Brain structures

The most commonly affected areas are associated with episodic and declarative memory such as the hippocampus,^[3] the diencephalon,^[4] and the temporal lobes.^[5]

• The hippocampus deals largely with memory consolidation,^[3] where information from the working memory and short-term memory is encoded into long-term storage for future retrieval. Amnesic patients with damage to the hippocampus are able to demonstrate some degree of unimpaired semantic memory, despite a loss of episodic memory, due to spared parahippocampal cortex.^[6] In other words, retrograde amnesics "know" an information or skill, but cannot "remember" how they do.
• The diencephalon and the surrounding areas' role in memory is not well understood. However, this structure appears to be involved in episodic memory recall.^[4]
• The temporal lobes are essential for semantic and factual memory processing. Aside from helping to consolidate memory with the hippocampus,^[5] the temporal lobes are extremely important for semantic memory. Damage to this region of the brain can result in the impaired organization and categorization of verbal material, disturbance of language comprehension, and impaired long-term memory. The right frontal lobe is critical for the retrieval of episodic information, while the left frontal region is more active for the retrieval of semantic information. [56] Lesions in the right hemisphere and right frontal lobes result in the impaired recall of non-verbal material, such as music and drawings.^[7] Difficulties in studying this region of the brain extends to its duties in comprehension, naming objects, verbal memory, and other language functions.^[8]

Brain plasticity has helped explain the recovery process of brain damage induced retrograde amnesia, where neuro-structures use different neural pathways to avoid the damaged areas while still performing their tasks.^[9] Thus, the brain can learn to be independent of the impaired hippocampus, but only to a certain extent.^[10] For example, older memories are consolidated over time and in various structures of the brain, including Wernicke's area and the neocortex, making retrieval through alternate pathways possible.^[2]

Types of retrograde amnesia

As previously mentioned, RA commonly results from damage to the brain regions most closely associated with episodic and declarative memory, including autobiographical information. In extreme cases, individuals may completely forget who they are. Generally, this is a more severe type of amnesia known as global or generalized amnesia.^[11] However, memory loss can also be selective or categorical, manifested by a person's inability to remember events related to a specific incident or topic. Patients also differ in durations of RA (how long they can't recall information) and durations of what is forgotten (past time frame for which information is unavailable).

Temporally graded retrograde amnesia

In temporally graded retrograde amnesia, victims eventually recover most memories following the onset of RA. This suggests that the hippocampal formation is only used in systematic consolidation for temporary, and short periods of time, until long-term consolidation takes place in other brain structures.[12] Here, the fact that damage to the hippocampal formation can eventually overcome RA suggests that other brain structures are able to take over the jobs of the malfunctioning regions. RA can also progress and further deteriorate memory recollection, as in the case of Korsakoff syndrome and Alzheimer's disease, due to the ongoing nature of the damage caused by the illnesses. The degree to which different patients recover from RA differs in time (some take a few days while others a few decades) and content (some will only remember certain specific instances while others more).