showed good reliability levels in our sample (α = .85).
The revised Lubben Social Network Scale (LSNS-R)
The LSNS-R is a 12-item scale that measures the size and complexity of social relationships 40. It consists of two scales, one tapping on kinship/family ties (e.g., “How many relatives do you see or hear from at least once a month?”), and other evaluating non-kin / friendship ties (e.g., “How many friends do you feel close to such that you could call on them for help?”). Items are rated on a scale from 0 to 5, with 0 indicating the lowest frequency and number of contact with others and 5 indicating the highest frequency/number of contacts with others. The total score is obtained by summarizing all items’ scores. The maximum total score is 60; with higher scores reflecting bigger and stronger social ties. The scale shows high reliability in old adults (α = 0.90) 41 and young populations (α = 0.83) 42. A similar level of reliability was obtained in our sample with a Cronbach coefficient of α = 0.85.
Images data collection
Images for this study were obtained from a Siemens Avanto 1.5 T scanner equipped with a standard head coil. We obtained 10-minute resting-state fMRI recordings from 61 participants, (data from 3 participants was excluded during pre-processing because of the low quality of their recordings). Functional spin-echo volumes were acquired in sequentially ascending order, parallel to the anterior-posterior commissures, covering the whole brain. The following parameters were used: TR = 3.3 sec; TE = 50 ms; flip angle = 90°; 36 slices, matrix dimension 4 x 64; voxel size in plane RL 3.59mm; AP 3.59mm; slice thickness = 4 mm; number of volumes = 190. Participants were instructed to lay still, keep their eyes closed, and not to think about anything in particular.
Data analyses
Self-report data
Descriptive data analysis for social adaptation, loneliness, social network, and executive functions are displayed in Table 1. We also conducted correlation analyses including sociodemographic data and variables of interest (see supplementary table 1)
Behavioral data were analyzed with R studio 43. We first conducted a hierarchical multiple regression to evaluate the predictive value of loneliness and social network on social adaptation. Hierarchical multiple regression models are useful to evaluate and compare the predictability of groups of independent variables that are entered at different steps of the analysis 44. In other words, the main idea of the analysis is to test whether variables entered in subsequent steps, have better predictive value compared to those entered in a former step of the analysis. As for the present analysis, we first specified a base model including our control variables (executive functions, age, education, and gender). These variables did not have any significant effect on social adaptation. In a subsequent step, our measure of loneliness was incorporated into the group of variables. For the last step, we specified a model that also included the social network measure (LSNS scores).
Table 1. Sample’s performance in the behavioral assessment
|
N = 64
(37 female; 27 male)
|
|
M (SD)
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Range
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SASS
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42.27 (7.12)
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27 - 55
|
UCLA
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8.65 (5.27)
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0 - 24
|
LSNS-R
|
34.53 (10.47)
|
8 - 51
|
IFS total score
|
20.92 (3.80)
|
8.5 - 28.0
|
Abbreviations: SASS: Social Adaptation Self-evaluation Scale); UCLA: University of California Loneliness Scale; LSNS-R: Lubben Social Network Scale; IFS: INECO Frontal Screening.
Resting-state fMRI data
Pre-processing
First, we discarded the first three volumes of each subject’s resting-state recording to ensure that magnetization achieved a steady state. Images were then pre-processed using the Data Processing Assistant for Resting-State fMRI (DPARSF V2.3) 45 an open-access toolbox that generates an automatic pipeline for fMRI analysis. The DPARFS processes the data by recruiting the Statistical Parametric Mapping (SPM12) and the Resting-State fMRI Data Analysis Toolkit (REST V.1.7). In line with recommendations 46, pre-processing included slice-timing correction (using the middle slice of each volume as the reference scan) and realignment to the first scan of the session to correct head movement (SPM functions). To reduce the effect of motion during image acquisition as well as physiological artifacts 47, we controlled two motion parameters (i.e. Translation, rotation; See supplementary table 2), CFS, and WM signals (REST V1.7 toolbox). Motion parameters were estimated during realignment, and data from three subjects were discarded due to exceeding the maximum head movement (3mm and 3o). CFS and WM masks were derived from the tissue segmentation of each subject’s T1 scan in native space with SPM12 (after co-registration of each subject’s structural image with the functional image). Then, images were normalized to the MNI space using the echo-planar imaging (EPI) template from SPM 48, smoothed using an 8-mm full-width-at-half-maximum isotropic Gaussian kernel (SPM functions), and bandpass filtered between 0.01 and 0.08 Hz. None of the participants showed movements greater than 3 mm (M = 0.05, SD = 0.04) and/or rotations higher than 3° (M = 0.05, SD = 0.03).
Functional connectivity analyses
We explored associations between resting-state functional connectivity data and scores from our predictor variables, loneliness (UCLA scores), and social network (LSNS-R score). First, for each subject, we extracted the mean time course of the BOLD signal in each of the 116 regions of the Automated Anatomical Labelling Atlas (AAL) 49, by averaging the signal in all voxels comprising each region. Second, we constructed a connectivity matrix for each subject indicating the strength of association between all pairs of regions (Pearson’s correlation coefficient; DPARSF toolbox). Third, we performed a Fisher z-transformation. The resulting functional connectivity (FC) correlation coefficients between all pairs of regions (AAL atlas) were used to perform Spearman’s correlations with the scores of each predictor: loneliness (UCLA score), and social network (LSNS-R score). Following procedures from recent research 50–52 significance thresholding of neuroimaging results was set to p ≤ .001 (uncorrected). Uncorrected statistical threshold has been used previously to avoid the detrimental effects of liberal primary thresholds on false positives (especially in modest sample sizes such as ours) 53–55.
Principal component analyses (PCA) of fMRI data
To reduce the dimensionality of the FC correlates of each variable of interest (loneliness, social network, and social adaptation) to include in path analyses, we performed three PCA in RStudio software. We retained the first component for each variable (i.e., the one that accounted for most of the variation in FC correlates) to include in a posterior path analysis model.
Multilevel Path analysis
To evaluate the combined effect of self-report and brain indicators of social adaptation, we performed a path analysis using the laavan package 56 in JASP statistical software. This technique aims to test a theoretical model characterized by hypothesized relationships between a set of variables. These patterns are specified a priori (following a given theoretical criterion), and the fit of said model is evaluated with the data obtained. Goodness of fit is indicated by various parameters, including the X2 statistic (non-significant), NFI(>0.95), GFI (>0.95), CFI (0.95-1.00), RMSEA (<0.08), IC(≤ 0.05), SRMR (<.08). Path models can be used as an extension of the multiple regression model but with the virtue of simultaneously analyzing the relationships between the independent and dependent variables 57. Based on this approach, we proposed a model incorporating brain variables with purely behavioral ones to generate an integrated model (multi-level perspective).