One of the best known memory disorders is amnesia. It causes important deficits in the functions of coding, archiving and retrieving the information obtained from the experience. The cognitive process behind memory is complex and multifactorial, which means that amnesia is sometimes a difficult symptom to explain.
In this sense, amnesia patients have been an important source of data. Studying the different deficits, in fact, helps us to understand more deeply the structure and functioning of memory. Most studies on this topic focus on amnesic syndrome, a disease involving episodic memory impairment.
Analyzing amnesia in episodic memory means, first of all, considering the temporal location of the damage. From this point of view, it is possible to identify two types of amnesia: (a) retrograde, if the disappearance of the memory concerns data or events that occurred before the illness or accident, (b) anterograde, if the damage affects the ability to generate new memories. Let’s look at these two types of amnesia in more detail.
It is characterized by the inability to access the memories preceding the mnemonic damage. The time frame involved is variable and oblivion can cover a few days or a lifetime. Importantly, the abnormalities and damage that occur in this type of amnesia do not affect the ability to create new memories. Other types of memory, such as implicit or procedural, also remain intact.
This disorder is usually organic in nature, ie it is the consequence of brain damage; in particular it can be caused by lesions of the hippocampus, basal ganglia or diencephalon. It is also possible that retrograde amnesia occurs without any type of injury, in this case we speak of psychogenic or functional amnesia.
The case of Clive Wearing
A famous and serious case of amnesic syndrome was that of Clive Wearing. This subject was unable to remember everything that had happened before 1985, the year in which following an accident he had completely injured the hippocampus and severely damaged the temporal lobes. In addition to retrograde amnesia, Clive suffered from anterograde amnesia that did not allow him to create new memories. This patient’s life was moving in an eternal present from which he could not escape.
Unlike the retrograde type, those suffering from anterograde amnesia are unable to create new memories, starting from the trauma. The information stored before the injury occurred remains available, but always subject to normal oblivion phenomena. As in the previous case, the disorder is usually linked to an organic lesion of the hypothalamic and temporal structures.
A special feature of this disorder is the fact that only the encoding of explicit memory remains impaired. In simple terms, it means that the subject, despite his inability to create memories, can learn new skills, procedural or implicit. For example, if a patient with anterograde amnesia plays the piano every day, it will improve over time; of course, he won’t explicitly remember playing the piano: for him every day is like the first.
The case of HM
One of the most famous cases of anterograde amnesia was that of HM (initials of Henry Molaison). This subject had to undergo surgery to alleviate a problem of severe seizures. The operation involved removing most of the hypothalamic structures. This put an end to his epilepsy attacks, but it also caused severe memory damage.
HM was unable to generate new memories after the operation, but his executive and procedural skills remained intact. In normal conversation he was consistent and, apparently, he didn’t seem to have any problems; but if the interlocutor left temporarily and then returned, HM had already removed him.
We said earlier that the two types of amnesia are independent: one can occur without the other. In this regard it should be remembered that the reality of the amnesic syndrome is much more complex. The most common situation is that the damage affects a large number of brain nuclei leading to amnesia of a mixed nature.
But also the study of “pure” amnesia is interesting and useful to better understand how our memory works.