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Striatal dopamine and motivated cognitive control
Conclusion
Imagine again that you are a squirrel. You are still in the forest, alternating between collecting
nuts and berries. You have just collected a nut, but now you see a larger berry (i.e. another
task) coming up. What happens in your squirrel brain? Well, I would speculate (based on the
existent literature and the work in this thesis) that the appearance of the large berry (i.e. a large
reward) will elicit a dopamine response in your midbrain. The projections of these dopamine
neurons to your ventral striatum will increase the sensitivity of your ventral striatum to input
from your prefrontal cortex. These reward-related signals in the ventral striatum will enhance
processing in the regions involved in flexible updating. This may occur via a number of routes.
First, these reward-related signals will alter signalling in the dorsal parts of the striatum via
the spiralling dopamine connections in the midbrain (thereby increasing the stimulation of
dopamine D2 receptors in the caudate nucleus, causing a D2-dominated state thereby enabling
you to quickly update the new task set). Alternatively (or concurrently) these signals in the
striatum act by changing the gating mechanism from the striatum to the ‘cognitive’ prefrontal
cortex and subsequently the other regions in the cognitive corticostriatal circuit. If the berry
you are about to collect is smaller, then the updating signals in your cognitive control network
will also be smaller, leading to less flexible updating of your task sets.
In conclusion,
the research in this thesis aimed to elucidate the causal role for striatal
dopamine and the corticostriatal network during the integration of reward and flexible
cognitive control.
We showed that dopaminergic manipulation with methylphenidate
indeed changed motivation-cognition signalling in the striatum. In addition, a causal role
for the ventral striatum in motivated cognitive control was established, followed by evidence
for frontal modulation of striatal processing during the integration of signals related to
motivation, cognition, and action across subparts of the striatum. Together, these results are
in line with a role for striatal dopamine in motivated cognitive control, and they show that
integration across corticostriatal circuits is involved in this process.