An act of memory comprises three distinct operations:
1. The fixation of a representation;
2. Its conservation;
3. Its revival, that is to say, its reappearance in the field of consciousness.
These may be disordered together or singly; hence the three forms of amnesia:
A. Amnesia by default of fixation (or simply amnesia of fixation), also known as anterograde amnesia;
B. Amnesia of conservation;
C. Amnesia of reproduction.
The latter two affect impressions previously acquired and constitute retrograde amnesia; there are therefore two varieties of retrograde amnesia: (1) by default of conservation, and (2) by default of reproduction.
The power of fixation (Merkfahigkeit of German authors) is dependent upon the distinctness of the perceptions. Therefore all conditions in which perceptions are vague and uncertain are accompanied by a more or less marked amnesia of fixation; such is the case in epileptic deliria and in acute confusional psychoses.
Distinctness of perception is therefore a condition necessary for the normal working of memory; it is, however, not in itself a sufficient condition. An impression, though very clear and very precise at the moment, may not become fixed in the mind. Thus a patient with polyneuritic psychosis may understand perfectly the questions put to him, execute properly the orders that are given him, so that on a superficial examination he may convey the impression of a normal person; but he preserves an incomplete recollection, or none at all, of the occurrences of the whole period of his illness. It seems, then, that for proper fixation is required, besides sufficient distinctness of perception, some other condition the nature of which is as yet undetermined.
An impression fixed in memory is preserved for a greater or lesser length of time, depending upon its nature and upon the individual capabilities of the subject. The memory of an important event persists longer than that of an insignificant one. Certain individuals possess a prodigious memory, others a very poor one or almost none at all; between these two extremes there are infinite gradations.
The disappearance, under the influence of some pathological cause, of impressions previously acquired, constitutes what we have termed amnesia of conservation. This destructive, and consequently incurable, form of amnesia is the principal factor of certain types of dementia, and is often the first sign that warns the patient's relatives of the beginning condition.
The disappearance of impressions may be more or less complete, depending upon the nature of the dementing process. While many precocious dements for a long time preserve a relatively good memory, general paralytics and senile dements present from the beginning of their illness marked amnesia.
Amnesia of conservation is generally associated with the other two forms of amnesia: amnesia of fixation and amnesia of reproduction.
In the normal state an impression fixed and preserved in memory possesses the property of being revived under certain conditions. In pathological conditions this power of reproduction may be suspended: the impressions exist, but they are dormant and cannot be revived. This form of amnesia is encountered in many acute psychoses, notably in manic depressive, acute confusional, and toxic psychoses. Its prognosis is of course much more favorable than is that of the preceding form.
The onset may be sudden or insidious; it is often sudden in amnesia of reproduction - pure or associated with amnesia of fixation - and almost always insidious in amnesia of conservation.
Amnesia may be stationary, retrogressive, or progressive; it is stationary when, certain impressions having become destroyed, the defect persists without increasing; retrogressive when the impressions, simply dormant, reappear little by-little; and progressive when, as the pathological process advances, the number of destroyed impressions becomes greater from day to day.
In progressive amnesia the disappearance of impressions occurs not at random, but in a definite order. "The progressive destruction of memory follows a logical course, a law. It descends progressively from the unstable to the stable: it begins with recent impressions which, fixed imperfectly upon the nervous elements, seldom repeated and therefore but feebly associated with others, represent the organization in its weakest degree; it ends with that instinctive, sensory memory which, stably fixed in the organism and having become almost an integral part of it, represents the organization in its strongest degree. From the beginning to the end the course of amnesia, governed by the nature of things, follows the line of least resistance, that is to say, the line of least organization." 1 In senile dementia, in which the law of amnesia is most perfectly demonstrated, the impressions of old age are the first to become effaced, later those of adult life, and finally those of youth and childhood. Some of the latter may remain intact long after the general ruin of memory and other faculties.
It is not uncommon to meet with advanced senile dements who, though incapable of recollecting the existence of their wife and children, are still able to relate with minute details the occurrences of their childhood or to recite correctly fragments from the works of classic authors.
The law of amnesia, though always the same, is difficult to demonstrate in those affections in which the destruction of memory progresses very rapidly, where many impressions, like other manifestations of intellectual life, disappear en masse. In general paralysis the course of amnesia is much more rapid and much less regular than in senile dementia. This fact, as we shall see, is an important element in diagnosis.
1 Ribot. The Diseases of Memory.
Amnesia is said to be partial when it involves only one class of impressions, for instance proper names, numbers, certain special branches of knowledge (music, mathematics), or a foreign language. A young man coming out of a severe attack of typhoid fever forgot completely the English language, which he had spoken fluently before the onset of his illness; other impressions were quite well preserved. When it involves verbal images the amnesia determines a particular form of aphasia, amnesic aphasia.
Amnesia is general when it affects equally all classes of impressions. Most of the progressive amnesias are general.
Amnesia may be limited to a certain period of existence. In such cases its onset is almost always sudden, and it is either anterograde, or retrograde by default of reproduction.
A recollection of an occurrence, once evoked, is usually easily localized by us as to its position in the past. This power of localization disappears in certain psychoses. The patients cannot tell on what date or even in what year some event occurred, an impression of which they have, however, preserved. The default of localization in the past combined with a certain degree of anterograde and retrograde amnesia produces disorientation of time.
In an illusion of memory a past event presents itself to consciousness altered in its details and in its relation to the patient, and exaggerated or diminished in importance. Thus one senile dement claimed to have superintended the construction of a Gothic cathedral several centuries old, holding, as he said, "the calipers in one hand and the musket in the other to defend myself against the Saracens." Upon inquiry it was found that the patient had really worked about thirty years previously on the restoration of an old cathedral.
An illusion of memory becomes a true hallucination when the representation perceived as a recollection does not correspond to any actual past occurrence. A patient who had been in bed during several weeks related once that on the previous day he assisted at the coronation of the Russian emperor: this is a representation without an object, a hallucination of memory.
Illusions and hallucinations of memory form the basis of pseudo-reminiscences which are met with in many psychoses, especially in the polyneuritic psychosis.
Pseudo-reminiscences are not infrequent in certain persons who are usually not classed with psychotics. In such cases the hallucinations and illusions of memory occur on a basis of abnormally vivid mental images which an inadequate auto-critique fails to correct.1
In some cases pseudo-reminiscences occur in such abundance as to constitute the principal symptom of the disease. Thus one patient imagined himself to have participated in all the important historical events of his epoch, particularly in the great military actions. He had taken part successively in the campaigns of Tonquin, Madagascar, and Dahomey, also in the Spanish-American war and in the Boer war, serving in different grades - now as corporal, now as sergeant-major, now as colonel. During all that time he had had several conferences with the German emperor, also with the empress, his cousin. When his reminiscence had bearing upon some historical event the patient would give details culled from magazines or from popular books, and related them with a degree of accuracy which indicated a good memory.
We would mention lastly a curious form of illusion of memory, which has been designated by the expression illusion of having already seen. "It consists in a belief that a state of consciousness that in reality is new was experienced before, so that when it first occurs it is thought to be a repetition." 2 One patient claimed that all the occurrences which he was witnessing had taken place a year previously, day by day. He made a great deal of noise at the marriage of one of his sisters, demanding to know why a ceremony which had already been performed a year ago was begun over again, and protesting that it was all a farce.1
1 Delbriick. Die pathologische Luge und die psychisch dbnormen Schwindler. - Koeppen. Ueber die pathologische Luge (Pseudologia phantastica). Charity Annul., Jan., 1898.
2 Ribot. Loc. tit.