Levodopa does not affect expression of reinforcement learning in older adults

J P Grogan,E J Coulthard,A Howat,H K Isotalus,N Irigoras Izagirre,L E Knight

doi:10.1038/s41598-019-42904-5

J P Grogan, E J Coulthard + Show 4 more

Open Access

https://doi.org/10.1038/s41598-019-42904-5

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Abstract

Dopamine has been implicated in learning from rewards and punishment, and in the expression of this learning. However, many studies do not fully separate retrieval and decision mechanisms from learning and consolidation. Here, we investigated the effects of levodopa (dopamine precursor) on choice performance (isolated from learning or consolidation). We gave 31 healthy older adults 150 mg of levodopa or placebo (double-blinded, randomised) 1 hour before testing them on stimuli they had learned the value of the previous day. We found that levodopa did not affect the overall accuracy of choices, nor the relative expression of positively or negatively reinforced values. This contradicts several studies and suggests that overall dopamine levels may not play a role in the choice performance for values learned through reinforcement learning in older adults.

Highlights

Dopamine has been heavily implicated in reinforcement learning[1,2,3], and recently evidence has shown that dopamine affects later choices based on these learned values[4,5,6]
When learning and choice trials were separated by a delay, which allowed Parkinson’s disease (PD) patients to learn off medication and be tested on or off medication, medication state during learning had no effect on expression of positive or negative reinforcement, but dopaminergic state during the choices did[4]
When PD patients learned a set of stimulus-stimulus associations, and only had the rewards mapped onto these stimuli after they had finished learning, they still showed a bias towards the most rewarded stimuli if they were on their medications during the entire session[5]

Summary

Introduction

Dopamine has been heavily implicated in reinforcement learning[1,2,3], and recently evidence has shown that dopamine affects later choices based on these learned values[4,5,6]. Other studies have failed to find effects of dopamine during choice performance, with dopamine during testing 24 hours after reinforcement learning not affecting the change in accuracy from the learning trials[7,8].

Results

Conclusion