Consolidation of Memory
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 consoli-dation 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 follows.
Rehearsal Enhances the Transference of Short-Term Memory into Long-Term Memory. Psychological studies haveshown that rehearsal of the same information again and again in the mind accelerates and potentiates the degree of transfer 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. There-fore, over a period of time, the important features of sensory experiences become progressively more and more fixed in the memory stores. This explains why a person can remember small amounts of information studied in depth far better than large amounts of infor-mation 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 themost important features of consolidation is that new memories are codified into different classes of infor-mation. During this process, similar types of informa-tion 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 con-solidation, the new memories are not stored randomly in the brain but are stored in direct association with other memories of the same type. This is necessary if one is 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
Hippocampus Promotes Storage of Memories—Anterograde Amnesia After Hippocampal Lesions. The hippocampus isthe 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 thereafter 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 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. 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 motiva-tions 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 struc-ture, 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 ofamnesia 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 engrained, 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 storehouses 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.
Hippocampi Are Not Important in Reflexive Learning. Peoplewith hippocampal lesions usually do not have difficulty in learning physical skills that do not involve verbal-ization 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 reflexivelearning; it depends on physically repeating therequired tasks over and over again, rather than on symbolical rehearsing in the mind.
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