How do reward signals strengthen synaptic connections in the human brain?
dopamine release changes the nature of synaptic long term potentiation (ltp) thought to underly long term memory. see http://www.sciencedirect.com/science/article/pii/S0896627305003971 for a paper i am familiar with, for an entry point to that field 

for a review article:
http://www.nature.com/nrn/journal/v5/n6/abs/nrn1406.html
see section titled "Dopamine and memory at the cellular level"