Baseline characteristics
The groups did not differ in average age or any of the baseline questionnaires (all p > .1; see Table 1). The sample demonstrated light depressive symptoms (BDI-II score: 18 0.855; range 0-48) where 71% of participants scored 14 (the cutoff value representing mild depression). At baseline, prior memory bias was assessed using the SRET. After exclusion of one participant (due to total memory failure), overall positive recall bias was high (0.74 0.23), while conditions did not differ significantly (F(2, 92)=2.497, p=.088, =.051).
Participants spent on average 27.56 (± 36.72, range 0 – 559) seconds per prompt and used the app on average 7.2 (± 0.65, range 1.27 – 22.93) minutes per day. Compliance rate was high (~ 96%) and did not differ between conditons (t(59)= 0.46, p =.65).
Table 1
Baseline characteristics. Counts, means, standard errors (SE) and group comparisons of baseline measurements
|
Positive training
n=32
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Sham training
n=32
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No-training
n=32
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Group comparisons
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Gender (F/M)
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30/2
|
24/8
|
26/6
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χ2 (2) = 4.2, p =.123
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Language (D/G)
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23/9
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21/11
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22/10
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χ2 (2) = .29, p =.865
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Age
|
23.3 ± 1.06
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23.2 ± 0.807
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25.5 ± 1.56
|
F(2, 93)=1.19, p=.309
|
BDI-II
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17.9 ± 1.73
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16.5 ± 1.17
|
19.5 ± 1.49
|
F(2, 93)=1.01, p=.37
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RRS
|
48.5 ± 1.98
|
52 ± 2.06
|
54 ± 2
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F(2, 93)=1.93, p=.151
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PMHS
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24.4 ± 0.983
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24.5 ± 0.797
|
23 ± 0.836
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F(2, 93)=0.96, p=.386
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DASS depression
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13.6 ± 1.77
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10.9 ± 1.37
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12 ± 1.46
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F(2, 93)=0.80, p=.451
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DASS anxiety
|
8.88 ± 1.33
|
9.69 ± 1.36
|
9.94 ± 1.47
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F(2, 93)=0.50, p=.61
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DASS stress
|
14.3 ± 1.34
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14.4 ± 1.52
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16.1 ± 1.51
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F(2, 93)=0.16, p=.852
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SRET
|
0.79 ± 0.23
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0.76 ± 0.23
|
0.67 ± 0.22
|
F(2, 93)=2.497, p=.088
|
Note. F = Female, M = Male, D = Dutch, G = German.
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|
Memory bias
Training effects on memory bias
To explore the influence of the training on overall memory bias, the average recalled event evaluation of the three memory bias measurements at the start and the end of the study were investigated, but no significant time x condition interaction was found (F(1, 60)=.003, p=.954, =.00002). A main effect of time was found, (F(1, 60)=20.417, p<.001, =.122), indicating an overall increase towards positive bias in both conditions ( M=10.64). A main effect of condition was found, (F(1, 60)=7.375, p=.009, =.068). Post-hoc pairwise comparisons revealed that the sham condition on average scored a more positive bias than the positive condition ( M=5.14, p=006, see Figure 1). Lastly, there were no modulating effects of prior positive bias and BDI-II score (see supplemental materials for more details).
Given the difference in positive memory bias between the positive and sham condition, we explored the difference between the conditions on the scores reported during the training (i.e. when either recalling a positive event or rating the contextual environment). After averaging all training recall scores per participant, conditions statistically differed from each other (F(1, 60)=4.475, p=.039, η2=.069), where the sham condition rated their contextual environment higher (M=20.9) compared to the positive training condition (M=14.8).
Training effects on memory bias proportional scores
After creating a sum score of overall positive memories per participants, a chi-square test for the positive training condition was performed, revealing no significant valence (4 levels) x time (2 levels) interaction ( (3, N=62)=6.31, p=.097). For the sham condition, however, a significant interaction was demonstrated (3, N=62)=10.02, p=.018), with an increase in positive scores (value=3) between baseline and post-training, as revealed by a post-hoc test on the standardized residuals (resid=3.088, p=.016; see Figure 2).
Changes in memory bias over time
Change of the memory bias score was investigated with a linear mixed-effects model, including a random intercept and slope per participant. No time x condition effect was found, (F(1, 60)=.388, p=.535). A main effect of time (F(1, 60)=16.21, p < .001) revealed a similar change in both conditions, with higher scores on the third day and lower scores on the fifth day (see Figure 3). This effect was further explored in a supplemental analysis on the effect of time of day on mood (see supplemental materials).
Changes in mood over time
Effects of the training on changes in mood over time were explored using a linear mixed-effects model. No time x condition interaction on positive mood (F(1, 59.98)=.407, p=.526) nor negative mood was found (F(1, 59.99)=1.099, p=.299). A main effect of time (F(1, 59.98)=9.36, p=.003) as well as a main effect of group was found (F(1, 59.995)=5.089, p=.028) on positive mood. Post-hoc pairwise comparisons did not show a significant difference in positive mood between the positive and sham condition, irrespective of time. A main effect of conidition on negative mood was found (F(1, 59.998)=6.75, p=.012), suggesting that the sham condition reported a lower negative mood compared to the positive condition ( M=-8.38, p=.036), irrespective of time. Lastly, as a supplemental analysis, we explored how changes in mood across the day differed between the two conditions, where moods were significantly more positive in the morning compared to the evening (see supplement materials for more details and results).
