Mechanisms Of Episodic Memory Encoding

Introduction

Episodic memory refers to the memory of autobiographical occurrences like associated emotions, places, times etc. It is a past personal experience(s) collection which occurred at a specific place and time. It is important in that; it allows an individual to recall memories which are sometimes vital part of life. This creates an insight of shared life history as well as people’s memories. There are various episodic memories which includes; episodic memories of certain occurrences. These include experiences of specific times from one’s past. Second is episodic memories of personal facts. There is also an episodic memory of unspecific occurrences and flashbulb memories. These are vivid and detailed "snapshots" of occurrences concerned with the discovery of particularly important news. Some moments are highly personal such as the moment one learns the death of his/her grandmother.

Episodic Memory steps and parts of the brain that are Involved

Studies indicate that the formation of episodic memory includes different steps, which also includes separate brain parts. The first process is encoding. This is a process undergone every time a person forms a new episodic memory (Loprinzi et al., 2017). Additionally, consolidation follows which involves stabilizing occurrences into long-term memory. In this way, the brain organizes and reorganizes itself in response to experiences, creating new memories prompted by experience, education or training. I most cases, after consolidation memory is not lost unless the brain suffers impairments. However, Murty, et al., (2016), claim that episodic memory can be affected by tumours, trauma as well as metabolic conditions and neurological diseases like Alzheimer’s disorders. Lastly is the memory recollection. (This elicits the contextual information retrieval especially those about an incident. Doesn’t make sense)

A study by Sneve et al., (2017) shows that new episodic memory formation needs the medial temporal lobe which involves the hippocampus. Without the hippocampus, an individual can create new procedural memories though cannot remember when the occurrence happened (Barbeau, 2017). Research by Wagner et al (2016) show that the prefrontal cortex also takes part in episodic memories formation-episodic encoding. The study also indicates that with damages to the prefrontal cortex an individual can learn new information though it is achieved in a disordered fashion which supports the studies by Sneve et al., (2017). As per Moscovitch et al. (2016), researchers agree that episodic memories hold memories; however, there is still debate on the storage time of long episodic memories in the hippocampus. While analysing literature, the study also suggests that scientists believe that episodic memories rely on the hippocampus while other studies show that the hippocampus only stores short term episodic memories and then memories are consolidated in the neocortex. Adult hippocampus’ neurogenesis enhances old memories removal while increasing the efficiency of fresh memory formation. This shows that there is a gap in the exact period of memory storage.

Mechanisms of Episodic Memory Encoding

Nevertheless, studies by Watrous & Ekstrom (2014), shows that the hippocampus is a critical region for episodic memory. It is thought to enhance episodic memories by binding stimuli’s representations with some unique spatiotemporal context underlying an occurrence. This step is perceived to include the neocortical regions and hippocampus’ interactions which represent the sensory information liked to the original occurrence (Nomura et al., 2019). Additionally, as per the study, a crucial element underlying encoding specificity is the similar phenomena’s reinstatement of neurons which were witnessed at the time of encoding. Context reinstatement effects (“context” is the external factors of the environment as well as internal dynamics which drives oscillatory activity and neural spiking into a certain state of the brain) is well-documented in studies of MRI (Sneve et al., 2015). Nevertheless, research shows that neurons which are active at the time of encoding are active at retrieval period. For example, the active neurons for fear memory encoding are also active while retrieving the memory in rodent hippocampus (Giannotti et al., 2019). This is an indication that there are a lot of similarities between encoding and retrieval of episodic memory. Additionally, the study indicates that memory-related reactivation is revealed in non-human primates. Similarly, in humans, hippocampal neurons which were active at the time of encoding were also active at the time of memory retrieval. This evidence proof that the context reinstatement is witnessed within a similar group of functionally liked neurons which are active during encoding as well as memory retrieval. Regardless of these studies, Besnard & Sahay, (2016), suggest that, though the specific overlap of cells which are active at the retrieval and encoding time is similar, there is variation by brain region as well as the hippocampal subfield. Since these variations are not adequately explored, this there is need for more studies.

These core neural findings and behavioural findings regarding the overlap of the neural patterns of activity, as well as the context in retrieval and encoding; leave important questions unresolved. For instance, how are certain assemblies of the cell created at the encoding period and reactivated electively at the time of retrieval of memories through traces of neocortical sensory? Additionally, mechanisms using the phase of cross-frequency coupling, oscillatory activity of the brain as well as phase synchronization seem to be hypothesized to underlie these processes. Studies by Martínez-Cancino et al., (2019) suggest that a low-frequency activity sometimes modulates the amplitude of high-frequency activity- cross-frequency (CFC) coupling enhances synaptic plasticity's optimal conditions, therefore, resulting to cell assembly formation. This, therefore, raises the question on the available evidence on the mechanism of episodic memory encoding.

