User:Dinosaurchicken03/Retrograde amnesia

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In neurology, retrograde amnesia (RA) is the inability to access memories or information from before an injury or disease onset occurred. RA differs from a similar condition called anterograde amnesia (AA), which is in the inability to form new memories following injury or disease onset. Although an individual can have both RA and AA at the same time, RA can also occur on its own; this 'pure' form of RA can be further divided into three types: focal, isolated, and pure RA. RA negatively affects an individual's episodic, autobiographical, and declarative memory, but they can still form new memories because RA leaves procedural memory intact. Depending on its severity, RA can result in either temporally-graded or more permanent memory loss. However, memory loss usually follows Ribot's law, which states that individuals are more likely to lose recent memories than older memories. Diagnosing RA generally requires using an Autobiographical Memory Interview (AMI) or observing brain structure using magnetic resonance imaging (MRI), a computed tomography scan (CT), or electroencephalography (EEG).[insert Reed, Squire Citation]

Temporally graded Retrograde Amnesia
Memory loss in patients with temporally graded RA strongly follows Ribot's law, meaning that one will experience more memory loss for events closer to the injury or disease onset. This type of RA is commonly triggered in individuals with Korsakoff syndrome due to a combination of effects from long-term alcohol use and Wernicke encephalopathy. Debate has risen about why this temporal gradient forms in the first place. Initial theories proposed that the hippocampus and medial temporal lobe are not nearly as important for long-term memories compared to short-term memories. As memory processing occurs in the brain, neocortical regions can directly communicate with each other over time and thus do not rely as heavily on the hippocampus for long-term memory storage. Hence, if an individual experiences retrograde amnesia that damages the hippocampus, they will lose less long-term memories compared to short-term memories. This theory has been challenged by the multiple-trace theory, which claims that the brain develops a hippocampal trace each time a memory is retrieved. Since more hippocampal traces are present for older memories, it is easier for older memories to remain intact when temporally graded RA occurs.

Focal, Isolated, and Pure Retrograde Amnesia
An absence of anterograde amnesia (AA) characterizes pure forms of RA, which fall into three main categories: focal, isolated, and pure RA. Slight differences in the use of these terms to describe a pure form of RA are summarized below:

A pure form of RA is rare as most cases of RA co-occur with AA. A famous example is that of patient ML. The patient's MRI revealed damage to the right ventral frontal cortex and underlying white matter, including the uncinate fasciculus, a band of fibers previously thought to mediate retrieval of specific events from one's personal past.

During consolidation, the hippocampus acts as an intermediate tool that quickly stores new information until it is transferred to the neocortex for the long-term. The temporal lobe, which holds the hippocampus, entorhinal, perirhinal and parahippocampal cortices, has a reciprocal connection with the neocortex. The temporal lobe is temporarily needed when consolidating new information; as the learning becomes stronger, the neocortex becomes more independent of the temporal lobe.

Studies on specific cases demonstrate how particular impaired areas of the hippocampus are associated with the severity of RA. Damage can be limited to the CA1 field of the hippocampus, causing very limited RA for about one to two years. More extensive damage limited to the hippocampus causes temporally graded amnesia for 15 to 25 years. Another study suggests that large medial temporal lobe lesions, that extend laterally to include other regions, produce more extensive RA, covering 40 to 50 years. These findings suggest that density of RA becomes more severe and long-term as the damage extends beyond the hippocampus to surrounding structures.

Diagnosis
Since RA affects people's memories to different degrees, testing is required to fully diagnose RA; these tests, however, are inherently limited if a patient's previous neuropathological medical history is unknown. As a result, some clinicians diagnose RA by testing patients about factual knowledge, such as current public events. This testing is limited because people's knowledge about current events varies. Furthermore, these tests must be adjusted to account for the time period that a patient is alive, which affects the amount of detail included in the questions asked. Since some information obtained from this testing is subjective, it is difficult to verify how accurately memories are recalled; this difficulty is especially true for memories from the distant past.

To avoid the previous issues, many researchers test for RA using the Autobiographical Memory Interview (AMI). The AMI asks patients targeted questions about three different time parts of their life: childhood, early adult life, and recent life. For each period of that individual's life, researchers ask questions that require the patient to use either their autobiographical or semantic memory. Through the AMI, researchers can understand the types of memories affected, as well as the degree of a patient's RA. AMI can be used in conjunction with functional brain imaging techniques like magnetic resonance imaging (MRI), computed tomography scans (CT) and electroencephalography (EEG). These techniques can be used to detect brain damage that contributes to RA.