Questions: Dopaminergic Pathways: Reward, Motivation, and Motor Control

Question 1

A monkey is trained to expect juice when it sees a light flash. Initially, dopamine neurons fire when juice arrives. After training, when does peak dopamine firing occur?

• A. When the juice is consumed
• B. When the light flash appears
• C. At a fixed rate throughout the trial
• D. When no juice is given despite the flash

Answer: (B)

This is the classic reward prediction error result (Schultz et al.). Once the animal learns that the light predicts juice, dopamine firing shifts from the reward delivery to the predictive cue. The cue now carries the 'prediction' signal — it is when the unexpected information arrives. When an expected reward is actually omitted, dopamine firing dips below baseline.

Question 2

Dopamine neurons fire most strongly in direct response to pleasurable sensations like taste or touch.

• T. True
• F. False

Answer: False

Dopamine neurons encode reward prediction error, not raw pleasure. They fire briskly to unexpected rewards or reward-predictive cues, but after learning, they stop firing to the reward itself (which is now fully predicted) and fire instead to the earliest cue. They also decrease firing below baseline when an expected reward fails to appear. Pleasure or 'liking' is mediated by different circuits, including opioid systems.

Question 3

What does a negative reward prediction error signal, and what change in dopamine activity reflects it?

Model answer: A negative reward prediction error means the outcome was worse than predicted (e.g., an expected reward did not arrive). It is reflected by a dip in dopamine neuron firing below baseline.

Dopamine encodes the difference between expected and received reward. Positive prediction error (better than expected) drives firing above baseline. Negative prediction error (worse than expected) depresses firing below baseline. This bidirectional signal is what allows dopaminergic circuits to update predictions and drive learning.