Parahippocampal gyrus
The parahippocampal gyrus is also known as the hippocampal gyrus. It is a
of the that surrounds the and is important in
and retrieval of . It collects a flow of information from the
association regions that it transfers to the hippocampus, mainly through the entorhinal
cortex. The and cortices are located at the front of the
parahippocampal gyrus, both of them d in ing information from
different sensory systems and are generally accepted to be essential in memory
functioning. (; ; )
Amygdala
Amygdalae are groups of which deeply located in the MTL as shown in figure 2. They are implicated in balancing the effects of emotional arousal on the strength of the memory for an event. The amygdalae are also involved in the modulation of as shown by many research and studies. (McGaugh, J.L., 1966 & 1992;)
Anatomical studies on structure of MTL and memory process
There were at least 50 years of evidence developed to support the relation between the
structure of MTL and memory processing. Mishkin et al. in 1983 firstly proposed
research that supported the hypothesis which states how information is encoded and
retrieved during memory processing. Squire et al. (1992) used positron emission
tomography (PET) scans to show that blood flow in the right hippocampus was higher
when participants were engaged in cued recall task. Anthony R. Mcintosh and his
colleagues also carried out a PET study of regional cerebral blood flow related with
remembering experienced events at a particular time in a particular place, i.e. the events
involved contents (“what”), the location (“where”) and the time (“when”).
It was found that the sensory information about qualities of objects (“what”) and
spatial information (“where”) are related to the perirhinal cortex and parahippocampal cortex. There are some connections between these two cortices and between the entorhinal regions; that two kinds of information (qualities of objects and spatial information) however mainly centralized within the hippocampus. H. Eichenbaum and colleagues (2007) made a conclusion:
The cortical outputs of hippocampal processing involve
feedback connections from the hippocampus successively
back to the entorhinal cortex, then perirhinal and
parahippocampal cortices, and finally, neocortical areas
from which the inputs to the MTL originated.
Nevertheless, it was suggested that if an item is presented as a cue for memory, a full picture of the event can be completed in the hippocampus as well as a retrieval of the contextual representation can be mediated in the parahippocampal cortex and medial entorhinal area. This shows that different kinds of memory may be controlled by different regions in the MTL of the brain.
Classic cases about the role of MTL on memory
Studies on this field were usually done by investigating patients with temporal lobe
injury. It was found that there would be resection on memory function if a person suffered
from bilateral hippocampal lesions. Two case studies are to be analysed in this section. It
is notable that these patients only had impairments in some aspects of memory but other
aspects may remain normal.
Case H.M.
The study of H.M. (Scoville & Milner, 1957; Milner, 1959) was one of the most classic
cases in neuropsychology. H. M. was born in 1926 and his development was normal, until
he was rendered unconscious for about five minutes after being knocked down by a
bicycle at the age of seven. He suffered with recurrent generalized seizures since he was
10 and these epileptic seizures deteriorated the quality of his life. His family then decided
that a surgery might be undertook to relieve this illness.
On September 1, 1953, the bilateral medial temporal lobe resection was carried out. The removals extended from the temporal tips for a distance of about 8 cm (see figure 3), two-thirds of the hippocampus, bilateral hippocampal gyrus with the entorhinal cortex, uncus and amygdale were destroyed in the operation.
The effect of the surgery on his memory was dramatically and instantly obvious.
H.M. was found to have a complete loss of memory for events that occurred prior to 19 months preceding the bilateral medial temporal lobe resection. Early memories however seems to be normal and there was no impairment of personality, emotion, or intelligence.
It was concluded by Scoville and Milner that the hippocampus was pivotal for the formation of memory to recent events and they also prudently consider that adjoining
structures such as the amygdala might also be involved.
Case N.A.
The case of N.A. was firstly described by Professor Teuber and his colleagues
(Teuber, Milner and Vaughan, 1968). N.A. was born in 1938 and his development was as
usual until an incident happened in December 1960. A foil accidently penetrated the
cribriform plate and upwardly went into the left forebrain. He lost his consciousness for a
few minutes and displayed a right-sided weakness and of the right eye
muscles after the accident. A surgery was then operated in order to have the dural tear
repaired. He gradually recovered, however anterograde amnesia for declarative memories
was retained.
It was showed in the magnetic resonance imaging (MRI) studies in 1986 that there
was a comprehensive damage to the thalamus and the MTL. N.A.'s memory from the time
of his injury to the present has remained impaired, and similar to H.M., N.A. showed
normal language skills or intelligence however both of them fail badly on formal tests on
learning new ability.
It can once again be concluded that the resection of the medial temporal lobe and regions can lead to memory impairment. In the other words, the medial temporal lobe and adjoining structures play an important role in memory processing.
Psychologists’ views on medial temporal amnesia
Milner in 1974 suggested that a positive correlation is shown by the gravity of the memory deficit with the extent of hippocampal region resection. For example in the case of H.M., the damage was extensive (involved the amygdale, uncus and the anterior hippocampus) would resulted in a severe and lasting amnesia. However in other cases of which the resection only included the amygdale and uncus, patients were not considered to be suffered from amnesia.
Another view was proposed by Mishkin in 1978 that connected damage to the hippocampus and amygdala is required to produce amnesia. Mishkin pointed out that damage to both these structures was always found in surgical amnesia cases in human. Thus, there are two possibilities on the evidence from the surgical cases: amnesia can caused by hippocampal damage alone or conjoint hippocampal-amygdala (H-A) damage is necessary. Evidence was gained that hippocampal lesion alone could lead to amnesia but the H-A damage produced a larger deficit. Nevertheless, the evidence was gained from studies on monkey and could not be generalized to human’s situation.
Although it has long been accepted that the MTL is vital for memory process, further research thus will be needed to determine the relation between the memory deficit and hippocampal lesion.
Conclusion
The title of this essay invites explanation and analysis of regions in the brain which play a role in memory processing. It is important to know the structure and function of those areas in order to understand how they work together to support memory.
It was found that the medial temporal lobe and adjoining structures were essential in memory processing as support by previous studies (H. Eichenbaum et al. 2007; Mishkin et al. 1983) Although most of the evidence didn’t directly point out the positive relation between the MTL and memory, they proposed that the damage in MTL and areas lead to deficit in memory (Scoville & Milner, 1957; Professor Teuber et al., 1968). Nonetheless, Milner in 1974 stated the relation of MTL and amnesia, as well as Mishkin’s study on the effect of resection in hippocampus and/or amygdale on amnesia in 1978, provided strong support for the hypothesis that the MTL play an essential role on memory process.
Importantly though, human brain is the most complicated region in the body and the concept of memory is not clearly defined yet. Our knowledge in this field has been improved but it is still not enough for us to make any judgment. Further research is vitally important so as to provide more evidence on the relation of MTL and memory process.
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