This approach attempts to address the question: why do people with bipolar disorder seem to cycle through emotional expectations and responses, which directly influence mood ups and downs, without a consistent pattern? Researchers have found that people with bipolar disorder show increased activity in the ventral striatum during rewards, explaining extreme mood swings.
Bipolar disorder is marked by phases of euphoria (mania or hypomania) and depression. These phases can be strongly influenced by reward processing—the way the brain responds to pleasurable experiences or the anticipation of future rewards.
However, until now, studies that have tried to isolate reward processing and mood instability have yielded inconsistent results, making it difficult to understand exactly how these characteristics are linked.
In this study, researchers at University College London wanted to see if there is a bias in reward processing in people with bipolar disorder, based on the idea that their brains can be heavily influenced by signals of changes in the environment as if things are always going to get better or worse.
This “momentum” is nothing more than a kind of tendency in the brain to continue “betting” on the perception of a positive or negative change, without fully evaluating reality.
To investigate this, participants (21 with bipolar disorder and 21 without the disorder, forming the control group) performed a specific reward task while undergoing functional magnetic resonance imaging (fMRI).
This scan allows the visualization of areas of the brain that are active during the performance of tasks, observing how they react to certain stimuli. The task consisted of receiving rewards probabilistically, that is, with a calculated chance of success.
An advanced computational model was used to measure how participants responded to “reward prediction errors” (RPEs). These errors represent the difference between the expected reward and the actual reward received and are a type of learning that the brain uses to adjust future expectations.
The researchers specifically looked to see if “momentum” was present in these prediction errors in people with bipolar disorder. They analyzed whether the brains of these participants (in particular, the striatum, an area linked to motivation and reward) reacted in a biased way, maintaining more intense expectations of improvement or worsening of the surrounding environment.
The results showed that participants with bipolar disorder were indeed biased toward processing RPEs based on momentum, more so than the control group. This means that they displayed a neural response that tended to heighten the expectation of reward (positive or negative) based on perceived environmental fluctuations.
This activity was located in the striatum, an area of the brain that is critical for reward processing. In addition, the team investigated the functional connectivity between two brain regions: the ventral striatum and the left anterior insula, which is a brain region that processes emotions and feelings from the body.
They found that in people with bipolar disorder, there was reduced connectivity between these two areas when momentum was present in the RPEs.
This disconnection was stronger in participants who were experiencing manic symptoms, suggesting that this reduction in connectivity between the striatum and insula may be linked to the manic symptoms of bipolar disorder.
Modulation of outcome activity by momentum-biased reward prediction errors in the ventral striatum. Only participants with bipolar disorder showed momentum-biased reward prediction errors (error bars: standard error of the mean). BD, bipolar disorder group; CG, control group. doi.org/10.1016/j.bpsgos.2024.100330
These results bring a new perspective to the study of bipolar disorder: they show that reward processing is indeed dysregulated and is influenced by a “momentum bias,” that is, a propensity to follow an expectation cycle, which interferes with mood balance.
The left anterior insula, which helps monitor changes in body and emotional state, appears to be unable to communicate effectively with the striatum, which contributes to an exaggerated response to changes in perception, intensifying the emotional instability typical of the disorder.
Activation in the left anterior insula tracked the degree of momentum of reward prediction errors. In both groups, momentum was tracked by the left anterior insula, and this was not significantly different between groups. BD, bipolar disorder group; CG, control group. doi.org/10.1016/j.bpsgos.2024.100330
These findings suggest that treatments focused on stabilizing reward processing and strengthening communication between these brain regions may hold promise.
In addition, this research also opens the door for further studies using computational models that analyze momentary fluctuations to better understand bipolar disorder and other psychiatric conditions.
READ MORE:
Misperceiving Momentum: Computational Mechanisms of Biased Striatal Reward Prediction Errors in Bipolar Disorder
Hestia Moningk and Liam Mason
Biological Psychiatry Global Open Science
Volume 4, Issue 4, July 2024, 100330
Abstract:
Dysregulated reward processing and mood instability are core features of bipolar disorder that have largely been considered separately, with contradictory findings. We sought to test a mechanistic account that emphasizes an excessive tendency in bipolar disorder to enter recursive cycles in which reward perception is biased by signals that the environment may be changing for the better or worse. Participants completed a probabilistic reward task with functional magnetic resonance imaging. Using an influential computational model, we ascertained whether participants with bipolar disorder (n = 21) showed greater striatal tracking of momentum-biased reward prediction errors (RPEs) than matched control participants (n = 21). We conducted psychophysiological interaction analyses to quantify the degree to which each group modulated functional connectivity between the ventral striatum and left anterior insula in response to fluctuations in momentum. In participants with bipolar disorder, but not control participants, the momentum-biased RPE model accounted for significant additional variance in striatal activity beyond a standard model of veridical RPEs. Compared with control participants, participants with bipolar disorder exhibited lower insular-striatal functional connectivity modulated by momentum-biased RPEs, an effect that was more pronounced as a function of current manic symptoms. Consistent with existing theory, we found evidence that bipolar disorder is associated with a tendency for momentum to excessively bias striatal tracking of RPEs. We identified impaired insular-striatal connectivity as a possible locus for this propensity. We argue that computational psychiatric approaches that examine momentary shifts in reward and mood dynamics have strong potential for yielding new mechanistic insights and intervention targets.
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