This begs the essential question of “what is a memory?” Current understanding suggests that no single hippocampal neuron, for example, embodies a memory but that the perhaps the connections between an ensemble of neurons in the medial temporal lobes and modality-specific neurons in the associative cortices are, in fact, the source of memory. Strengthening synaptic connections among this network serves to establish the memory. This may occur through long-term potentiation, as the work of Kandel has shown in experimental models. It is not clear if a hippocampal neuron is the trigger to the memory ensemble or the entirety of hippocampal system serves a generic role in cohering all memories. These cellular mechanisms involved in learning and the formation of memories are only beginning to be understood. Whether physiologic phenomena such as long-term potentiation or anatomic changes in the dendritic structure of neurons are at the center of memory storage is not known; certainly both are likely to be involved. The neurochemical systems that are activated during formation and recall of memory are also obscure. Kandel has provided a detailed review of information on this subject. The anatomic and physiologic mechanisms that govern immediate registration, which remains intact in even the most severely damaged patients with the Korsakoff amnesic syndrome has not been fully deciphered.
Certain psychologic features of human memory that must be accounted for by any model purporting to explain this function are the importance of cueing in eliciting learned material and the imprecision of past memories, allowing for unwitting embellishment and false recollection, to the point of fabrication. The latter aspect has been a topic of considerable importance in children who have (or have not) been subjected to sexual abuse and in adults and children whose memories of past abuse have been suggested by the examiners (see Schacter).
Each of the amnesic states listed in Table 20-5 is considered at an appropriate point in subsequent chapters of this book. The only exception is the striking syndrome of transient global amnesia, the nature of which is not certain. It cannot be included with any assurance with the epilepsies or the cerebrovascular diseases or any other category of disease and is therefore considered here.
This was the name applied by Fisher and Adams to a transient disturbance of memory they observed in more than 20 middle-aged and elderly persons. The condition is characterized by an episode of amnesia and bewilderment lasting for several hours. The syndrome has its basis in amnesia for events of the recent past coupled with ongoing anterograde amnesia. During the attack, there is no impairment in the state of consciousness, no other sign of confusion, and no seizure activity; personal identification is intact, as are motor, sensory, and reflex functions. The patient’s behavior is normal except for a very characteristic incessant, repetitive questioning about his immediate circumstances—usually of the identical question over and over at intervals of 20 to 60 s after a response to the query has already been given by the examiner (e.g., “What am I doing here?”; “How did we get here?”). Curiously, even the inflection used in questioning is repeated. Unlike psychomotor epilepsy, the patient is alert, in contact with his surroundings, and capable of high-level intellectual activity and language function during the attack. As soon as the episode has ended, no abnormality of mental function is apparent except for a permanent gap in memory for a large part of the period of the attack and for a brief period (hours or days) preceding it. The patient may be left with a mild headache. It is possible there are incomplete or mild attacks as brief as 1 h in duration but they are not common (we have observed three such patients) and a typical episode is longer. It is difficult to determine when precisely a given attack began and when it ends is difficult. The condition is among the most curious in medicine and may be mistaken for a psychiatric episode.