. Forgetting, reminding, and remembering: the retrieval of lost spatial memory. PLoS Biol. 2004 Aug;2(8):E225. PubMed.

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  1. A fundamental question about memory is: What happens to a memory after it is initially stored? Studies of patients with amnesia have found that a strengthening process known as consolidation must be occurring on some of our memories. While some memories fade with time, others appear to become more firmly ingrained. These “consolidated” memories survive damage to the memory systems of the brain, producing the phenomenon of “temporally graded retrograde amnesia,” which is often observed when conversing with an amnesic patient. The patient will often have no difficulty recalling memories from his or her youth, but will have great difficulty remembering more recent events, even those that occurred before the patient became amnesic. For this patient, the consolidation process had not yet run its course on those recent memories and they were lost during the event that produced the amnesia. The more distant memories have had years to become consolidated and they are intact, even after damage to the memory system.

    The mechanism by which the brain consolidates our memories is not known. One hypothesis is that this occurs naturally during sleep. A separate suggestion has been that conscious reminding, or thinking about the memory, is the key event in consolidation. These two ideas are not incompatible; perhaps during sleep, some sort of unconscious reminding occurs to help consolidate memories. But it is difficult to assess the effect of reminding, particularly in animal models of memory dysfunction that are extremely helpful in understanding the neurobiology of memory. The recent report by de Hoz, Martin and Morris (2004) provides an elegant experimental approach to examining the effect of reminding in an animal model of amnesia.

    Rats have good memory for spatial location and quickly learn to perform a task known as a “water maze.” In this task, rats swim in a tub of water to a platform that allows them to “escape” and perch above the water (rats are good swimmers, but find it annoying). The rat’s memory is tested by clouding the water (e.g., with powdered milk) and they show that they remember where the escape platform is by quickly swimming to it, even when it is not visible. Rats whose memory system is impaired perform very poorly at this task; they cannot remember where the platform is in between tests.

    The critical finding of de Hoz et al. (2004) is that rats with a mild memory impairment can be reminded of their past experience and this improves their performance. Rats were first shown the platform at position A. Memory-impaired rats were not very good at remembering to search at A when tested later. A separate group was reminded of their experience with another swim in the maze, this time with the platform at a new place, B. Surprisingly, the effect of this practice trial was to cause the rats to search in position A in a later test. The memory impairment caused the rats to forget the time they escaped at position B, but reminded and strengthened the memory of the time they escaped at place A. This effect did not occur with severe memory impairments, indicating that some memory of searching at A has to survive for the reminding to work.

    This result provides strong evidence that reminding or thinking about a past event strengthens the memory of that event and makes it easier to recall later, even when the brain’s memory system is partially damaged. This suggests that frequent reminding about recent events might help a patient with a memory disorder retain some memories. After onset of a syndrome that impairs memory, such as Alzheimer disease, frequent reminding about events that occurred before onset could help reduce the effect of retrograde amnesia and loss of those memories.

    View all comments by Paul Reber

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