期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2016
卷号：113
期号：46
页码：13251-13256
DOI：10.1073/pnas.1605719113
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceWhen it comes to memory, our brain needs to deal with two opposing demands: remaining plastic to acquire new information rapidly and maintaining old information faithfully over long periods. It is assumed that two distinct brain systems are in charge of these functions: the fast-learning hippocampus and the slow-learning neocortex. The interaction between these two systems has traditionally been seen as very slow, requiring weeks or months to build a neocortical memory trace. In this study, brain activity during virtual-reality navigation shows that contributions of hippocampus and parietal neocortex to memory are changing substantially already at the time a spatial memory representation is built. Notably, we show that the posterior parietal cortex fulfills all criteria for a hippocampus-independent memory representation. Previous evidence indicates that the brain stores memory in two complementary systems, allowing both rapid plasticity and stable representations at different sites. For memory to be established in a long-lasting neocortical store, many learning repetitions are considered necessary after initial encoding into hippocampal circuits. To elucidate the dynamics of hippocampal and neocortical contributions to the early phases of memory formation, we closely followed changes in human functional brain activity while volunteers navigated through two different, initially unknown virtual environments. In one condition, they were able to encode new information continuously about the spatial layout of the maze. In the control condition, no information could be learned because the layout changed constantly. Our results show that the posterior parietal cortex (PPC) encodes memories for spatial locations rapidly, beginning already with the first visit to a location and steadily increasing activity with each additional encounter. Hippocampal activity and connectivity between the PPC and hippocampus, on the other hand, are strongest during initial encoding, and both decline with additional encounters. Importantly, stronger PPC activity related to higher memory-based performance. Compared with the nonlearnable control condition, PPC activity in the learned environment remained elevated after a 24-h interval, indicating a stable change. Our findings reflect the rapid creation of a memory representation in the PPC, which belongs to a recently proposed parietal memory network. The emerging parietal representation is specific for individual episodes of experience, predicts behavior, and remains stable over offline periods, and must therefore hold a mnemonic function.
