fornix the ROC analysis was based on KN’s

fornix transection in macaques impaired their
ability to learn association between visual stimuli (what) and their reactions
that involved “where” and “which” information (Brasted et al., 2003). In later work
by Eacott and Gaffan (2005), they analysed the results of studies on memory in
rats, and found a clear double dissociation: while lesion in hippocampal system
impaired episodic-like memory and spared what-which or what-where recognition,
post-rhinal cortical lesion caused the reverse. Employing ROC analysis, spared
familiarity and impaired recollection were found to be underlying rats’
preserved recognition in similar tasks (Eichenbaum et al., 2010). There is no
doubt that episodic-like memory and neuroanatomy in animals are not identical
to episodic memory and neural system in human. Nevertheless, it seems that to
date animal study is the most feasible way to investigate impacts of confined
brain lesion on memory. As a result, the dilemma between single dissociation in
human cases and double dissociation in animal studies poses a difficulty in
determining which model is better.

 

Apart from the limitation of the models
themselves, the interdependent nature of subtypes within declarative memory has
also posed difficulty in dissociating them in tests. Returning to the case of
KN, although Aggleton (2005) suggested dissociation between recollection and
familiarity within preserved recognition, it was statistical results instead of
real behavioural dissociation. In particular, the ROC analysis was based on
KN’s confidence rating of performance. However, it should be cautious that KN’s
rating might not be reliable. In the study by Vann et colleagues (2008), the
case of patient DN with lesion in fornix was reported. Although DN attained a
high hit rate of 0.9 in recognition task on words, he was unable to classify
whether his responses were based on recollection or familiarity. Unlike recall
and recognition, familiarity and recollection are just two pathways rather than
two kinds of memory. Therefore, it is difficult to test them separately using
different tasks. Aside from recollection and familiarity, episodic and semantic
memory are also closely related. According to Squire’s model, they are both
under the single system of declarative memory (Squire et al., 2004); whereas,
A&B would argue that semantic memory is supported by perirhinal cortex
while episodic memory is supported by Papez circuit (Graham & Hodges,
1999). In contrast to A&B’s model, sometimes amnesic patients with
relatively focal damage in hippocampus show impairments in semantic memory
(Manns et al., 2003). Nevertheless, this may reflect patients’ disadvantage in
episodic memory in fact. Functional imaging in healthy subjects has shown
overlapping in brain regions activated during semantic and memory retrieval
(Rajah & Mclntosh, 2005). Semantic memory is usually tested by recalling
word lists. This is possible that healthy controls could gain advantage from
recollecting learning episodes in semantic tests, and this exaggerates the
relative impairment of semantic memory in amnesic patients. Moreover, amnesic
patients’ deficits in acquiring recent facts do not reflect pure impairments in
semantic memory. In the study by Manns and colleagues (2003), subjects were
asked to judge whether celebrities were dead and to recollect the memory of
hearing their death. Despite high accuracy rate, healthy subjects failed to
recall the corresponding episodic memory for more than 80% of their responses. A
considerable proportion of semantic memory might in fact be converted from
episodic memory with the passage of time. This means that tasks of factual
knowledge recollection do not test semantic knowledge exclusively, and again
exaggerated amnesic patients’ relative impairments in semantic memory. Although
it is uncertain if declarative memory is dissociable, its subtypes are at least
functionally closely related. As a result, it leads to lack of testability and
makes it difficult to determine whether familiarity and semantic memory are
independently preserved.

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In addition to the nature of memory, plasticity
of brain and adoption of compensatory strategies also make it difficult to
determine the neural substrates of cognitive process. In Brizzolara’s work
(2003), it was suggested that an associative model could no longer withstand
given that semantic knowledge could develop in children with early-onset
amnesia, which also known as developmental amnesia (DA). There are a number of
cases showing impaired episodic memory with preserved semantic memory. For
instance, the three patients suffered from developmental amnesia caused by
hippocampal lesion reported in the classic study by Vargha-Khadem and
colleagues (1997) were able to study in mainstream schools despite impaired
episodic memory; similar selective impairment was also observed in adult-onset
amnesic patients (Balthazar et al., 2007). Nevertheless, this again is single
dissociation which insufficient to support A’s model. It is possible that
semantic memory is more accessible than episodic memory, and it would be
impaired only when the lesion is severe enough, like H.M. (O’Kane et al.,
2004). Not only the single dissociation makes Brizzolara’s
claim not convincing, functional reorganization within the brain and/or
compensatory strategies could also explain the preserved ability to acquire
semantic knowledge. In a recent review paper (Jaimes-Bautista et al., 2015), it was reported
that patients with right temporal lobe epilepsy showed increased activity in
contralateral hippocampus during successful semantic retrieval, while patients
with left temporal lesion failed in the task and did not show similar
compensatory activation in related brain regions. The contrast is possibly due
to functional reorganization in the former case. Since human brain develops
rapidly during early age, it has greater plasticity before becoming fully
matured. Consistent with this, preserved semantic memory is more frequently
found in DA cases (Blumenthal et al., 2017). Jon, a well-documented case of DA, showed normal
verbal IQ despite impaired episodic memory (Vargha-Khadem et al., 1997), Although reduced
hippocampal volume was observed, it was found that Jon showed increased
activity in cerebrum and prefrontal cortex during memory retrieval (Manning,
2008). The extent of increased brain activity’s contribution is uncertain, but functional
reorganization in brain during early development should allow some kinds of
compensation. Since functional reorganization only appears in some patients but
not the others, patterns of memory impairments may be inconsistent despite
similar brain lesion. Therefore, it is difficult to determine whether a brain
region is necessary 

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