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Temporal gradient

Temporal gradient is a pattern of retrograde amnesia in which recent memories are being disrupted more in comparison to older memories. In this case temporal explains the dependence on time, whereas gradient explains the magnitude of a score dependent on the point of measurement.

Temporal gradient means there is a difference in the disruption of memories based on when the memories have been formed, in this case a bigger disruption of newer memories in comparison to older memories. Contrary to temporal gradient is flat gradient. In the case of flat gradient, there would be no difference in the magnitude of disruption, an old and new memories are being equally disrupted. The finding of the temporal gradient is in line with the findings that are described as the standard consolidation theory.

According to the standard consolidation theory, the hippocampus and the temporal lobes are needed for episodic memory storage and retrieval in the early stages of forming memories. These episodic memories consist of many different components (for example: smell, sound, sight and other context related characteristics) which are all stored in different parts of the cerebral cortex. As time progresses the cortex in itself becomes capable of retrieving memories through memory consolidation. Memory consolidation is the process in which the different components of episodic memories form a direct link to each other and thus do not need mediation from the hippocampus to be retrieved. Before consolidation these parts of episodic memories are separate pieces only unified through the hippocampus. This explains why old memories are less disrupted in comparison to newer memories.

Test methods

The temporal gradient in retrograde amnesia is shown in many researches such as with depressed psychiatric patients receiving electroconvulsive therapy (ECT). There are some tests that can be used to describe such a temporal gradient in retrograde amnesia.

Remote memory test

The remote memory test is a test method developed to assess the temporal gradient in retrograde amnesia. This test has initially been used on depressed psychiatric patients before and after they received five ECT treatments. The test is constructed in a multiple choice format in which the titles of several television programs broadcast for the period of 1957 to 1972 were being asked. Patients had undergone one version of the remote memory test before receiving ECT treatments to help cope with the depression and received the other version of the test after having received full ECT treatments. Results indicated that patients were relatively better at remembering names of television programs broadcasted four to seventeen years ago than programs broadcast one to three years ago. These results indicate that due to electroconvulsive therapy there was a temporal gradient in the amnesia, concluding for far reaching amnesia for longterm memories that have been formed relatively short before.

The results of the remote memory test can be interpreted as being caused by an impairment due to electroconvulsive therapy and not depression, because the test has also been carried out under a control group which resulted in a slightly higher but not significant difference.

Experiments on animals

Using experimental animals results in great experimental control, including the locus and extent of the lesion. When researching temporal gradient in retrograde amnesia by experimenting on animals, studies usually focus on the effects of hippocampal formation or fornix lesions on memories acquired at different times before surgery. Animals in these studies are given equivalent amounts of training at two or more times before surgery. These memories are required by a wide variety of spatial and nonspatial tasks, for example; context fear conditioning, trace eyeblink conditioning, object discrimination learning, and place learning in a water maze. Most of the studies use either cats, monkeys, rabbits, rats or mice.

After the surgery these tasks are performed again. The performances on the tasks involving remotely acquired memories (memories acquired longer before the surgery) or recently acquired memories (memories acquired closer before the surgery) say something about if the retrograde amnesia is graded or not.

Brain substrates

There are a couple of brain substrates (being certain areas/regions in the brain) responsible for memory. The medial temporal lobe, and especially the hippocampus is vital for memory. Temporally graded amnesia is often associated with damage to structures of the medial temporal lobe. If the hippocampus, located in the medial temporal cortex, is damaged, temporal gradient has a chance to arise.

Hippocampal activity is associated with the early stages of memory storage and the retrieval of episodic memories. Hippocampal activity decreases strongly with time. An episodic memory becomes independent of the hippocampus, because a cortical circuit for the memory forms, according to the standard consolidation theory. The connections between the cortical areas of the memory grow stronger every time the memory is retrieved. After numerous retrievals the cortical circuit becomes independent of the hippocampus and the memory can thus be retrieved without hippocampal activity. The multiple trace theory explains this almost identical, but with a small difference. This theory states that an episodic memory becomes more semantic every time the memory is retrieved, so the memory would become independent of the hippocampus as well. These principles explain temporal gradient, because when a patient suffers from a lesion to the hippocampus, they forget more recent memories and tend to remember older ones. Because older memories have become independent of the hippocampus, damage to this structure does not affect these memories. More recent memories are dependent on the hippocampus and thus get affected more by damage to this structure. Extensive damage, limited to the hippocampus, can cause temporally graded amnesia for memories as much as 15-25 years before the damage.

