Models of systems memory consolidation postulate a fast-learning hippocampal store and a slowly developing, stable neocortical store. Accordingly, early neocortical contributions to memory are deemed to reflect a hippocampus-driven online reinstatement of encoding activity. In contrast, we found that learning rapidly engenders an enduring memory engram in the human posterior parietal cortex. We assessed microstructural plasticity via diffusion-weighted magnetic resonance imaging as well as functional brain activity in an object-location learning task. We detected neocortical plasticity as early as 1 hour after learning and found that it was learning specific, enabled correct recall, and overlapped with memory-related functional activity. These microstructural changes persisted over 12 hours. Our results suggest that new traces can be rapidly encoded into the parietal cortex, challenging views of a slow-learning neocortex. 7 4 z-transformed single-subject correlations, P < 0.001, n = 39. (D) Conjunction of the minimum statistic of all three analyses, green. Clusters exhibited significant peak-level effects at full-volume-corrected P FWE < 0.05 and exceeded 10 voxels. No masking. Beta values were corrected for baseline activation. Data are means ± SEM. Corresponding data from encoding are shown in fig. S1 and table S2. (E) Experimental design. An object-location learning task was trained for eight encoding (E)-recall (R) runs with fMRI. DW-MRI was measured at t0 to t2. For the control condition, the learning task was omitted.

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.long-term memory | posterior parietal cortex | precuneus | memory systems consolidation | virtual reality L earning enables adaptive and effective interaction with the environment based on past experience. How this essential capability of the brain to encode, store, and later retrieve new information is implemented on the systems level has been the focus of many studies. However, although there is consistent evidence that specific brain regions are involved in learning and memory, the interactions between these regions and their temporal dynamics remain unclear. For declarative memory, one influential model proposes complementary roles of the hippocampus and neocortex in supporting memory representations (1-3). It assumes that the highly plastic hippocampus serves as a fast learner, transiently storing newly encountered information. Later on, this information is gradually integrated into more stable neocortical networks (4).Many experiments in animals and humans have confirmed decreased hippocampal but increased neocortical contributions to memory retrieval with longer consolidation intervals (5-7). Concerning the time frame during which hippocampal independence of a memory is established, accounts diverge widely. In the case of patients with medial temporal lobe (MTL) dama...

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