The Physical Basis of Memory.—The power to reproduce a once-known fact depends ultimately on the brain. This is not hard to understand if we go back a little and consider that brain activity was concerned in every perception we have ever had, and in every fact we have ever known. Indeed, it was through a certain neural activity of the cortex that you were able originally to know that Columbus discovered America, that your house is white, and that it rained on a day in the past. Without this cortical activity, these facts would have existed just as truly, but you would never have known them. Without this neural activity in the brain there is no consciousness, and to it we must look for the recurrence in consciousness of remembered facts, as well as for those which appear for the first time.
How We Remember.—Now, if we are to have a once-known fact repeated in consciousness, or in other words remembered, what we must do on the physiological side is to provide for a repetition of the neural activity which was at first responsible for the fact's appearing in consciousness. The mental accompaniment of the repeated activity is the memory. Thus, as memory is the approximate repetition of once-experienced mental states or facts, together with the recognition of their belonging to our past, so it is accomplished by an approximate repetition of the once-performed neural process in the cortex which originally accompanied these states or facts.
The part played by the brain in memory makes it easy to understand why we find it so impossible to memorize or to recall when the brain is fatigued from long hours of work or lack of sleep. It also explains the derangement in memory that often comes from an injury to the brain, or from the toxins of alcohol, drugs or disease.
Dependence of Memory on Brain Quality.—Differences in memory ability, while depending in part on the training memory receives, rest ultimately on the memory-quality of the brain. James tells us that four distinct types of brains may be distinguished, and he describes them as follows:
Brains that are:
(1) Like marble to receive and like marble to retain.
(2) Like wax to receive and like wax to retain.
(3) Like marble to receive and like wax to retain.
(4) Like wax to receive and like marble to retain.
The first type gives us those who memorize slowly and with much heroic effort, but who keep well what they have committed. The second type represents the ones who learn in a flash, who can cram up a lesson in a few minutes, but who forget as easily and as quickly as they learn. The third type characterizes the unfortunates who must labor hard and long for what they memorize, only to see it quickly slipping from their grasp. The fourth type is a rare boon to its possessor, enabling him easily to stock his memory with valuable material, which is readily available to him upon demand.
The particular type of brain we possess is given us through heredity, and we can do little or nothing to change the type. Whatever our type of brain, however, we can do much to improve our memory by obeying the laws upon which all good memory depends.
2. THE FOUR FACTORS INVOLVED IN MEMORY
Nothing is more obvious than that memory cannot return to us what has never been given into its keeping, what has not been retained, or what for any reason cannot be recalled. Further, if the facts given back by memory are not recognized as belonging to our past, memory would be incomplete. Memory, therefore, involves the following four factors: (1) registration, (2) retention, (3) recall, (4) recognition.