Dopamine neurons encode trial-by-trial subjective reward value in an auction-like task

Area of Science:

• Neuroscience
• Behavioral Economics
• Decision Making

Background:

• Dopamine's role in reward prediction error is established but typically studied in aggregate.
• Individual choices vary trial-by-trial, reflecting instantaneous subjective reward value.

Purpose of the Study:

• To investigate if dopamine neuron activity tracks instantaneous subjective reward value at the individual trial level.
• To determine if dopamine signals reflect subjective value beyond objective reward magnitude.

Main Methods:

• Utilized the Becker-DeGroot-Marschak (BDM) mechanism with experienced male rhesus monkeys bidding for juice rewards.
• Recorded midbrain dopamine neuron activity during BDM bidding.
• Employed Support Vector Regression to correlate dopamine responses with bids.

Main Results:

• Monkey bids for identical rewards varied trial-by-trial, demonstrating subjective value fluctuations.
• Dopamine neuron responses tracked these trial-by-trial bid variations, even with constant predicted rewards.
• Dopamine responses were similar for similar bids, irrespective of physical reward differences.
• Support Vector Regression accurately predicted bids from dopamine neuron activity.

Conclusions:

• The phasic dopamine reward signal encodes instantaneous subjective reward value, not just aggregate or predicted outcomes.
• This finding bridges understanding between dopamine's role in reward processing and individual decision-making.