2.1 Participants
Fifty-five participants (26 IGD and 29 RGUs) were recruited from college campuses through WeChat and advertisements. All the participants were right-handed college students and free of any substance dependences (e.g., cocaine and alcohol) and other behavioral addictions (e.g., problematic gambling). None of them reported historical or current psychiatric diseases (e.g., depression and anxiety), brain surgery/brain injury, and neurological disorders. Additionally, six participants were excluded due to choosing the same option in more than 90% of the trials (1 RGU) or larger head motion than 3 mm/degree in any direction (2 IGD and 3 RGUs) during fMRI scanning. Thus, twenty-four IGD participants (10 women and 14 men) and twenty-five matched RGU participants (6 women and 19 men) were included in the final data analyses. The demographic characteristics of the IGD and RGU groups are listed in Table 1.
Table 1
Demographic and clinical characteristics of IGD (n = 21) and RGU (n = 23) groups
|
IGD (M ± SD)
|
RGU (M ± SD)
|
t value
|
p value
|
Age(years)
|
20.17 ± 1.97
|
21.00 ± 2.52
|
-1.29
|
0.20
|
Gender(F/M)
|
10/14
|
6/19
|
n/a
|
0.19
|
Year of education
|
16.54 ± 1.38
|
17.12 ± 2.00
|
1.17
|
0.25
|
Time spent on games per week (hour)
|
17.29 ± 4.47
|
16.00 ± 3.67
|
1.11
|
0.27
|
IAT
|
63.25 ± 6.17
|
41.08 ± 7.49
|
11.28
|
< 0.001
|
DSM
|
5.63 ± 0.88
|
2.80 ± 1.44
|
8.24
|
< 0.001
|
Self-Esteem
|
27.83 ± 4.28
|
30.28 ± 3.61
|
-2.17
|
0.035
|
BDI
|
11.08 ± 8.12
|
6.76 ± 5.33
|
2.21
|
0.032
|
The participants were selected based on the types of online games they played, Internet Addiction Test (IAT) developed by Young (41), nine-item criteria proposed by the DSM-V committee (2) and amounts of time for game playing. The type of multi-player competitive game was chosen (e.g., Stimulate the battlefield, Arena of Valor); in these game environments, the player could interact with other players via the online environment. The Young’s IAT comprised of 20 (5-point) Likert-type items; the total score ranging from 20 to 100 reflects the severity of IGD. Both the IAT and DSM-V criteria were exactly translated into Chinese for the sake of Chinese participants. According to previous studies, the participants were classified as IGD according to the following criteria: (1) scored 55 or higher on Young’s IAT; (2) met at least five DSM-V criteria; (3) played online games for at least two hours a day and had at least two years of game experience (38, 39). For the RGU participants, they scored lower than 50 on the Young’s IAT and met less than five DSM-V criteria. To control the effect of the patterns of gaming, the RGU group also satisfied the third criteria mentioned above and had almost the same playtime as the IGD group. However, the RGU group reported that they prioritized their academic work/examination over games without developing psychological dependence.
The two groups showed no significant difference in age, education year, sex ratio or time spent on Internet games (Ps> 0.05; Table 1). However, the IGD group reported higher scores on Young’s IAT and more items of DSM criteria than the RGU group (Ps< 0.001, Table 1). All the participants were paid after completing the whole experiment.
2.2 Task and procedure
Task and Materials
In the present study, we adopted a modified version of a causal attribution task (26). In the original task, the participants were presented with a series of sentences describing self-relevant (e.g., I like John, John hits me) and other-relevant (e.g., Lily likes David, Lily hits David) interpersonal events. Each sentence comprised a subject, a verb and an object. In self-related events (happening between the self and another person), the participants were involved in the situation; in other-related events (happening between two other persons), the participants were observers, and the other persons were not known to them (strangers). The participants were instructed to imagine the event happening, and then evaluate using a four-point Likert scale that asked how likely was it that they attributed the cause of an event to themselves or others (1= very unlikely, 2= moderately unlikely, 3= moderately likely, 4= very likely).
