In neurology, retrograde amnesia (RA) is the inability to access memories or information from before an injury or disease occurred. 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. Diagnosing RA generally requires using an Autobiographical Memory Interview (AMI) and observing brain structure through magnetic resonance imaging (MRI), a computed tomography scan (CT), or electroencephalography (EEG). 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. When an individual experiences focal RA, a combination of their episodic and semantic memories may be affected. In one case study, a middle-aged woman experienced focal RA after significant head trauma. Similar to other forms of RA, the inability to recall past information characterizes the isolated form. It is often confused with peritraumatic amnesia that commonly follows mild concussions, but the severity and duration of PRA differs from that of peritraumatic amnesia. 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).

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. or childhood brain damage (e.g., shaken baby syndrome).

TBI varies according to impact of external forces, location of structural damage, and severity of damage ranging from mild to severe.

Specific cases, such as that of patient ML, support the evidence that severe blows to the head can cause the onset of RA. In this specific case there was an onset of isolated RA following a severe head injury. The brain damage did not affect the person's ability to form new memories. Therefore, the idea that specific sections of retrograde memory are independent of anterograde is supported. Normally, there is a very gradual recovery, however, a dense period of amnesia immediately preceding the trauma usually persists. Primarily referred to as psychogenic amnesia or psychogenic fugue, it often occurs due to a traumatic situation that individuals wish to consciously or unconsciously avoid through intrapsychic conflicts or unconscious repressions. The onset of psychogenic amnesia can be either global (i.e., individual forgets all history) or situation specific (i.e., individual is unable to retrieve memories of specific situations).

Patients experiencing psychogenic amnesia have impaired episodic memory, instances of wandering and traveling, and acceptance of a new identity as a result of inaccessible memories pertaining to their previous identity.

Studies of specific cases, such as 'AMN', support evidence of traumatic experiences as a plausible cause of RA. AMN escaped a small fire in his house, did not inhale any smoke, and had no brain damage. Nevertheless, he was unable to recall autobiographical knowledge the next day. This case shows that RA can occur in the absence of structural brain damage.

After a traumatic head injury, emotional disturbances can occur at three different levels: neurological, reactionary, and long-term disturbances. Neurological disturbances can change emotional and motivational responses. Reactionary disturbances effect emotional and motivational responses as well, but reflect the failure to cope with environmental demands. Someone with this might withdraw from the environment that they are placed in because they no longer know how to handle the cognitive resources.

Nutritional deficiency

RA has been found among alcohol-dependent patients who have Korsakoff's syndrome. Korsakoff's syndrome patients develop retrograde amnesia due to a thiamine deficiency (lack of vitamin B1). Also, chronic alcohol use disorders are associated with a decrease in the volume of the left and right hippocampus.

Surgery

Henry Molaison had epilepsy that progressed and worsened by his late twenties. The severity of his condition caused him to undergo surgery in an effort to prevent his seizures. Unfortunately, due to a lack of overall known neurological knowledge, Molaison's surgeons removed his bilateral medial temporal lobe, causing profound AA and RA. The removed brain structures included the hippocampus, the amygdala, and the parahippocampal gyrus, now called the medial temporal lobe memory system. Tests show that information from days and weeks before the ECT can be permanently lost. The results of this study also show that severity of RA is more extreme in cases of bilateral ECT rather than unilateral ECT. Impairments can also be more intense if ECT is administered repetitively (sine wave simulation) as opposed to a single pulse (brief-pulse stimulation).

  • Electroconvulsive shock (ECS): The research in this field has been advanced by using animals as subjects. This is done to further understand RA.

Diagnosis

Since RA affects people's memories to varying degrees, testing is required to fully diagnose RA; these tests, however, are inherently limited if a patient's previous neuropathological medical history is unknown. 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. The AMI asks patients targeted questions about three different portions of their life: childhood, early adult life, and recent life.

Brain structures

The most commonly affected areas are associated with episodic and declarative memory such as the hippocampus, the diencephalon,

  • The hippocampus deals largely with memory consolidation, In other words, retrograde amnesics "know" about 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.
  • The temporal lobes are essential for semantic and factual memory processing. Aside from helping to consolidate memory with the hippocampus, Lesions in the right hemisphere and right frontal lobes result in the impaired recall of non-verbal material, such as music and drawings. Difficulties in studying this region of the brain extends to its duties in comprehension, naming objects, verbal memory, and other language functions.

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. Thus, the brain can learn to be independent of the impaired hippocampus, but only to a certain extent. many individuals with brain damage have volunteered to undergo countless tests to advance our scientific knowledge of the human brain. For example, Henry Molaison (HM) was someone with significant brain damage and participated in a lot of neurological research. Furthermore, he was also the most tested person in neuropsychology. All living people who participate are referred to in literature using only their initials to protect privacy.

Each case of RA has led to different symptoms and durations, where some patients have exhibited an inability to describe future plans, whether in the near future (e.g., this afternoon) or in the distant future (e.g., next summer) because of their inability to consolidate memories.

A few case examples are:

  • After a head injury, AB had to relearn personal information. Many of AB's habits had also changed.
  • CDA is a 20-year-old man who fell and experienced head trauma after being unconscious for a little less than an hour. He had a self-identity loss and a retrograde deficit limited to the autobiographical events 5 years before the trauma. He often showed signs of spontaneous speech that was iterative and sometimes incoherent. When he saw his family and friends, he was shocked at how old they looked because he remembered them from 5 years earlier. This case also included amnesia for procedural skills like the fear of shaving or driving, which ultimately was overcome. There were no psychological, neuropsychological, or brain damage problems. His recovery of memory was progressive and spontaneous, where after several months the amnesia was limited to the two years preceding the trauma. This was a classic case of PRA.

Although it may seem that people living with brain damage have great difficulty continuing the usual day-to-day aspects, they still can accomplish many feats. People with RA are able to lead a normal life. For instance, KC is a man who has many functional aspects intact; normal intelligence, unaffected perceptual and linguistic skills, short-term memory, social skills, and reasoning abilities. This describes a problem encoding, storing, or retrieving information that can be used in the future. These two conditions can, and often do, occur in the same patient simultaneously, TGA is very difficult to study because of the patients' quick recovery. This form of amnesia, like AA, remains distinct from RA.

Post-traumatic amnesia (PTA) is a state of confusion that occurs immediately following a traumatic brain injury in which the injured person is disoriented and unable to remember events that occur after the injury.

Psychogenic amnesia, or dissociative amnesia, is a memory disorder characterized by sudden retrograde autobiographical memory loss, said to occur for a period of time ranging from hours to years.

See also

  • Amnesia
  • Anterograde amnesia
  • Dissociative amnesia

References

  • Journal of Neuroscience
  • The strange case of Jonathan Overfeld from The State We're In radio show