Brain functional and structural changes over learning and sleep

Traditionally, learning and memory consolidation are considered to rely on initial encoding by the hippocampus and subsequent, gradually developing, stable storage in neocortical circuits. Recently, we have shown rapidly emerging memory-related functional activity in the posterior parietal cortex (PPC) that over learning repetitions becomes independent of hippocampal signaling and fulfills necessary criteria for a long-term memory representation. In the current study, we assessed whether these findings actually reflect neuronal plasticity by tracing learning-induced changes in gray matter microstructure. Additionally, we were interested in the impact of sleep on both functional and structural changes. In a day-wake, night-sleep design two groups of subjects (n=41) learned object-place associations over eight learning-recall repetitions in two sessions spaced 13 hours apart. Neural activity during learning and recall was measured with fMRI. To assess structural changes, we acquired diffusion weighted images at three time points: immediately before the first learning session, 90 minutes after the first learning session and again before the second learning session, after a day of waking or a night of sleep. In line with the results on functional brain activity, mean diffusivity in the PPC decreases after learning, which has been suggested as a marker for long-term potentiation. The simultaneous investigation of functional and structural changes confirms the rapid build-up of a long-term memory representation in the neocortex, which is further stabilized by sleep. The contribution of the hippocampus to encoding, however, seems to be confined to the very first encounter with new information