Amnesia: Types And Characteristics

One of the best known memory disorders is amnesia. This is a clinical problem that causes significant deficits when coding, storing and retrieving information from the experience. However, the basic psychological process of memory is complex and multifactorial, which results in amnesia being sometimes a symptom that is difficult to explain with precision.

In this sense, patients with amnesia have been an invaluable source when it comes to understanding a little better how our memory works. Investigating the different deficits that we can find helps us to understand more in depth the structure and functionality of memory. Most of the study on this topic has been done around amnesic syndrome, a syndrome in which episodic memory is impaired.

Now, when we investigate amnesia in episodic memory, we do so around a temporal organization of the damage. That is, we can organize amnesia into two types: (a) retrograde, if you have forgotten what happened before the illness or accident, and (b) anterograde, if the ability to generate new memories is impaired. In the next sections we are going to explain these two categories in a broader way.

Retrograde amnesia

Retrograde amnesia is characterized by the inability to access memories prior to the time the memory damage occurs. The period of time affected can be variable, from forgetting a few days to a lifetime. It is important to bear in mind that the syndromes, disorders and injuries that occur with this type of amnesia, the ability to create new memories is intact; Other types of memory, such as implicit and procedural, are also intact.

The origin and cause of this disorder is usually organic in nature. That is, it is the consequence of brain damage, specifically it can be caused by injuries to the hippocampus, basal ganglia or diencephalon. Although it is no less true that retrograde amnesia can occur without any type of injury, which has been called psychogenic or functional amnesia.

One of the most studied patients with a severe case of amnesic syndrome was Clive Wearing. This subject was unable to recall any information that had occurred prior to the accident in 1985. Accident that resulted in the destruction of his hippocampus and severe damage to his temporal lobes. In addition to retrograde amnesia, Clive also suffered from anterograde amnesia that did not allow him to create new memories; The life of this patient meant living in a continuous present from which he could not escape.

Anterograde amnesia

Unlike retrograde-type disorder, anterograde amnesia involves the inability to create new memories after injury. Of course, any information that has been stored before the injury remains stored in memory subjected to normal forgetfulness phenomena. Like the previous one, in most cases this injury is the product of an organic injury, where the hypothalamic and temporal structures are damaged.

A special quality of this disorder is that only the explicit memory encoding is impaired. This means that the subject, despite his inability to create memories, can learn new procedural or implicit skills. For example, if a patient with anterograde amnesia plays the piano every day, he will improve over time; Of course, he will not explicitly remember having played the piano, for him every day would be like the first day.

One of the most famous cases of anterograde amnesia was the case of HM (the acronym for patient Henry Molaison). This subject had to undergo surgery to alleviate severe epileptic seizures. In this surgical procedure, a large part of the hypothalamic structures was removed. This spelled the end of his epileptic seizures, but it also seriously damaged his memory.

HM was unable to remember anything new after the operation, but his executive and procedural skills were intact. When it came to engaging in conversation with him, he was consistent and at first glance didn’t seem to have any problems; But when I left the conversation and returned to him, he no longer remembered you.

Earlier we mentioned that the two types of amnesia are independent: one can appear without the other appearing. It should be remembered in this sense that, in particular,  the reality of the amnesic syndrome is much more complex. The most normal thing is that the lesions affect a multitude of brain nuclei, which triggers amnesias of a mixed nature. Even so, the study of these “pure” amnesias is interesting to better understand how our memory works.