Category-specific item encoding in the medial temporal lobe and beyond: The role of reward

Forming new memories is a fundamental part of human life, and the medial temporal lobe (MTL) is central to memory formation. Recent research suggests that within MTL, the perirhinal and parahippocampal cortices (PRC, PHC) process object and scene memory, respectively, whereas the hippocampus (HC) is agnostic to stimulus category. It is unclear, however, whether MTL category specificity extends to item encoding. Furthermore, MTL does not act in isolation: Reward-related memories are formed in interplay with the dopaminergic midbrain (substantia nigra/ventral tegmental area, SNVTA) and amygdala (AMY), but it is unclear whether reward modulates neural item encoding in a category-specific way. To address these questions, we had 39 healthy volunteers (27 for all memory-based analyses) undergo functional magnetic resonance imaging while they solved an incidental encoding task, which paired objects or scenes with high or low reward, followed by a next-day surprise recognition test. Behaviourally, high reward preferably enhanced object memory. Importantly, neural activity in PRC and PHC reflected item encoding of objects and scenes, respectively. Moreover, AMY encoding effects were selective for high-reward objects, with a similar pattern in PRC. SNVTA and HC showed no clear evidence of item encoding. The behavioural and neural asymmetry of reward-related encoding effects may be conveyed through an anterior-temporal memory system, including AMY and PRC, potentially in interplay with the ventromedial prefrontal cortex (vmPFC).