To detect the self-serving bias in both the real-world and game-world contexts, we reconstructed interpersonal events that could occur not only in the real world but also in the game world. We screened 40 Chinese verbs to construct one-sentence interpersonal events. First, a series of positive and negative two-character verbs were collected and rated by 30 college students on a 9-point Likert scale for arousal, familiarity and valence. Next, ten game players (not participating in formal experiment) were invited to choose the verbs depicting events that could occur in both real world and game world. Finally, forty verbs (20 positively valenced and 20 negatively valenced) were selected by all the players and determined to construct interpersonal events. These two types of verbs differed in valence (t = 26.17, p < 0.001) but not in arousal or familiarity (ps > 0.05).
Additionally, each participant’s real name and game name were collected and used to describe self-related events that could occur in the real-world and game-world contexts, respectively. Similarly, the others’ real names (randomly generated using Chinese common first names and last names) and game names (randomly collected via Internet search engines) were used to describe other-related events that could occur in real-world and game-world contexts, respectively.
Each verb was used four times to construct four different categories of interpersonal events according to the combinations of Target (self, other) and Context (real world, game world). Thus, 160 interpersonal events were obtained — 40 positive self-related events, 40 positive other-related events, 40 negative self-related events and 40 negative other-related events. In the self-related events, the participants’ real/game names were used to represent the ‘self’ in the real/game world, respectively. That is, the word ‘I’ or ‘me’ in the original task was replaced with the participant’s real name or game name. The other persons’ names in the self-related events were also real names/game names in the real/game world, respectively. In other-related events, the others’ real/game names were used to represent the ‘other’ in the real/game world, respectively.
Procedure
The total time of this experiment was approximately an hour. Upon arriving at the laboratory, the participants completed a series of self-reported questionnaires, including basic demographic information, the Internet Addiction Test, DSM-V criteria, Self-Esteem Scale (42) and Beck Depression Inventory (BDI) (43). Following the task instruction, a shortened sample (ten trials) of the task was conducted outside the scanner to familiarize the participants with the experimental procedure. Afterwards, the participants were asked to complete the formal task inside the scanner.
The formal experiment included two runs — that is, the real-world context and game-world context. At the beginning of each run, the participants were instructed to imagine that the interpersonal events occurred either in the real world or game world and then made causal attributions on a four-point scale. The order of the contexts was counterbalanced between the participants in each group: 12 IGD and 12 RGU participants were randomly assigned to the game-world context first, and the remaining participants completed the real-world context before the game-world context. In each run, 80 trials with 20 trials for each of the four experimental conditions (e.g., self-positive, self-negative, other-positive and other-negative) were pseudorandomized. In terms of self-related trials, ‘self’ was assigned to the subject position or object position and was the target of evaluation; in terms of other-related trials, the subjects or objects would be the targets of evaluations. The positions of the targets were counterbalanced across trials. The experiment began with a black fixation cross presented on a white screen for 2000 ms, followed by the presentation of stimulus interface displaying one interpersonal event and a 4-point scale (Fig. 1). The participants were required to respond within 6000 ms for each trial, and after which a red circle would appear around the selected option (lasting for 1000 ms). Between trials, an inter-trial interval of 200–3200 ms (jittered) was used, during which a fixation cross was shown.
2.3 Behavioral data analysis
All behavioral data analyses were conducted using Statistical Package for the Social Sciences version 23.0 (SPSS Inc, Chicago, IL, USA). Considering that individuals may show different levels of self-other bias in causal attribution, the attribution rating difference scores between self and other were initially calculated for each participant, taking the other condition as the baseline (26). Next, Group (2; IGD, RGU) × Context (2; real world, game world) × Valence (2; Negative, Positive) repeated-measures analyses of variance (ANOVA) was performed, with Context and Valence as with-in subject variables and Group as the between-subject variable; the attribution rating difference score (self minus other) was defined as dependent variable, with BDI and self-esteem scores included as covariates. Bonferroni correction was employed for multiple post hoc comparisons. Additionally, independent sample t-test and chi-squared test were used to compare the demographic data between the groups. All the results with significant effects were reported at the p < 0.05 level.
Pearson’s correlation analyses were conducted for the attribution rating difference scores and questionnaire scores (Self-Esteem Scale and BDI) for RGU and IGD groups, respectively. The correlation results were considered significantly after correction using Sequential Bonferroni correction (44). Fisher’s Z-test was applied to compare the correlations between these two groups.
