Euthymic patients with bipolar disorder show decreased reward learning in a probabilistic reward task

Abstract

Background: Bipolar disorder (BPD) features cycling mood states ranging from depression to mania with intermittent phases of euthymia. Bipolar disorder subjects often show excessive goal-directed and pleasure-seeking behavior during manic episodes and reduced hedonic capacity during depressive episodes, indicating that BPD might involve altered reward processing. Our goal was to test the hypothesis that BPD is characterized by impairments in adjusting behavior as a function of prior reinforcement history, particularly in the presence of residual anhedonic symptoms.

Methods: Eighteen medicated BPD subjects and 25 demographically matched comparison subjects performed a probabilistic reward task. To identify putative dysfunctions in reward processing irrespective of mood state, primary analyses focused on euthymic BPD subjects (n = 13). With signal-detection methodologies, response bias toward a more frequently rewarded stimulus was used to objectively assess the participants' propensity to modulate behavior as a function of reinforcement history.

Results: Relative to comparison subjects, euthymic BPD subjects showed a reduced and delayed acquisition of response bias toward the more frequently rewarded stimulus, which was partially due to increased sensitivity to single rewards of the disadvantageous stimulus. Analyses considering the entire BPD sample revealed that reduced reward learning correlated with self-reported anhedonic symptoms, even after adjusting for residual manic and anxious symptoms and general distress.

Conclusions: The present study provides preliminary evidence indicating that BPD, even during euthymic states, is characterized by dysfunctional reward learning in situations requiring integration of reinforcement information over time and thus offers initial insights about the potential source of dysfunctional reward processing in this disorder.

Figure 1

Response bias as a function of block (block 1: trials 1–100; 2: trials 101–200; 3: trials 201–300) for healthy comparison (n = 25) and euthymic BP (n = 13) subjects. Error bars represent standard errors.

Figure 2

Mean accuracy for the rich and lean stimulus across the three blocks (panels A and B) and averaged across the three blocks (panel C) for healthy comparison (n = 25) and euthymic BP (n = 13) subjects. In (C), arrows denote significant post-hoc tests; error bars represent standard errors.

Figure 3

Probability of miss rates for healthy comparison (n = 25) and euthymic BP (n = 13) subjects as a function of whether the preceding rich or lean trial was rewarded or not. Arrows denote significant post-hoc tests; error bars represent standard errors.

Figure 4

Pearson correlation (r = −0.59, p < 0.010) for the entire BPD sample (n = 18) between ΔResponse Bias and the residualized BDI-II anhedonic subscore, which was computed by removing variance associated with the total YMRS score and the two MASQ anxiety scores (MASQ AA and MASQ GDA).