Parceling Human Accumbens into Putative Core and Shell Dissociates Encoding of Values for Reward and Pain

In addition to their well-established role in signaling rewarding outcomes and reward-predictive cues and in mediating positive reinforcement, there is growing evidence that nucleus accumbens (NAc) neurons also signal aversive events and cues that predict them. Here we use diffusion tractography to subdivide the right NAc into lateral-rostral (putative core, pcore) and medial-caudal (putative shell, pshell) subdivisions in humans. The two subregions exhibited differential structural connectivity, based on probabilistic tractography, to prefrontal cortical and subcortical limbic regions. We also demonstrate unique roles for each of the two subdivisions for monetary reward and thermal pain perception tasks: pshell signaling impending pain and value predictions for monetary gambles and pcore activating with anticipation of cessation of thermal pain (signaling reward value of analgesia). We examined functional connectivity for resting state, monetary reward, and thermal pain tasks, and for all three conditions observed that pcore and pshell of right NAc exhibit distinct patterns of synchrony (functional connectivity) to prefrontal cortical and subcortical limbic targets within the right hemisphere. To validate the NAc segregation, we mirrored the coordinates of right NAc pcore and pshell onto the left hemisphere and examined structural and resting state connectivity in the left hemisphere. This latter analysis closely replicated target-specific connections we obtained for the right hemisphere. Overall, we demonstrate that the human NAc can be parceled based on structural and functional connectivity, and that activity in these subdivisions differentially encodes values for expected pain relief and for expected monetary reward.