Final answer:
DA neuron activity signals the subjective value of reward at the time of CS.
Step-by-step explanation:
The activity of dopamine (DA) neurons signals the subjective value of a reward at the time of a conditioned stimulus (CS). DA neurons are part of the reward system in the brain and are involved in associative learning and memory encoding. When a rewarding event occurs, such as receiving a desired outcome or experiencing pleasure, DA neurons are activated and release dopamine, which then signals to the brain the value and significance of the reward.