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Psychogenic amnesia, also known as functional or dissociative amnesia, is a disorder characterized by abnormal memory functioning in the absence of structural brain damage or a known neurobiological cause; severe cases are very rare.[1] It is defined by the presence of retrograde amnesia or the inability to retrieve stored memories and events leading up to the onset of amnesia and an absence of anterograde amnesia or the inability to form new long term memories.[2][3][4] In most cases, patients lose their autobiographical memory and personal identity even though they are able to learn new information and perform everyday functions normally. Other times, there may be a loss of basic semantic knowledge and procedural skills such as reading and writing.[5]
There are two types of psychogenic amnesia, global and situation-specific.[5][6] Global amnesia, also known as fugue state, refers to a sudden loss of personal identity that lasts a few hours to days.[4] This is preceded by severe stress and/or depressed mood. Fugue state is very rare, and usually resolves over time, often helped by therapy.[7] Situation-specific amnesia is a type of dissociative amnesia occurs as a result of a severely stressful event, as in post-traumatic stress disorder. Dissociative amnesia is due to psychological rather than physiological causes and can sometimes be helped by therapy.[7]
There are three types of memory – sensory, short-term, and long-term memory. Sensory memory lasts up to hundreds of milliseconds and short-term memory lasts from seconds to minutes while anything else longer than short-term memory is considered to be a long-term memory.[2][8]
The information obtained from the peripheral nervous system (PNS) is processed in four stages – encoding, consolidating, storage, and retrieval.[2] During encoding, the limbic system is responsible for bottlenecking or filtering information obtained from the PNS. According to the type of information given, the duration of consolidating stage varies drastically. Majority of consolidated information gets stored in the cerebral cortical networks where the limbic system record episodic-autobiographical events. These stored episodic and semantic memories can be obtained by triggering the uncinate fascicle that interconnects the regions of the temporofrontal junction area.
Emotion seems to play an important role in memory processing in structures like the cingulated gyrus, the septal nuclei, and the amygdala that is primarily involved in emotional memories.[2][9] Functional imaging of normal patients reveal that right-hemisperic amygdala and ventral prefrontal regions are activated when they were retrieving autobiographical information and events. Additionally, the hippocampal region is known to be linked to recognizing faces. Researchers have found that emotional memories can be suppressed in non-mentally ill individuals via the prefrontal cortex in two stages – an initial suppression of the sensory aspects of the memory, followed by a suppression of the emotional aspect.[10] It has also been proposed that glucocorticoids can impair memory retrieval, though to date this has only been tested in rats.[11]
Traumas can interfere with several memory functions. van der Kolk divided these functional disturbances into four sets, traumatic amnesia, global memory impairment, dissociative processes and traumatic memories’ sensorimotor organization. Traumatic amnesia involves the loss of remembering traumatic experiences. The younger the subject and the longer the traumatic event is, the greater the chance of significant amnesia. Global memory impairment makes it difficult for these subjects to construct an accurate account of their present and past history. Dissociation refers to memories being stored as fragments and not as unitary wholes. Not being able to integrate traumatic memories seems to be the main element which leads to PTSD. In the sensorimotor organization of traumatic memories, sensations are fragmented into different sensory components.[12]
Clinically, psychogenic amnesia is characterized by the loss of the ability to retrieve stored memory without having damages to the brain while organic amnesia is characterized by damages to the medial or anterior temporal and/or prefrontal regions caused by stroke, traumatic brain injury, ischemia, and encephalitis.[2][5] Some characteristics that define organic amnesia is the maintenance of personal identity, basic semantic knowledge and procedural skills as well as neuroradiological images showing cerebral damage to the cortical and/or subcortical areas known to be associated with long-term memory while some characteristics that define psychogenic amnesia is the loss of personal identity, semantic knowledge, and procedural abilities at least in the early phase of amnesia as well as damage directly affecting cerebral areas critical for memory functioning that cannot be detected in clinical history or neuroradiological exams.[5]
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