The hippocampus is not the only part in the medial temporal lobe that is crucial for memory. There is evidence for the importance of other structures in the medial temporal lobe in memory. The region of the medial temporal lobe is known as vulnerable. If the medial temporal lobe is damaged, this often results in amnestic alterations. This damage is common with epilepsy, Alhzheimer’s disease, senile dementia or Pick’s disease. The system of the medial temporal lobe includes anatomitically related structures that are essential for declarative memory. The hippocampal region and the adjacent perirhinal, entorhinal and parahippocampal cortices are part of the system.

The region around the hippocampus, but within the temporal lobe is also vital for memory. One finding is that retrograde amnesia is greater when there is more damage to the medial temporal lobe than when the damage is limited to the hippocampus. Retrograde amnesia following large medial temporal lobe lesions can cover 40-50 years. Besides that, lesions lateral to the medial temporal lobe sometimes increase the extent of retrograde amnesia.

When damage occurs in the adjacent cortices and the hippocampus, this will most of the time also lead to a more profound and extended retrograde amnesia. The difference between this and retrograde amnesia following damage limited to the hippocampus, suggests a role for the adjacent cortices in memory.

It seems that the hippocampus and other structures in the medial temporal lobe are relevant for memory and when damaged result in temporally graded retrograde amnesia, which means they are also crucial for the temporal gradient.

Causes

There are different causes of temporal gradient including amnesia's, Koraskoff's syndrome and confabulation.

• Korsakoff’s syndrome is a rare illness that occurs when a person has a deficiency in thiamine, which can arise from chronic alcohol abuse. It can cause severe retrograde amnesia and confabulation which can be attributed to temporal gradient. The condition generally damages two parts of the brain, the mediodorsal nucleus and the mammillary bodies. Patients with Korsakoff’s syndrome might also tell fictional stories about events they have previously forgotten. Meaning that their memory is not strong enough to distinguish new and old memories.

• Temporary global amnesia (TGA), which contains anterograde and retrograde amnesia, is usually present in a transient, or temporary disruption of memory. There are no proven causes, such as a head injury or epilepsy. The underlying causes are unknown, but about a third of patiënts have an obvious precipitating event such as physical exercise, cold water swimming, or emotional tension. Because of retrograde amnesia for incidents that happened during the previous decade or so, this can be attributed to the temporal gradient.

• Temporal gradient in confabulation is the result from damage to the ventromedial prefrontal cortex. Because of this damage, an impairment of strategic retrieval processes occurs which are associated with confabulation. The strategic retrieve processes help with guiding and initiating search in semantic and episodic memory, but also with organizing and monitoring the output of the semantic and episodic memory systems. A well known patient who suffers from temporal gradient in confabulation is P.L.

• Functional amnesia (also known as psychogenic amnesia) is a sudden, massive retrograde memory loss that appears to be caused by psychological factors. Dissociative amnesia is one form of functional amnesia, in which patients lose memory of a particular traumatic event. a more dramatic type is dissomatic amnesia, in which patients lose memory of a specific, traumatic event.

• Focal retrograde amnesia is an uncommon neurocognitive disorder characterized by an isolated loss of retrograde memory. Specifically the loss of retrograde memory in contrast to normal learning of new knowledge. This syndrome has been linked to a variety of neurological illnesses, including traumatic brain injury, encephalitis, hypoxia, and epilepsy. Related lesions range in size and location, and include neocortical, limbic, and brain stem components. It is suggested that the medial temporal, temporopolar, and frontal cortices play a crucial role in remote memory processes and, as a result, should be involved in the pathophysiology of focal retrograde amnesia. There are two well known patients, T.H. and K.N., who suffer from focal retrograde amnesia.

