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 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. The possible function of the thalamus in this process is discussed further in Chapter 58.
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.
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