In order to represent the world around us in a precise fashion, our sensory and cognitive systems need to adapt to the context in which we find ourselves. The processing of reward is one such mechanism where the subjective value of a given gain strongly depends on the other available rewards. For example, you might be quite content with your salary, until you learn your colleague is earning more… Adaptive coding of reward is this process by which neurons adapt their response to context, allowing a more precise representation of available incentives. Importantly, adaptive coding is thought to play a role in motivation for goal-directed behaviours. Accordingly, deficits in adaptive coding of reward have been reported in psychiatric conditions with motivational negative symptoms (i.e. apathy), such as schizophrenia and bipolar disorder.
In the present work we aimed to assess the neural bases of adaptive coding of reward in a large sample of patients with schizophrenia, and establish whether any deficits in this mechanism are stable in time and across two experimental sites, and how they relate to motivational negative symptoms. We used the Monetary Incentive Delay (MID) task during an fMRI session to assess reward processing. Participants performed the task twice, once upon inclusion, and a second time about three months later. Participants were recruited in Geneva and in Berlin. A total of 86 patients and 66 healthy controls matched on age, gender and parental level of education were recruited.
The MID task has a narrow reward range condition (participants can win rewards between 0 and 40 cents) and a large reward range condition (rewards between 0 and 2 CHF). Larger rewards produce an increased BOLD response, and the slope of this increase is shallower in the large range condition where more rewards need to be represented, and steeper in the narrow range, where fewer rewards need to be represented. The difference between the slopes in the two conditions is a marker of adaptive coding of reward.
Patients with schizophrenia showed reduced adaptive coding in the striatum – a major hub for processing reward related information. This reduced adaptation in patients was driven by shallower slopes in the narrow reward range condition, as compared to controls (t=2.67, p=0.039). There was no difference between groups in the large reward range condition (t=0.26, p=0.9). Adaptive coding didn’t change in controls between the two experimental sessions (t=1.4, p=0.18). In patients, however, we observed an increase in slope in the narrow reward range (t=-2.8, p=0.006), such that patients’ adaptation was at the same level as that of controls. Interestingly, patients’ adaptive coding at the second session correlated with motivational negative symptoms.
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