Main findings
Two weeks of ABM training significantly reduced blinded clinician-rated symptoms as compared to a control condition in a group of patients previously treated for depression and with various degrees of residual symptoms. Relatively more positive change in AB was associated with symptom improvement. ABM versus placebo did not differentiate changes in AB without taking symptom changes into account, meaning that a beneficial effect of ABM despite no intervention dependent change in AB was found. This means either than ABM is working by some other mechanism or that the measurement of AB is not very robust. Corroborating cognitive models of emotional disorders, the degree of symptom improvement increased with degree of relatively more positive bias across groups. AB change may therefore be a useful clinical measure since it is sensitive to individual differences in clinician rated symptoms, and may predict treatment outcome.
A larger intervention independent effect was also found for self-reported depression and self-reported anxiety. Post hoc exclusion of participants with high HRSD scores had no impact on the ABM effect, and further analyses showed that the results are robust against imputations and compliance rates.
In addition to being among the strongest predictors for recurrence in depressive disorder [
4] residual symptoms also cause significant functional impairments, manifested in a variety of domains, including work and leisure activities [
18]. The latter is important because one aim of depression treatment is to restore the patients’ previous level of functioning. However, residual symptoms in depression have traditionally not been the target of treatment trials. Given the recurrent nature of the illness, we suggest that this should change and that our study provides an example of how this may be done.
It is not clear why ABM versus placebo did not differ as measured by the BDI-II self-report scale, but instead showed a relatively large improvement in both the ABM- and placebo. In another recent study, fifty-two participants in a major depressive episode (MDE) was recruited for ABM training and a similar significant change in BDI-II in both the ABM- and placebo was found [
19]. The lack of association between ABM and self-reported symptoms was not expected from the pre-registered hypothesis and interpreting the source of this lack of evidence is speculative. However, self-report may be more influenced by placebo or expectation effects. A general effect of the intervention may also be linked to cognitive training effects present in both training conditions. In contrast to general improvement in self-reported symptoms, increased depression was found within the placebo group for clinician rated depression. Further explorations of this pattern showed that a HRSD change was only statistically significant within the ABM group, indicating that the negative changes within the placebo group did not drive the observed ABM effect. The intervention may target the exact thing that worsens in the placebo group, but may also improve something else and thereby influence different domains that cancel each other out. Hereunder, clinical assessment might drive the participants’ attention towards depression symptoms and both positive- and negative assessment effects could therefore influence the change patterns in ABM and placebo. An assessment only group will help in distinguishing assessment effects- from ABM in future studies.
Symptom assessment based on both syndrome diagnoses, self-rated and blinded clinician-rated symptoms represent strength in the current study and the results clearly underpin the importance of comprehensive clinical evaluations in future research. The current study used a training task that earlier has shown to reduce self-rated symptoms first after 4 weeks [
14]. The sample size (statistical power) and sample characteristics, like degree of residual symptoms, may explain why the current study found intervention dependent symptom changes immediate after training.
Participants completed the training in their homes, which is an advantage as it makes it more feasible as a treatment. Furthermore, compliance was high (see Additional file
2) which is important and contrary to the conclusion in a recent review of meta-analyses [
11]. These authors conclude that ABM paradigms are most effective when delivered in the laboratory rather than at home. Specifying the scheduling of the training sessions individually might have increased motivation to do and focus on the task.
Implications
The results suggest that ABM does indeed exert an effect on blinded clinician-rated symptoms. The small effect sizes could be considered clinically non significant in treatment trials. However, it is not clear how to interpret clinical relevance of small HRSD score changes in this group. The HRSD and BDI-II is only moderately correlated and may partly reflect different depression constructs [
20]. It is unclear whether the effect of ABM would increase and generalize across self-rated and clinician-rated symptoms if treatment continued for longer. Although residual symptoms are predictive of risk of relapse, it will be important to test in longer-term prevention studies whether the observed beneficial effect of ABM could translate into clinical relevance like reduction of relapse.
There is a pressing need to improve treatment and thus clinical trials should focus not only on efficacy, but also on identification of the underlying mechanisms through which treatments operate [
21]. We observed a number of people with large changes in AB that go along with symptoms improvement, but also with no change in HRSD or even worsening of HRSD scores. Identification of this individual variability may be useful in evaluations of treatment efficacy and in personalized treatment. The current study reports the pre-registered primary outcome, but stratification based on degree of AB, changes in dispersion and/or distribution of AB, or paradigms sensitive to the temporal expression of AB may help advance basic knowledge on the conditional benefit of ABM [
22].
Limitations
The study has several limitations that should be mentioned. Sparse research on this population impede calculations of sample sizes from prior studies but would further increase the stringency of the study design in accordance with CONSORT guidelines [
23]. Inclusion of 37 patients that also fulfilled the formal criteria for current depression represents a deviation from the pre-registered protocol. However, sensitivity analyses that excluded participants at the higher end of symptom scores did not explain the results and increases the generalizability of the reported findings. Residual depression was widely defined as the study did not assess symptom change during former treatment and classify all symptoms as residual independent of treatment response or time since the last episode. The stimuli used for ABM training- and AB assessment was adopted from earlier studies and this combination of stimuli was not cross-balanced or validated based on ratings of stimulus valence- and arousal.