NOVELTY and EPISODIC ENCODING: Laminar activity in the Hippocampus and Entorhinal Cortex While novelty processing is strongly related to HC–EC input pathways, the memory fate of a novel stimulus depends more on HC–EC output nature/ncomms/2014/141126/ncomms6547/full/ncomms6547.html For the recall of experiences and facts, various parts of the brain have to work together. Much of this interdependence is still undetermined, however, it is known that memories are stored primarily in the cerebral cortex and that the control center that generates memory content and also retrieves it, is located in the brain’s interior This happens in the hippocampus and in the adjacent entorhinal cortex. “It is been known for quite some time that these areas of the brain participate in the generation of memories. This is where information is collected and processed. The HC–EC circuitry shows a quantitative segregation of anatomical directionality into different neuronal layers. Whereas superficial EC layers mainly project to dentate gyrus (DG), CA3 and apical CA1 layers, HC output is primarily sent from pyramidal CA1 layers and subiculum to deep EC layers. Our study has refined our view of this situation,” explains Professor Emrah Düzel, site speaker of the DZNE in Magdeburg and director of the Institute of Cognitive Neurology and Dementia Research at the University of Magdeburg. Multivariate Bayes decoding within HC/EC subregions shows that processing of novel information most strongly engages the input structures (superficial EC and DG/CA2–3), whereas subsequent memory is more dependent on activation of output regions (deep EC and pyramidal CA1) “We have been able to locate the generation of human memories to certain neuronal layers within the hippocampus and the entorhinal cortex. We were able to determine which neuronal layer was active. This revealed if information was directed into the hippocampus or whether it traveled from the hippocampus into the cerebral cortex. Previously used MRI techniques were not precise enough to capture this directional information. Hence, this is the first time we have been able to show where in the brain the doorway to memory is located.” There is converging evidence that novelty encoding in the HC should primarily support the ability to later recollect associative information about the context and episodic details with which a novel item was encountered, such as time and location A key aspect of our study is that successfully memorized items are novel items at the time of encoding. Therefore, according to our data the HC receives novelty signals for all new items (signalled via superficial EC) but produces an output from CA1 preferentially for some of them (signalled via pyramidal CA1 and deep EC). Owing to this selectivity, CA1 is not only acting as a generic novelty detector but also generates novelty responses that are tightly linked to successful long-term encoding. In contrast, a mere increase in familiarity recognition due to previous exposure can be preserved even after bilateral hippocampal lesions , suggesting that familiarity per se does not critically depend on the HC. Other studies suggest that familiarity also depends on the HC–EC circuitry Our results did show the strongest relationship between hippocampal activity levels and later recollection, as well as high confidence memory scores. Familiarity estimates, on the other hand, did not show any significant correlation with our activity measures. In this study, we focused on the laminar organization of novelty processing and DM effects within the HC–EC circuitr Another functional segregation within the MTL concerns a potential separation of item- and space-related inputs to HC–EC circuitry. Within this scheme, item information from PRC converges to lateral EC, whereas spatial information from PHC converges to medial EC . In monkeys, this dissociation appears to be more dominant between posterior and anterior portions of EC These results are compatible with longstanding evidence that generic novelty signals do not require CA1 and can be computed without the HC For example, novelty signals can be preserved in cases of bilateral hippocampal injury4 and extrahippocampal MTL regions can show novelty responses to visual stimuli such as scenes and in fact compute this very rapidly Thus our results suggest that selectivity could distinguish hippocampal outputs for novel stimuli from novelty signals at the input level. An internal comparator process as proposed for CA1 could be one mechanism by which such selectivity could be implemented.
Posted on: Mon, 08 Dec 2014 04:12:30 +0000