The Hippocampal Complex and Its Necessity for the Retrieval of Episodic Memories

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As suggested by Deawyler (1984), it is widely accepted that hippocampus plays an important role in storing and retrieving memory in human brain. Various studies showed that hippocampal lesions disrupted the retrieval of episodic, semantic and spatial memories to a certain extent. (Addis, Moscovitch,Crawley & McAndrews,2004; Bayley, Gold, Hopkins & Squire,2005; Cipolotti, Shallice, Chan, Fox, Scahill, Harrison, Stevens & Rudge, 2001; Hirano, Noguchi, Hosokawa & Takayama,2002; Maguire & Frith,2003; Moscovitch, Nadal, Winocur, Gilboa & Rosenbaum,2006; Nadel,Samsonvich,Ryan & Moscovitch,2000; Pinel,2006; Steinvorth, Levine & Corkin,2005). However, Maguire and Frith (2003) pointed out the time-scale of hippocampal involvement in the retrieval of episodic memories is still datable. There are three main theories, the standard model of consolidation (SMC), multiple trace theory (MTT) and the cognitive map (CM) theory attributing importance of hippocampal complex in retrieval of episodic memory to a different extent. Although the study conducted by (Bayley et al., 2005) suggested that hippocampal complex is only essential for the retrieval of recent episodic memories since remote one has already consolidated and retrieval depended on other neocortical areas. However, the flaws such as the lack of experience near details in his decide made the result inconclusive. (Steinvorth et al., 2005). Evidences from neuropsychological and neuroimaging studies largely supported MTT. In neuropsychological aspect, the case studies of amnesic patients V.C. (Cipolotti et al., 2001), Y.K. (Hirano et al., 2002), H.M. and W.R. (Steinvorth et al.,2005) and others with damages in medial temporal lobe (Nadel et al.,2000) suggested that hippocampal complex was necessary for recollecting and re-experiencing high quality episodic memories, no matter how remote they were. In functional neuroimaging account, studies revealed there was extensive activation in hippocampal complex when participants recalled their autographical memories, though they showed some extent of lateral asymmetry in the hippocampal response. (Addis et al., 2004; Maguire & Frith, 2003).

Theories of hippocampal complex in remote memory
Hippocampal complex, located in the medial temporal lobe consists of “hippocampus proper, subiculum, entorhinal, peri-rihinal and paprahippocampal cortices.” (Cipolotti & Bird,2006, p.594). Studies with the amnesic patient H.M. who had temporal lobe lobectomy revels that the medial temporal lobe plays an important role in memory. Among the various structures of medial temporal lobe, researchers are particular interested in investigating the function and importance of hippocampal complex related to episodic memory retrieval. The consolidation theory predicts the newly learned information will be consolidated in the hippocampus and then the will gradually become independent on neocortical storage sites. Therefore, recent memories which not yet become consolidated would be impaired much severely impaired than the remote one. (Cipolotti & Bird,2006). In contrast, multiple trace theory suggests hippocampal complex is always crucial for recollecting episodic memory. The hippocampus sparely encodes the new information and binds other neurons to form memory trace. When the original memory is recalled, a new engram is established to link the previous memory, making the old memory more resistant to disruption since it has a stronger trace. The process is mediated by the hippocampal complex and thus lesions to it would lead to ungraded impairment to episodic memory. (Pinel,2006). O’Keefe and Nadel (1978, cited in Pinel,2006) proposed the cognitive map theory (CM) which suggested the specific role of hippocampus for constructing and storing allocentric spatial representations. They suggested that spatial images had a close linkage with the episodic memory since it provided rich contextual details for episodic events. Also, the theory proposes no...
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