Dopamine reward prediction error signal encodes effort cost as negative reward value

Monkey midbrain dopamine neurons encode joystick resistance as the negative value of effort cost, showing inhibition when effort exceeds prediction and excitation when it is lower.

• Why it matters: Reward-maximizing decisions require weighing benefits against effort costs, but it was unclear whether dopamine reward prediction error signals represent effort cost in a value-consistent way.
• What they did: Monkeys made repeated binary choices between options that varied in reward amount and effort; dopamine activity was measured at effort-predicting stimuli and at experienced effort, compared with reward prediction error signals.
• The result: Effort-related dopamine signals appeared both at stimuli and outcomes and contrasted with positive reward prediction error, matching temporal-difference reinforcement learning formalism.

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