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New Paper: Catecholaminergic challenge uncovers distinct Pavlovian and instrumental mechanisms of motivated (in)action

Posted 1/5/2017

Congratulations to Jennifer Swart on her beautiful new paper just accepted in eLife. See below for a lay summary and scientific abstracts. First paper in a series on methylphenidate, stay tuned!

Lay summary

When we see a threat, we tend to hold back. When we see a reward, we have a strong urge to approach. Most of the time, these hardwired tendencies – or biases – are the right thing to do. However, our behaviour is not all hardwired; we can also learn from our previous experiences. But might this learning be biased too? For example, we might be quicker to believe that an action led to a reward, because actions often do bring rewards. Conversely, we might be less likely to attribute a punishment to having held back, because holding back usually helps us to avoid punishments.
Swart et al. have now tested whether rewards and punishments influence our actions solely via hardwired behavioural tendencies, or whether they also bias our learning. That is, are we biased to learn that taking action earns us rewards, while holding back spares us punishments? Previous work has shown that chemical messengers in the brain called catecholamines help us to take action when we anticipate a reward. Swart et al. therefore also examined whether catecholamine levels contribute to any bias in learning.
One hundred young healthy adults twice performed a task in which they could earn rewards and avoid losses by taking or withholding action. By using a mathematical model to work out what influenced the choices made by the volunteers, Swart et al. found that rewards and punishments did indeed bias learning. Moreover, this learning bias became stronger when the volunteers took methylphenidate (also known as Ritalin), a drug that increases catecholamine levels and which is used to treat ADHD and narcolepsy. The volunteers varied markedly in how strongly methylphenidate affected their choices. This emphasises how important it is to account for differences between people when evaluating the effects of medication.
Motivations are what get us going and keep us going. The findings of Swart et al. mean that we now have a better understanding of how motivations, such as desired rewards or unwanted punishments, influence our behaviour. A future challenge is to understand how we can overcome these motivations when they work against us, such as in addiction or obesity.


Scientific abstract

Catecholamines modulate the impact of motivational cues on action. Such motivational biases have been proposed to reflect cue-based, ‘Pavlovian’ effects. Here, we assess whether motivational biases may also arise from asymmetrical instrumental learning of active and passive responses following reward and punishment outcomes. We present a novel paradigm, allowing us to disentangle the impact of reward and punishment on instrumental learning from Pavlovian response biasing. Computational analyses showed that motivational biases reflect both Pavlovian and instrumental effects: reward and punishment cues promoted generalized (in)action in a Pavlovian manner, whereas outcomes enhanced instrumental (un)learning of chosen actions. These cue- and outcome-based biases were altered independently by the catecholamine enhancer melthylphenidate. Methylphenidate’s effect varied across individuals with a putative proxy of baseline dopamine synthesis capacity, working memory span. Our study uncovers two distinct mechanisms by which motivation impacts behaviour, and helps refine current models of catecholaminergic modulation of motivated action.

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