How could dopamine tell a synapse, seconds later, that it helped? · Grey Matter

Dopamine neurons fire more when an outcome is better than expected and less when it is worse, a reward prediction error broadcast to large parts of the brain.

How could dopamine tell a synapse, seconds later, that it helped?

Dopamine neurons fire more when an outcome is better than expected and less when it is worse, a reward prediction error broadcast to large parts of the brain. In three-factor learning rules, a synapse whose two neurons were active together sets a temporary chemical tag, an eligibility trace that fades over about a second or more, and the synapse changes only if dopamine or another modulator arrives while the tag is still there. The tag says which synapses were involved and the modulator says whether it went well, which assigns credit across the delay between an action and its result, with experimental support in the striatum, hippocampus and cortex.