The Hippocampal Complex Is Essential for the Retrieval of Episodic Memories.вð‚ñœ Critically Evaluate This Statement, Focusing on Evidence from Neuropsychological Studies

Introduction

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 distinction between remote and recent spatial memory and so damage to hippocampus determines both spatial and episodic memories regardless of their age. (Moscovitch et al., 2006).

Episodic memory

Memory composes of multiple separable systems and it is stored diffusely throughout the structure of the brain. (Pinel,2006). There are different kinds of memory and one of the classifications is semantic memory and episodic memory. Semantic memory refers to memory for “general knowledge about language, the world and oneself” where episodic memory or sometimes called autobiographical memory is the specific events or episodes that one has experienced.” (Moscovitch et al., 2006, p.180). Some also proposed that episodic memory had a more detailed spatial context where semantic memory was largely context free. (O’Keefe and Nadel,1978, cited in Moscovitch et al., 2006). To test the retrieval of episodic memory, some studies (Bayley et al., 2005; Hirano et al., 2002), would also categorize the recalled memory into “remember” or “know”. Hirano et al. (2005, p.6) suggested only memory that “consists of both episodic remembering and remembering accompanied by the subjective feelings of ones own experience” can be considered as episodic memory. Therefore, to examine the role of hippocampal complex in recollecting episodic memory, the qualities of the recalled events should be taken into account.

Limitations on previous study

In Bayley et al. study (2005), patients who had medial temporal lobe performed as well as the control group on the tests remote episodic memory. Severe impairment in retrieving remote memory only showed on patients who had brain lesions beyond the hippocampal complex. The result appeared to support the standard model of consolidation. However, there are many limitations in this study. First, the test limited patients’ potential to recall the autobiographical events by themselves. Prompts were extensively given before and during the narrative recollection, which might not easy to figure out the differences of performance between control and patients. Second, although patients classified most of the retrieved events as “remember”, the term was vaguely defined as “actually remember the event as if you were there and you can mentally travel back in time to the place that the incident occurred and imagine that you are there.” (Bayley et al., 2005, p.808). It happened that even for events categorized as “remember”, they still lacked “experience near details”