A fast track to the neocortex: long-term memory representations in the parietal cortex

Traditional models of systems memory consolidation postulate two interacting memory stores, with rapid encoding of new information supported by the hippocampus and a gradually developing, stable storage in neocortical circuits. Recently, the posterior parietal cortex (PPC), particularly the precuneus, has been proposed as a cardinal location of neocortical long-term memory. We have shown functional activity in this area over repeated learning that is memory specific, long-term stable and related to memory accuracy. To conclusively identify the PPC as a location of memory storage, learningcontingent, lasting structural changes have to be demonstrated as well. Here, we used diffusion MRI to assess changes in brain microstructure, which reject neuronal plasticity. 41 participants learned object-place associations over 8 learning-recall repetitions in two sessions. Task-related activity was tracked with fMRI. Structural changes were assessed with dMRI at three time points (before, 90 minutes and 13 h after learning). A non-learning condition measured at the same times was employed as control. Functional PPC activity increases with learning repetitions, remains stable over a 13-h period and strongly correlates with recall performance. Furthermore, decreases in mean diffusivity indicate structural changes in the same area, which also develop after learning, remain stable for over 12 hours and correlate with behavioral performance. We thus show functional and structural changes in the PPC that fulfill all requirements for a neocortical long-term memory representation: learning specificity, long-term stability and behavioral relevance. The confirmation of structural plasticity in particular proves the importance of the PPC as a site of neocortical memory storage