Evidence of a diurnal rhythm in implicit reward learning

ABSTRACT

Many aspects of hedonic behavior, including self-administration of natural and drug rewards, as well as human positive affect, follow a diurnal cycle that peaks during the species-specific active period. This variation has been linked to circadian modulation of the mesolimbic dopamine system, and is hypothesized to serve an adaptive function by driving an organism to engage with the environment during times where the opportunity for obtaining rewards is high. However, relatively little is known about whether more complex facets of hedonic behavior – in particular, reward learning – follow the same diurnal cycle. The current study aimed to address this gap by examining evidence for diurnal variation in reward learning on a well-validated probabilistic reward learning task (PRT). PRT data from a large normative sample (N = 516) of non-clinical individuals, recruited across eight studies, were examined for the current study. The PRT uses an asymmetrical reinforcement ratio to induce a behavioral response bias, and reward learning was operationalized as the strength of this response bias across blocks of the task. Results revealed significant diurnal variation in reward learning, however in contrast to patterns previously observed in other aspects of hedonic behavior, reward learning was lowest in the middle of the day. Although a diurnal pattern was also observed on a measure of more general task performance (discriminability), this did not account for the variation observed in reward learning. Taken together, these findings point to a distinct diurnal pattern in reward learning that differs from that observed in other aspects of hedonic behavior. The results of this study have important implications for our understanding of clinical disorders characterized by both circadian and reward learning disturbances, and future research is needed to confirm whether this diurnal variation has a truly circadian origin.

KEYWORDS:

• Reward learning
• anhedonia
• circadian rhythms
• dopamine

Declaration of interest

Over the past three years, Dr. Pizzagalli received consulting fees from Akili Interactive Labs, BlackThorn Therapeutics, Boehreinger Ingelheim, Pfizer and Posit Science for activities unrelated to the present study. Dr. Pizzagalli has a financial interest in BlackThorn Therapeutics, which has licensed the copyright to the Probabilistic Reward Task through Harvard University. Dr. Pizzagalli’s interests were reviewed and are managed by McLean Hospital and Partners HealthCare in accordance with their conflict of interest policies. No from these entities was used to support the current work, and all views expressed are solely those of the authors. All other authors report no conflicts of interest. The authors alone are responsible for the content and writing of this article.

Contributions

DAP designed the original studies that contributed the data for the current analyses; AEW, MM and MLI performed the data analysis with critical input from DAP; AEW and DAP drafted the manuscript and GM provided expert input on later drafts of the manuscript.

Additional information

Funding

The current analyses were supported by R01MH068376, R01MH101521, and R37MH068376. AEW is supported by funding from the National Health and Medical Research Council, the Brain and Behavior Research Foundation, McLean Hospital and the National Institute of Mental Health.