Essay Example on Amnesia as an organic clinical disorder








Amnesia as an organic clinical disorder caused by brain damages refers to permanent disturbance of declarative memory despite preserved cognitive functions Markowitsch 2001 Moscovitch 2004 Patients with anterograde amnesia AA consistently show different degrees of impairment in forming declarative memory with normal implicit and working memory Studying amnesia provides insights on cognitive and biological mechanism underling memory and a representative model is vital for understanding existing data and making predictions Although researches on amnesia have progressed a lot since the famous case of H M a convincing and widely accepted model has yet to be established One of the main debates concerning it is whether the major impairment in amnesia declarative memory is relating to the damage of an associative or dissociative neural system The presentation of amnesia varies across patients and data are inconsistent Hence it causes a great challenge of model s generalizability Since both of them are rather inconclusive in explaining all the existing data it is difficult to tell whether one is superior to the other Apart from the models themselves external factors have also posed difficulties in determining whether one of them is better

Although declarative memory could be subdivided the interdependent nature of the subcomponents makes it difficult to test them separately in a clean way Furthermore difficulty in making accurate predictions is also a barricade To predict impairments from brain lesions it is essential to localize the neural substrates of memory in the corresponding brain regions However the plasticity of brain and adoption of compensatory strategies make it difficult to map the impairments and preserved abilities in brain In this essay the associative model proposed by Squire and the dissociative model proposed by Aggleton and Brown A B would be used to illustrate the difficulties in determining whether one of them better describes amnesia It is argued that both of them are inconclusive in explaining all the of existing data Moreover the interdependent nature of subtypes within declarative memory and plasticity of brain make both of the models not testable or predictable To compare on the same ground Squire s model has relative strengths in explaining single dissociation in human cases while double dissociation in animal studies seems to support A B s model instead According to the medial temporal lobe memory model proposed by Squire declarative memory is supported by a unitary system which includes hippocampus entorhinal perirhinal and parahippocampal cortex Squire Zola Morgan 1991

As a single entity degrees of impairment should be proportionated to the extent of lesion in the system regardless of location Squire et al 2004 Whereas A B advocates a dissociative model concerning the hippocampal diencephalic system As claimed by this model recollection and familiarity are two distinct pathways Aggleton Brown 1999 To recall episodic memory recollection supported by the Papez circuit is required This circuit is connected by fornix and mammillothalamic tract and includes hippocampus mamillary bodies MB and anterior thalamus nuclei ATN Whereas recognition is subserved by recollection and or familiarity As long as recognition does not involve relational binding between elements familiarity pathway that supported by cortices perirhinal entorhinal and parahippocampal cortex and medial dorsal thalamic nuclei MDTN is sufficient Brown Aggleton 2001 In order to validate A B s idea double dissociation between recognition and recall is required The clinical evidence that A B s model initially built upon were cases showing selective impairments in recall which were suggested to be the results of sole damages in recollection but not familiarity pathway Aggleton Brown 1999 Further support was provided by subsequent cases that consistently presented similar selective impairments For instance patient KN and YR who had selective bilateral lesions in hippocampus performed far below average in recall tasks but relatively normal in recognition of both verbal and visual materials Aggleton et al 2005 Kopelman et al 2007

Similarly patient DN had recollection pathway disrupted by fornix compression also presented selective impairments in recall Vann et al 2008 Nevertheless as what endorsed by A B in their later review 2006 to date there is no recorded case showing the opposite selective impairments Although receiver operating characteristic ROC analysis has successfully differentiated recollection and familiarity pathway underlying patients preserved memory in recognition Aggleton et al 2005 Cipolotti et al 2006 such statistical dissociation is still qualitatively different from clinical dissociation This single dissociation does not provide convincing evidence for A B s model but is more likely to go with Squire s instead According to his model recognition could be independently preserved since it is cognitively less demanding and would be impaired only when pathology is severe enough Squire et al 2004 In other words it is not the location but the extent of lesion that is important for explaining patterns of impairments If this is the case then impairments in recognition and recall should be proportionate to each other since they belong to one single system 

Consistent with this studies adopted MRI scanning found volume in medial temporal lobe and hippocampus negatively correlated with impairments in both recall and recognition Kopelman et al 2007 Kramer et al 2005 However the discussed evidence is still inconclusive since anatomical lesion may not be equivalent to functional lesion Bachevalier 1996 Aggleton 2008 Animal studies enable more confined focal brain lesion and they could complement lesion studies in human to investigate the neuroanatomy underlying patterns of memory impairment In contrast to human cases animal studies have clearly presented double dissociation between recall and recognition Eacott and Norman 2004 defined episodic like memory in animals as the interaction of eventual what temporal spatial where and contextual which information Without either where or which component animal s memory of what could be supported by familiarity alone The speciality of episodic like memory is probably subserved by the relational binding function of hippocampus which enables mental construction of scenes Maguire et al 2010 Supporting this idea

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