Journal Article

Effects of dopamine on reinforcement learning and consolidation in Parkinson’s disease

Tsivos, Demitra, Coulthard, Elizabeth J, Smith, Laura, Knight, Brogan E, Whone, Alan, Bogacz, Rafal, Grogan, John P

2017

Overview

Emerging evidence suggests that dopamine may modulate learning and memory with important implications for understanding the neurobiology of memory and future therapeutic targeting. An influential hypothesis posits that dopamine biases reinforcement learning. More recent data also suggest an influence during both consolidation and retrieval. Eighteen Parkinson’s disease patients learned through feedback ON or OFF medication, with memory tested 24 hr later ON or OFF medication (4 conditions, within-subjects design with matched healthy control group). Patients OFF medication during learning decreased in memory accuracy over the following 24 hr. In contrast to previous studies, however, dopaminergic medication during learning and testing did not affect expression of positive or negative reinforcement. Two further experiments were run without the 24 hr delay, but they too failed to reproduce effects of dopaminergic medication on reinforcement learning. While supportive of a dopaminergic role in consolidation, this study failed to replicate previous findings on reinforcement learning. Brain cells release a naturally occurring chemical called dopamine. The release of this chemical affects how people respond to their ever-changing environment, including how they learn from rewards and punishments. Parkinson’s disease is a condition where the brain cells that make dopamine start to die, and so the levels of dopamine in the brain begin to drop. Parkinson’s disease patients are routinely given drugs to bring their dopamine levels back up to near-normal levels. About 13 years ago, researchers found that when patients with Parkinson’s disease were given dopamine-medication they were better at learning from rewards and worse at learning from punishments. If the patients were withdrawn from their dopamine-medications they were worse at learning from rewards but better at learning from punishments. However, it was not clear if this was because the dopamine affects the learning process, or if it affects how people remember what they learned and how they make choices later on. To better understand how dopamine is involved in learning in people with Parkinson’s disease, Grogan et al. looked at the effects of dopamine on memory over a timescale of 24 hours. People with Parkinson’s disease and healthy volunteers were shown a choice of symbols and given the chance to learn which gave a reward – a picture of a smiling face – and which gave a punishment – a frowning face. If the Parkinson’s disease patients had taken their dopamine-medication before learning the task, their memory did not worsen over the next 24 hours. This suggests that having dopamine in the brain around the time of learning helped the patients to store the memory. The patients, however, were not any better at learning from rewards when taking their medication, which contradicts some earlier studies. To explore this further, Grogan et al. copied the exact same task from the 13-year-old study, and still did not find that patients were better at learning from reward when taking dopamine. These findings could help scientists to better understand what dopamine does during learning and memory, and how the brain normally works. Finally, Parkinson’s disease causes problems with memory. A clearer picture of the types of memory problems patients have, and of how their dopamine-medication can help, might make it easier for clinicians to treat patients with Parkinson’s disease.

• Publisher: eLife Science Publications, Ltd,eLife Sciences Publications Ltd,eLife Sciences Publications, Ltd
• Subject: Accuracy / Aged / Brain research / Care and treatment / Development and progression / Dopamine / Dopamine Agents - administration & dosage / Dopamine receptors / Feedback / Female / Humans / Learning / Male / Memory / Memory - drug effects / Nervous system / Neuroscience / Neurosciences / Parkinson disease / Parkinson Disease - drug therapy / Parkinson's disease / Psychological aspects / Reinforcement / reinforcement learning / Reinforcement learning (Machine learning) / Reinforcement, Psychology / reward / Therapeutic targets / Treatment Outcome