Early reduction in aversive Pavlovian bias mediates anhedonia improvement during Behavioural Activation in realistic treatment settings

Background: Behavioral Activation therapy is an effective treatment formajor depressive disorder. Conceptually, the mechanisms through which it actsare thought to involve alterations to reinforcement learning, and several recentstudies have supported this in laboratory settings. However, it remains unclear whether reinforcement learning mechanisms are involved in a realistic treatment setting.Methods: In a randomized controlled observational study in the UK NHSTalking Therapy service, 152 participants with low mood received reinforcementlearning assessments using an affective Go/No-Go task. The assessment timepointswere randomized between subjects, with assessments occurring either before(control; n = 78) or during BA therapy (active; n = 74). Changes in Pavlovianbiases were quantified using computational modeling. Results: Anhedonia improved during treatment, but not before ($p=0.047;d=0.53$). The active treatment group showed significant changes in Pavlovianparameters compared to controls ($p_{\text{FDR}} = 0.012; d=0.65$). Pavlovian biases became more positive in the active group ($M = 0.44$, $SD =0.81$) and more negative in the control group ($M = -0.12$, $SD = 0.70$).Changes in aversive Pavlovian bias fully mediated treatment effects on symptomsof anhedonia ($\beta = -1.77$, $p = 0.036$, bootstrap; p = 0.0280). Conclusions: Behavioral Activation modified Pavlovian biases early intreatment by simultaneously strengthening appetitive and diminishing aversiveresponses. Reduction in aversive bias mediated improvements in anhedonia,suggesting a specific cognitive mechanism through which Behavioral Activation exerts itstherapeutic effects. The study provides evidence that online measurements ofcognitive function may have potential as mechanistic biomarkers.