Consolidation, Strength of Memory, and Forgetting

Some investigators (eg, McGaugh 2000) have hypothesized that long-term memory differs from long-lasting memory. While long-term memory lasts from hours to months, long-lasting memory stays with the animal for time spans in the range of months to its entire lifetime. Long-lasting memory seems to differ from long-term memory in the strength of the associations formed between the remembered item and other memories. At the structural level, one hypothesis is that periodic reinforcement of an item in long-term memory causes the synaptic connections associated with that item to persist in time, thus strengthening the memory. Periodic reinforcement is a critical component of the process of memory consolidation, in which long-term memory is converted into long-lasting memory.

As with the difference between short- and long-term memory, consolidation is an important process in separating valuable from expendable information. Access to experiences that are not reinforced and consolidated are, after a time, lost. It may in fact be that the memory itself is not lost, but its synaptic connections disappear, so there is no route, within the brain, to find the information. In humans, this would explain why memories can be "dredged up" when a person is presented with an appropriate set of associations.

The strength of a memory may also be linked to specific contexts; the brain may be set up to facilitate strong, long-lasting memory of important items. Specific aversions, for example, are easily learned and difficult to forget. Humans and animals probably are primed in similar ways to place important information about social interactions, such as information used to recognize close relatives, into long-lasting memory quickly and with little or no reinforcement. Another example of strong memories comes in the context of imprinted information; the presence of a narrow time period in which the animal imprints and the subsequent accurate recall point to neural priming to easily create strong, long-lasting memories.

When items are lost from memory, they seem to disappear at an exponential rate (White 2001). Forgetting is largely a function of failure to reinforce a memory through repeated experiences; reinforcement is critical in the transition of memory from short-term to long-term and from long-term to long-lasting. While forgetting is not exactly a mirror image of learning, ethologists can often document a rate of forgetting in contexts in which discovering the rate of learning is difficult or impossible, as in caching animals forgetting their cache locations.

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McGaugh JL 2000 Neuroscience - Memory - a century of consolidation SCIENCE 287 (5451): 248-251
Sangha S, McComb C, Lukowiak K 2003 Forgetting and the extension of memory in Lymnaea JOURNAL OF EXPERIMENTAL BIOLOGY 206 (1): 71-77
Sargisson RJ, White KG 2003 The effect of reinforcer delays on the form of the forgetting function. JOURNAL OF THE EXPERIMENTAL ANALYSIS OF BEHAVIOR 80 (1): 77-94
White KG 2001 Forgetting functions ANIMAL LEARNING & BEHAVIOR 29 (3): 193-207