2.4 Imaging acquisition and pre-processing
Brain images were acquired using a Siemens Trio 3T scanner at the Functional MRI Laboratory (East China Normal University, Shanghai). An 8-minute structural scan was performed before 24 minutes of task-related scan for the normalization and coregistration of the functional data. Structural images were collected using a T1-weighted three-dimensional spoiled gradient-recalled sequence (192 slices; slice thickness= 1.0 mm; skip= 0 mm; echo time TE= 2.98 ms; repetition time= 2530 ms; flip angle= 7°; inversion time= 1100 ms; field of view= 256×256 mm; in-plane resolution= 1×1 mm) for whole-brain coverage. The functional MRI data were obtained using a gradient-echo EPI T2-sensitive pulse sequence in 33 slices (interleaved sequence, 3 mm thickness; echo time TE= 30 ms; flip angle= 90°; repetition time= 2000 ms; field of view= 220×220 mm; matrix 64×64; gap = 0.3 mm).
Data preprocessing was conducted using the statistical parametric mapping software package (SPM8, http://www.fil.ion.ucl.ac.uk/spm/spm8). The first five functional images were discarded due to scanner equilibrium effects. The remaining images were manually reoriented to the anterior-posterior commissure (AC-PC) line, slice-timed, and realigned to the first volume with a mean functional image created. Next, structural images were co-registered to the mean image and spatially normalized to the MNI space, resulting in an isometric voxel size of 2×2×2 mm3 and spatially smoothed using an 8-mm full-at-half-maximum Gaussian kernel.
2.5 Imaging data analysis
2.5.1 First-level regression analysis
In the first-level analysis, two different general linear model (GLM) models were created for each participant. First, a factorial model (GLM1) was applied to confirm the participants’ blood oxygen level dependence signal corresponding to each task condition. For both groups, eight conditions were determined according to Context (Real, Game), Valence (Positive, Negative) and Target (Self, Other). The timings of stimulus onset and durations of the response time were convolved with the canonical haemodynamic response function. Second, we also built a parametric model (GLM2) to investigate the brain responses associated with the self-attribution ratings. The rating score was adopted as a parametric regressor to the different weights of the self-related positive/negative trials for each context. In both models, the six head-movement parameters were included as regressors of no interest. To improve the signal-to-noise ratio, a high-pass filter (cut-off period = 128 s) was applied by filtering out low-frequency noise. Error trials were excluded, and the GLM models were independently applied to each voxel to identify the voxels that were significantly activated. The statistical images created at the individual level were then entered into further analyses.
2.5.2 Second-level group analysis
Region of interest (ROI)-based analysis
ROI-based analysis was performed to detect group differences between conditions for GLM1. The ROIs were determined based on functional-defined areas that were identified previously as being important for self-serving bias. The regions including the dmPFC (x = 12, y = 56, z = 44), vmPFC (x = 12, y = 52, z = - 10) and precuneuspcc (ax = 10, y = - 50, z = 30) described in (28, 45) were selected, which also have been used to investigate the processes of self-positivity bias (45). All the ROI masks were 10-mm-radius spheres centered at the standard MNI coordinates.
For each ROI, the beta values of the eight conditions were extracted from the statistical images generated from the first-level analysis using the REST toolbox (Version 1.8, https://www.nitrc.org/projects/rest/) and then were subjected to analysis of variance to assess the main effects and interactions. In examining brain activations specific to the self (relative to other) during the process of causal attribution, we calculated the activation difference between self and other and simultaneously tested how the activation difference was modulated by Group, Context and Valence, analogy with behavioral data analyses. Thus, for each ROI, Group (2; IGD, RGU) × Context (2; real world, game world) × Valence (2; Negative, Positive) repeated-measures ANOVAs was conducted, with the difference scores of beta values between self and other defined as the dependent variable. Bonferroni correction was applied when multiple statistical tests were performed simultaneously. Correlation analyses were performed between the difference scores of beta values (self minus other) in specific conditions that demonstrated group differences and attribution rating difference scores.
Parametric analysis
For group-level analysis of GLM2, two-sample t tests were conducted based on the subject-specific estimates of the parametric regressors at each voxel. This allowed us to identify the brain areas that showed differential associations with self-attribution rating scores in the IGD and RGU groups. The results were reported when significant at a voxel-level threshold of p < 0.001 uncorrected and a cluster-level threshold of p < 0.05 family wise error (FWE) corrected.