CFC is evident in many sections in various tasks therefore; it is maybe an unspecified recurring phenomena/motif in the neuronal computation. Additionally, inter-area the population spike (PS) is likely to support coordination of areas of the brain which may enhance the interaction of hippocampal-neocortical at the time of memory retrieval (Dimitriadis & Marimpis, 2018). As per this evidence, we can conclude that the whole mechanism of action is not fully explored therefore there is a need for more studies to enhance understanding of the Episodic memory encoding mechanisms.

Additionally, recent evidence shows that low-frequency oscillatory activity is reinstated at the time of memory retrieval (Sotero, 2016). The study shows that frequency-specific input patterns which occur at the time of encoding are reinstated during the retrieval period. Therefore, this indicates that retrieval which is successful in lead lessons under these circumstances recapitulated the frequency-specific patterns of phases of oscillation which was experienced at encoding period. In conclusion there is prove that encoding aspects relies on the frequency-specific input pattern which is experienced in the initial stimulus processing, then lead to frequency-specific oscillatory activity patterns. Additionally, there are many gaps in this subject.

References

• Barbeau, E.J., Chauvel, P., Moulin, C.J., Regis, J. and Liégeois‐Chauvel, C., 2017. Hippocampus duality: Memory and novelty detection are subserved by distinct mechanisms. Hippocampus, 27(4), pp.405-416.
• Besnard, A. and Sahay, A., 2016. Adult hippocampal neurogenesis, fear generalization, and stress. Neuropsychopharmacology, 41(1), p.24.
• Dimitriadis, S.I. and Marimpis, A.D., 2018. Enhancing performance and bit rates in a brain–computer interface system with phase-to-amplitude cross-frequency coupling: evidences from traditional c-VEP, Fast c-VEP, and SSVEP designs. Frontiers in neuroinformatics, 12, p.19.
• Giannotti, G., Heinsbroek, J.A., Yue, A.J., Deisseroth, K. and Peters, J., 2019. Prefrontal cortex neuronal ensembles encoding fear drive fear expression during long-term memory retrieval. Scientific reports, 9(1), p.10709.
• Loprinzi, P.D., Edwards, M.K. and Frith, E., 2017. Potential avenues for exercise to activate episodic memory‐related pathways: a narrative review. European Journal of Neuroscience, 46(5), pp.2067-2077.
• Martínez-Cancino, R., Heng, J., Delorme, A., Kreutz-Delgado, K., Sotero, R.C. and Makeig, S., 2019. Measuring transient phase-amplitude coupling using local mutual information. NeuroImage, 185, pp.361-378.
• Moscovitch, M., Cabeza, R., Winocur, G. and Nadel, L., 2016. Episodic memory and beyond: the hippocampus and neocortex in transformation. Annual review of psychology, 67, pp.105-134.
• Murty, V.P., FeldmanHall, O., Hunter, L.E., Phelps, E.A. and Davachi, L., 2016. Episodic memories predict adaptive value-based decision-making. Journal of Experimental Psychology: General, 145(5), p.548.
• Nomura, T., Asao, A. and Kumasaka, A., 2019. Transcranial alternating current stimulation over the prefrontal cortex enhances episodic memory recognition. Experimental brain research, pp.1-7.
• Sneve, M.H., Grydeland, H., Amlien, I.K., Langnes, E., Walhovd, K.B. and Fjell, A.M., 2017. Decoupling of large-scale brain networks supports the consolidation of durable episodic memories. NeuroImage, 153, pp.336-345.
• Sneve, M.H., Grydeland, H., Nyberg, L., Bowles, B., Amlien, I.K., Langnes, E., Walhovd, K.B. and Fjell, A.M., 2015. Mechanisms underlying encoding of short-lived versus durable episodic memories. Journal of Neuroscience, 35(13), pp.5202-5212.
• Sotero, R.C., 2016. Topology, cross-frequency, and same-frequency band interactions shape the generation of phase-amplitude coupling in a neural mass model of a cortical column. PLoS computational biology, 12(11), p.e1005180.
• Wagner, I.C., van Buuren, M., Bovy, L. and Fernández, G., 2016. Parallel engagement of regions associated with encoding and later retrieval forms durable memories. Journal of Neuroscience, 36(30), pp.7985-7995.
• Watrous, A.J. and Ekstrom, A.D., 2014. The spectro-contextual encoding and retrieval theory of episodic memory. Frontiers in human neuroscience, 8, p.75.

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