BASELINE DOPAMINE IN RAT PREFRONTAL CORTEX REFLECTS THE OPPORTUNITY COST OF TIME INVESTMENT

Many decisions have delayed outcomes and require investing time to reap the benefits. According to normative theories, time investment should be scaled with its opportunity cost and the expected value of outcomes. While phasic dopamine response signals reward prediction error, recent studies have suggested that the slow fluctuation in cortical dopamine may signal this opportunity cost by estimating the local average reward rate. However, it remains unclear whether this signal guides time investment. Here we recorded cortical dopamine dynamics using fiber photometry and the genetically-encoded dopamine sensor GRAB-DA_3h, while rats performed a dynamic foraging task. This task required investing time into choices for earning uncertain and delayed rewards. We applied a Q-learning model with additional forgetting and choice stickiness mechanisms to infer the subjects’ trial-by-trial choice values and their local reward rate estimates. Preliminary results demonstrate that rats adjusted their choices in proportion to the relative frequency of reward, consistent with the matching law. Aligned with normative theories, their time investment scaled with the model-inferred choice values, providing a trial-by-trial behavioral readout of their subjective value. The relative dopamine level, measured by the relative fluorescence, prior to the choice in the baseline period best correlated with the rats’ local reward rate estimate, but not the chosen choices’ values. It also predicted the dynamic adjustment of time investment on top of the choice value. Our results suggest that baseline dopamine in rat prefrontal cortex may encode an internal estimate of the opportunity cost, potentially informing dynamic adjustments in time investment