Hippocampal memory encoding and retrieval: Difference between revisions

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The [[hippocampus]] participates in the [[encoding (memory)|encoding]], [[memory consolidation|consolidation]], and retrieval of memories.<ref name=a /> The hippocampus is located in the medial temporal lobe (subcortical), and is an infolding of the medial temporal cortex.<ref name=a>Gazzaniga, Michael S., Richard B. Ivry, and G. R. Mangun. "Chapter 9: Memory." Cognitive Neuroscience: The Biology of the Mind. 4th ed. New York: W. W. Norton, 2014. 378-423. Print.</ref> The hippocampus plays an important role in the transfer of information from [[short-term memory]] to [[long-term memory]] during encoding and retrieval stages. These stages do not need to occur successively, but are, as studies seem to indicate, and they are broadly divided in the neuronal mechanisms that they require or even in the hippocampal areas that they seem to activate. According to Gazzaniga, "encoding is the processing of incoming information that creates memory traces to be stored."<ref name=a /> There are two steps to the encoding process: "acquisition" and "consolidation". During the acquisition process, stimuli are committed to short term memory.<ref name=a /> Then, consolidation is where the hippocampus along with other cortical structures stabilize an object within long term memory, which strengthens over time, and is a process for which a number of theories have arisen to explain the underlying mechanism.<ref name=a /> After encoding, the hippocampus is capable of going through the retrieval process. The retrieval process consists of accessing stored information; this allows learned behaviors to experience conscious depiction and execution.<ref name=a /> Encoding and retrieval are both affected by [[neurodegenerative]] and [[anxiety disorders]] and [[epilepsy]].

==Theories and reasoning==

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===HIPER (hippocampal encoding/retrieval) model===

Meta-[[positron emission tomography]] (PET) analysis has lent support toward a division of the hippocampus between caudal and rostral regions.<ref name=b>{{cite journal | last1 = Lepage | first1 = M. | last2 = Habib | first2 = R. | last3 = Tulving | first3 = E. | year = 1998 | title = Lepage, M., Habib, R. & Tulving, E. Hippocampal PET Activations of memory encoding and retrieval: the HIPER model" ''Hippocampus'' 8, 313-322 | journal = Hippocampus | volume = 8 | issue = 4| pages = 313–22 | doi = 10.1002/(SICI)1098-1063(1998)8:4<313::AID-HIPO1>3.0.CO;2-I | pmid = 9744418 | s2cid = 14507536 }}</ref> Scans have demonstrated a uniform variation in blood flow distribution within the hippocampus (and the medial temporal lobe broadly) during the separate processes of episodic encoding and retrieval.<ref name=b /> In the hippocampal encoding/retrieval (HIPER) model, episodic encoding is found to take place within the rostral region of the hippocampus whereas retrieval takes place in the caudal region.<ref name=b /> However, the divide between these regions need not be disjoint, as [[functional magnetic resonance imaging]] (fMRI) data has demonstrated encoding processes occurring within the caudal region.<ref name=b />

HIPER is a model resulting from and therefore a reflection of certain experimental phenomena, but cannot completely explain hippocampal encoding and retrieval on its own.<ref name=b /> Nevertheless, the model suggests a broad division of labor in encoding and retrieval, whether they involve separate regions of the hippocampus or act simultaneously or independently within a single, more inclusive process.

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===Theta phase separation===

In a framework first developed by Hasselmo and colleagues, theta phase separation implies that the [[hippocampal theta rhythm of the hippocampus]] occurs in cycles and various phases of the rhythm entail encoding and retrieval as separate processes.<ref name=c>{{cite journal | last1 = Hasselmo | first1 = ME | last2 = Bodelon | first2 = C | last3 = Wyble | first3 = BP | year = 2002 | title = A proposed function for hippocampal theta rhythm: separate phases of encoding and retrieval enhance reversal of prior learning | journal = Neural Computation | volume = 14 | issue = 4| pages = 793–817 | doi = 10.1162/089976602317318965 | pmid = 11936962 | s2cid = 9128504 }}</ref><ref name=m>{{cite journal | last1 = Kunec | first1 = S | last2 = Hasselmo | first2=ME | last3 = Kopell | first3 = N | year = 2005 | title = Encoding and Retrieval in the CA3 Region of the Hippocampus: A Model of Theta-Phase Separation | journal = Journal of Neurophysiology | volume = 94 | issue = 1| pages = 70–82 | doi = 10.1152/jn.00731.2004 | pmid = 15728768 | citeseerx = 10.1.1.333.5452 }}</ref> An extra-hippocampal structure, the septum, initiates and regulates the theta rhythm and its associated memory processes. GABAergic activity within the septum inhibits certain classes of CA3 cells (a region of the hippocampus), the divide often drawn between basket cells, pyramidal cells, and interneurons, to distinguish encoding from retrieval mechanisms. The study emphasizes and models the CA3 subfield of the hippocampus as a primary inducement towards encoding and retrieval. Encoding as a procedure begins when septal GABAergic inhibition is at minimum, freeing basket cells to act within CA3, and during brief dis-inhibition periods, other cells receive input: a proximal entorhinal input toward pyramidal cells and a coincident [[dentate gyrus]] input toward interneurons.<ref name=c /><ref name=m /> On the other hand, retrieval as a procedure begins when septal GABAergic inhibition is at maximum, occluding basket cell activity and enabling pyramidal cells to signal.<ref name=c /> During this period, Oriens- Lacunosum Moleculare (O-LM) cells disambiguate memory for retrieval.<ref name=m />

CA3 is significant as it is allows auto-associative processes through a recurrent, collateral system.<ref name=c /> The theta phase separation model agrees generally with others on the significance of CA3 but is the first to attribute both the processes of encoding and retrieval to the subfield.<ref name=c /><ref name=m />

===Reconsolidation hypothesis===

{{Main|Memory reconsolidation}}

The reconsolidation hypothesis claims that objects encoded into long term memory experience a new period of consolidation, or the time and resource expended to stabilize a memory object, upon each recollection. This is in opposition to the classical consolidation hypothesis which regards consolidation as a one-time event, following the first encoding of a memory. A memory item in this hypothesis, upon reactivation, destabilizes for a brief period and thereafter invokes the neuronal processes requisite for stabilization.<ref name=d>{{cite journal | last1 = Morris | first1 = R. G. M. | last2 = Inglis | first2 = J. | last3 = Ainge | first3 = J. A. | last4 = Olverman | first4 = H. J. | last5 = Tulloch | first5 = J. | last6 = Dudai | first6 = Y. | last7 = Kelly | first7 = P. A. T. | year = 2006 | title = Memory reconsolidation: Sensitivity of spatial memory to inhibition of protein synthesis in dorsal hippocampus during encoding and retrieval | journal = Neuron | volume = 50 | issue = 3| pages = 479–489 | doi = 10.1016/j.neuron.2006.04.012 | pmid=16675401| doi-access = free }}</ref>