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Memory and the brain
How Memory Works

Amnestic Syndromes
The modern era of memory research

People who consume alcohol to the point of acute intoxication can experience “blackout” episodes: periods in which they engage in conversations and perform tasks, but of which they have no memory once they have sobered up. Contrary to what was previously believed, their problem is not in retrieving the memories, but rather in having failed to store them in the first place. Sedatives such as barbiturates and benzodiazepines can also produce this kind of amnesia.

The various types of memory involve various structures in the brain, and their destruction causes differing amnesic syndromes.

The best known of these is the global amnesic syndrome experienced by patient H.M., which was characterized by severe anterograde amnesia and more moderate retrograde amnesia. This syndrome results from bilateral lesions of the medial portion of the temporal lobe, and more specifically, of the hippocampus and its neighbouring structures (the parahippocampal, entorhinal, and perirhinal cortexes). These lesions can be due to surgical ablation, as in the case of H.M., or to other causes such as tumours, ischemic episodes, head traumas, and various forms of encephalitis.

Another well-known form of amnesia is Korsakoff’s syndrome, encountered for the first time in chronic alcoholics. Korsakoff’s syndrome is similar to global amnesic syndrome, except that people with Korsakoff’s are more prone to confabulation to cover up gaps in their memories of their own past. Korsakoff’s syndrome is also known as diencephalic amnesia, because the vitamin B1 deficiency that results from alcoholism causes bilateral damage to the mammillary bodies of the hypothalamus. Similar symptoms are also produced by damage to the dorsomedial thalamic nuclei, the mammillothalamic tract, and the upper portion of the brainstem. Once again, other etiologies, such as strokes and tumours, can affect the same structures and produce the same results.

Brain affected by Wernicke-Korsakoff syndrome.
Note the pigmentation of the grey matter around the third ventricle.
Source: University of Texas (Houston)

There is also an amnesia of the frontal lobe due to damage at this site. People with this disorder do not suffer from global amnesia, but do show a memory deficit in tasks involving temporal planning of sequences of events. These people also have problems with the sources of newly acquired knowledge and have deficient meta-memory (they cannot make judgments about their memory’s contents).

Other types of damage to the cortex can cause forms of amnesia that are sometimes highly specific. For example, if the part of the cortex that perceives colours is damaged, people can lose their knowledge of colour. And since the memory of colours is reconstructed at this same location, this memory disappears as well.

A specific injury to the amygdala can prevent people from recording memories of traumatic events. In normal people, such memories are formed when particularly stressful conditions make certain details of a scene practically unforgettable.

Other localized cortical lesions can prevent people from accessing certain items in their semantic memory and thus cause all sorts of specialized aphasias.

Lastly, certain transitory global amnesias can be triggered suddenly, causing people to completely lose their memory for a few hours. Though these transitory amnesic episodes are frightening, they are brief and do not cause any permanent damage to the brain. They seem to be due to a temporary vascular insufficiency in the brain tissue.

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