For short-term memory to be converted into long-term memory that can be recalled weeks or years later, it must become “consolidated.” That is, the short-term memory, if activated repeatedly, will initiate chemical, physical, and anatomical changes in the synapses that are responsible for the long-term type of memory. This process requires 5 to 10 minutes for minimal consolidation and 1 hour or more for strong consolidation. For instance, if a strong sensory impression is made on the brain but is then followed within a minute or so by an electrically induced brain convulsion, the sensory experience will not be remembered. Likewise, brain concussion, sudden application of deep general anesthesia, or any other effect that temporarily blocks the dynamic function of the brain can prevent consolidation.
Consolidation and the time required for it to occur can probably be explained by the phenomenon of rehearsal of the short-term memory, as described in the following section.
Rehearsal Enhances the Transference of Short-Term Memory Into Long-Term Memory. Studies have shown that rehearsal of the same information again and again in the mind accelerates and potentiates the degree of trans fer of short-term memory into long-term memory and therefore accelerates and enhances consolidation. The brain has a natural tendency to rehearse newfound information, especially newfound information that catches the mind’s attention. Therefore, over a period of time, the important features of sensory experiences become progressively more and more fixed in the memory stores. This phenomenon explains why a person can remember small amounts of information studied in depth far better than large amounts of information studied only superficially. It also explains why a person who is wide awake can consolidate memories far better than a person who is in a state of mental fatigue.
New Memories Are Codified During Consolidation. One of the most important features of consolidation is that new memories are codified into different classes of information. During this process, similar types of information are pulled from the memory storage bins and used to help process the new information. The new and old are compared for similarities and differences, and part of the storage process is to store the information about these similarities and differences, rather than to store the new information unprocessed. Thus, during consolidation, the new memories are not stored randomly in the brain but are stored in direct association with other memories of the same type. This process is necessary for one to be able to “search” the memory store at a later date to find the required information.
Role of Specific Parts of the Brain in the Memory Process
The Hippocampus Promotes Storage of Memories— Anterograde Amnesia Occurs After Hippocampal Lesions Are Sustained. The hippocampus is the most medial portion of the temporal lobe cortex, where it folds first medially underneath the brain and then upward into the lower, inside surface of the lateral ventricle. The two hippocampi have been removed for the treatment of epilepsy in a few patients. This procedure does not seriously affect the person’s memory for information stored in the brain before removal of the hippocampi. However, after removal, these people have virtually no capability there after for storing verbal and symbolic types of memories (declarative types of memory) in long-term memory, or even in intermediate memory lasting longer than a few minutes. Therefore, these people are unable to establish new long-term memories of those types of information that are the basis of intelligence. This condition is called anterograde amnesia.
But why are the hippocampi so important in helping the brain to store new memories? The probable answer is that the hippocampi are among the most important output pathways from the “reward” and “punishment” areas of the limbic system, as explained in Chapter 59. Sensory stimuli or thoughts that cause pain or aversion excite the limbic punishment centers, and stimuli that cause pleasure, happiness, or sense of reward excite the limbic reward centers. All these together provide the background mood and motivations of the person. Among these motivations is the drive in the brain to remember those experiences and thoughts that are either pleasant or unpleasant. The hippocampi especially and to a lesser degree the dorsal medial nuclei of the thalamus, another limbic structure, have proved especially important in making the decision about which of our thoughts are important enough on a basis of reward or punishment to be worthy of memory.
Retrograde Amnesia—Inability to Recall Memories From the Past. When retrograde amnesia occurs, the degree of amnesia for recent events is likely to be much greater than for events of the distant past. The reason for this difference is probably that the distant memories have been rehearsed so many times that the memory traces are deeply ingrained, and elements of these memories are stored in widespread areas of the brain.
In some people who have hippocampal lesions, some degree of retrograde amnesia occurs along with anterograde amnesia, which suggests that these two types of amnesia are at least partially related and that hippocampal lesions can cause both. However, damage in some thalamic areas may lead specifically to retrograde amnesia without causing significant anterograde amnesia. A possible explanation of this is that the thalamus may play a role in helping the person “search” the memory store houses and thus “read out” the memories. That is, the memory process not only requires the storing of memories but also an ability to search and find the memory at a later date. The possible function of the thalamus in this process is discussed further in Chapter 59.
Hippocampi Are Not Important in Reflexive Learning. People with hippocampal lesions usually do not have difficulty in learning physical skills that do not involve verbalization or symbolic types of intelligence. For instance, these people can still learn the rapid hand and physical skills required in many types of sports. This type of learning is called skill learning or reflexive learning; it depends on physically repeating the required tasks over and over again, rather than on symbolical rehearsing in the mind.