Patients

• Patient P.L. suffers from temporal gradient in confabulation, she developed an amnesic confabulatory syndrome which occurred in both semantic and episodic memory tasks. Her confabulatory syndrome is not part of a general decrease of cognitive functions, because her results on tasks which are sensitive to frontal dysfunction are not supporting this. On one task, the temporal gradient on her performance came to light. She was asked to identify different photographs of events and people who were very familiar to her. The results showed that she recognized photographs which were taken a long time ago very well. As time went up, more to present time, the results got worse. These results show that memories are stored in a temporal criterion and not in a random way. The temporal gradient presumably reflects the stability and strength in which episodic memories are stored according to a temporal criterion. According to this criterion, recent memories are stored less stronger than older memories.
• Patient T.H. is a male born in 1974 who developed focal retrograde amnesia. T.H. crashed his vehicle and when he was rescued, he was unconscious and transported to the hospital. He recovered consciousness the next day and was able to carry on a modest conversation, but he was unable to provide his name or age. He did not recognize his mother when she came to see him until the hospital staff pointed her out to him. His memory loss for autobiographical events was gradually apparent by the fact that he responded "don't know" for practically all situations from his life prior to the trauma when he spoke with his parents or friends. In addition, his knowledge of common place objects was shattered. Given a pen and a toothbrush, for example, he could not name them, understand their names, or illustrate how to use them. When he was told the names and functions of these things, he was able to relearn them quickly but not perfectly. With time, he was able to remember his own name and his mother's name, and when his name was called, he reacted quickly and naturally. He seems to recognize his mother as well. His demeanor was childlike, in the sense that he occasionally used phrases associated with early childhood and did not recognize the seriousness of his condition. His mother, on the other hand, reported that his personality, which was happy and gregarious, remained mostly constant. His speech rate was a little slow, but there were no signs of dysarthria or agrammatism. Although there were rarely anomic or paraphasic errors in dialogue. When T.H. was asked about personal incidents that happened multiple days after de trauma that caused the focal retrograde amnesia, T.H. said that he only had a weak feeling-of-knowing about the mentioned incidents. T.H, could remember other incidents such as the first time he left the hospital. This may indicate relative preservation of T.H.'s ability to acquire and remember new information.
• Patient K.N. is a female born in 1973 who developed focal retrograde amnesia. When K.N. was a college student, she had general malaise and rapid weight gain. A diuretic helped her recover temporarily, but she was admitted to a local hospital in January 1997, where she was diagnosed with idiopathic edema which is a cause of fluid retention and swelling with an unknown cause. Anorexia, dyspnea, and recurrent hyperthermia were among her other symptoms. She experienced right-sided motor disturbance and mild dysarthria on May 17 2000, following an episode of hyperthermia. Plantar responses were negative, while deep tendon reflexes were normal. An MRI scan of the brain revealed no lesions. Despite the fact that she faced numerous challenges, it was believed that she was functioning relatively well and had no significant psychiatric issues. K.N. later got a fever of 41.7 °C with no identified cause. The next day, her fever was virtually gone, and she was able to speak again, but she had forgotten her name. She pretended she did not recognize her parents when she saw them. Her autobiographical memory appeared to be completely ravaged, she could not recall any life events and claimed, "I have lost all my memories." In addition, she was unable to recognize or utilize common objects such as a television, tape recorder, or the buttons on her clothing. Despite the fact that she quickly relearned these item names and usages in reaction to instruction, her performance was not flawless. A few days after the onset of her amnesia, K.N. was evaluated in depth. She was alert but disoriented at the same time. She remembered her name and seemed to identify it on the first day of the assessment, but she could not remember her mother's name. She gradually remembered her parents' names, but when asked about them, she said she did not know much about them. She acted like a child, using a kid's speech, being overly polite at times, and appearing unconcerned about her diseases. Her premorbid personality (cheerful and friendly) was mostly preserved, according to her mother. Dysprosody and agrammatism were not noted, despite the fact that she was dysarthric. Substantial word finding issues were detected, and she frequently inquired about the meanings of words used by the examiners. K.N. was also asked about incidents that happened several days after de trauma that caused the focal retrograde amnesia, K.N. could remember most incidents, but her recall of dates was off by multiple days. This indicates that K.N.'s ability to learn and recall new information is still somewhat intact.