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Memory Engram in the Posterior Parietal Cortex Science 30 Nov 2018, Vol. 362, Issue 6418, pp. 1045-1048 Fast track to the neocortex: A memory engram in the posterior parietal cortex S. Brodt, et.al. Institute of Medical Psychology and Behavioral Neurobiology, University of Tübingen, Tübingen, Germany. Max-Planck-Institute for Biological Cybernetics, Tübingen, Germany. Biomedical Magnetic Resonance, Universitätsklinikum Tübingen, Tübingen, Germany. Princeton Neuroscience Institute, Princeton University, Princeton, NJ, USA. [paraphrase] 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. Systems memory consolidation is considered a slow process of neuronal reorganization. Fresh memories rely on the hippocampus, which reinstates the cortical ensembles that were active during encoding, whereas neocortical memory develops more slowly, through frequent reactivation. Recent findings suggest that the posterior parietal cortex (PPC) can acquire a memory representation rapidly during learning. It is unclear whether these early contributions go beyond an online reinstatement of previous activity or whether they originate from a true neocortical engram. Methodological advances have made it possible to track engrams in rodents, yet they have remained elusive in humans. In humans, multivariate analysis of functional magnetic resonance imaging (fMRI) can assess active memory representations during encoding and retrieval, but this method is unable to distinguish between activity originating within a region and activity reinstated through input from another region. It thus cannot unequivocally reveal the permanent location of the dormant trace. We used fMRI and DW-MRI to demonstrate the dynamic contributions of neocortical areas to memory during two sessions of four encoding–recall repetitions of an object–location association task. We identified posterior parietal areas that fulfilled all defining conditions of a memory engram, i.e., they showed functional responses that were specifically related to the memory, were persistent over longer offline periods, and were relevant for later memory recall. These regions also showed structural–plastic changes that conformed to the same criteria. Thus, a true neocortical engram developed rapidly, after only four rounds of rehearsal. Similarly, studies in rodents have revealed that neocortical engram cells are already tagged during encoding and have detected experience-dependent microstructural changes as early as 1 hour after learning. We suggest that such rapid learning-induced neocortical plasticity arises from multiple encoding–recall repetitions. The PPC’s ability to accumulate new information over several minutes and learn associations between well-known object schemata might allow particularly fast neocortical memory formation. Although there is still debate about the functions of the different subregions of the PPC and their roles in working memory, memory-related attention, or reinstatement of previous experience, our study highlights the role of the medial PPC. Observing microstructural changes in the precuneus takes us from memory processing and reinstatement to the memory engram itself. The fast temporal dynamics that we observed challenge traditional models of slow systems consolidation and suggest that new traces are encoded rapidly in the neocortex from the onset of learning. In addition, we detected learning-specific, persistent microstructural changes upstream along the dorsal and ventral visual pathways, which is in line with the notion of distributed neocortical memory traces. Apart from their role in perception, visual areas process memory content, suggesting memory storage also at this level. Indeed, many accounts regard perception and memory not as faculties of different systems but as being localized within the same distributed neural circuits. Combining functional imaging with diffusion imaging might help transform our view of how the brain translates perception into memory. [end of paraphrase]
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