Autobiographical memory
The average number of specific memories recalled per condition did not significantly differ (F(2, 92)=.393, p=.676, =.008; see Figure 4). In addition, the influence of baseline memory bias, as measured by the SRET, on the number of specific memories recalled per training condition was explored. No pre-memory bias score x condition interaction on the number of specific memories was found (p=.935).
Next, we explored if there was a differential effect of the number of recalled specific memories per condition on positive and negative memories. No significant valence x condition interaction was found (F(2, 92)=1.425, p=.246, =.006). A main effect of valence was found (F(1, 92)=8.732, p=.004, =.019), suggesting that irrespective of condition, more specific positive memories (M=3.53) compared to negative memories (M=3.14) were recalled.
False memory
Studied items were recognized better than chance at baseline, (mean d-prime=0.92, versus chance level of 0), t(95)=17.71, p < .001), as well as endorsement of critical lures, (mean d-prime critical lures=1.2, versus chance level of 0), t(95)=19.84, p<.001). The number of falsely endorsed critical lures differed per valence at baseline, (F(2, 186)=3.985, p=.02, =.017) with a higher endorsement for positive compared to negative critical lures, t(95) =2.92, p=.004. The time x condition interaction was neither significant for recognition (F(2, 92)=.767, p=.467, =.007), nor for recall data (F(2, 91)=.425, p=.655, =.004). No main effect of time or condition was found (p>.5; see Figure 5).
Next, we explored the influence of baseline memory bias, as measured by the SRET, on the false recognition memory bias difference score between the two sessions for the two training conditions. No pre-memory bias score x condition interaction effects were found on false recognition memory bias, (p=.899) or on the recall data (p=.055).
Symptoms
There were no significant time x condition interaction effects for the BDI-II (F(2, 92)=.932, p=.397, =.003), RRS (F(2, 92)=.434, p=.649, =.0008), PMHS (F(2, 92)=.2, p=.819, =.0004), and the three subscales of the DASS (depression: F(2, 92)=1.348, p=.265, =.004; anxiety: F(2, 92)=.569, p=.568, =.002; stress: F(2, 92)=.967, p=.384, =.004). Significant main effects of time were found for the BDI-II (F(1, 92)=14.815, p<.001, =.023, M=2.71), RRS (F(1, 92)=5.391, p=.022, =.005, M=1.65, and the three DASS subscales (depression: F(1, 92)=4.958, p=.028, =.007, M=1.41; anxiety: F(1, 92)=7.365, p=.008, =.01, M=1.39; stress: F(1, 92)=10.973, p=.001, =.022, M=1.65), indicating an overall decrease in these measures.
In addition, the influence of SRET baseline memory bias on the post-BDI and post-RRS score was examined for the two conditions. No pre-memory bias score x condition interaction effects on the BDI score, (p=.157) nor on the RRS score (p=.132) were found.
Since both conditions yielded similar increases in positive memory bias, we explored the transfer to depressive symptoms after combining both training conditions and compared this to the no-training (i.e., control) condition. The time (baseline, post-training) x condition (trainings vs. no-training) interaction effect was not significant for the BDI (p=.243), RRS (p=.621), PMHS (p=.619), and DASS subscales (depression: p=.103; anxiety: p=.815; stress: p=.472).
Follow-up after COVID-19 outbreak
Participants repeated the PMHS and the DASS questionnaires online. Conditions did not differ from each other on either the PMHS or DASS scales (all p-values >.2). In addition, there were no time (post-training, follow-up) x condition interaction effects (all p-values >.8), indicating that the three conditions did not differentially change in depressive symptoms, anxiety symptoms, stress and positive mental health from post-training to follow-up (see supplements for Figure S3 and S4).
Since training effects were similar in both the positive and sham training, we combined the active and sham group and compared those who responded most to the training (i.e., responders) to those who did the least (i.e., non-responders). To identify responders and non-responders, participants were classified as having more change towards a positive memory bias, as measured by the difference score in ESM memory bias between baseline to post-training, compared to the median change (median=7.67; i.e., responder) versus less change or change towards a negative processing style (i.e., non-responder). A time (post-training, follow-up) x respond type (responder,non-responder) ANOVA revealed no significant interactions for the PMHS (p=.807), DASS depression subscale (p=.093), and DASS anxiety subscale (p=.161). However, a significant interaction effect for the DASS stress subscale was observed (F(1, 25)=6.088, p=.021, =.079). A post-hoc pairwise comparisons t-test showed that the non-responder group showed a significant increase in stress levels between post measurement (ΔM=8.4, p=.007), whereas the responder group remained stable over time (ΔM=-1.5, p=.712; see Figure 6).