The hippocampal-VTA loop: The role of novelty and motivation in controlling the entry of information into long-term memory

Abstract

The role of dopamine has been strongly implicated in reward processes, but recent work shows an additional role as a signal that promotes the stable incorporation of novel information into long-term hippocampal memory. Indeed, dopamine neurons, in addition to being activated by reward, can be activated by novelty in the absence of reward. The computation of novelty is thought to occur in the hippocampus and is carried to the dopamine cells of the VTA through a polysynaptic pathway. Although a picture of novelty-dependent processes in the VTA and hippocampus is beginning to emerge, many aspects of the process remain unclear. Here, we will consider several issues: (1) What is relationship of novelty signals coming to the VTA from the superior colliculus, as compared to those that come from the hippocampus? (2) Can dopamine released by a reward enhance the learning of novel information? (3) Is there an interaction between motivational signals and hippocampal novelty signals? (4) What are the properties of the axons that generate dopamine release in the hippocampus? We close with a discussion of some of the outstanding open issues in this field.