5/17/2006

Lost keys: Memory Search Failures

The "seven sins of memory" can be more productively viewed as three types of memory failure. Although maintenance failure was discussed yesterday, the second type of memory failure, and by far the most well-known, is the topic of today's post: failures of memory search. This category includes long-term forgetting, in which information that was initially remembered is now inaccessible, either temporarily or permanently.

Originally, these phenomena had been seen by Schacter (author of the "seven sins of memory") as an instance of the sin of transience, but there are subtle problems with this account. Under normal circumstances, information is probably not ever actually “lost” from storage. Instead, I argue that all memory search failure results from two distinct causes: information is either inaccessible because the proper “search parameters” are not being used as cues for the memory system, or the search mechanism itself is suppressing certain “search results.”

In the first type of memory search failure – failures of cue specification – memories appear to be lost only because attempts at retrieval are either providing the wrong retrieval cues, or these cues are insufficiently precise. This conceptualization of forgetting provides a natural explanation for one of the most common task paradigms in the memory literature, the AB-AC task. In this case, the recall of one list of word pairs is impaired by the subsequent studying of a second list of word pairs when both lists share a cue. In this case, the “A” cue is ambiguous in a memory search, and thus results in the appearance of forgetting for the first list, in addition to slowed learning for the second list of words.

Other evidence for the idea that cue specification failure underlies some types of forgetting is that successful recall can be predicted on the basis of how well the current state of neural activity matches the neural activity at the time of encoding (Polyn, et al., 2005). The striking neuroimaging used to confirm this hypothesis provides a window onto exactly the kind of “cue specification” process that is frequently at fault in forgetting phenomena.

Cue specification failure also explains phenomena such as transfer-appropriate processing, in which memory is improved in tasks where the cues provided to a memory search more closely match the content of neural activity at the time of encoding (Morris et al., 1977). Furthermore, a “deeper” level of processing may provide a memory advantage because it results in more processing, such that any given cue is more likely to activate a related memory. Therefore, one interpretation of the levels-of-processing advantage is that “width” as opposed to “depth” of processing is the critical feature that results in improved memory search.

Finally, cue specification failure may underlie some aspects of infantile amnesia. Although there is reason to think that encoding failures are also involved (Bauer, 2004), one interpretation of infantile amnesia is that waves of neural pruning during childhood make it essentially impossible to provide memory search cues that are sufficiently similar to the state of neural activity at the time of encoding. Hippocampus is already well developed by 1 year of age (Bauer, 2004), and although prefrontal cortex undergoes development well into the teens, most people would probably report fairly good memory before that age. Therefore, I argue that infantile amnesia results primarily from a failure to reinstantiate the neural context that was present during encoding in infancy – in other words, to provide sufficiently precise memory cues.

The second cause of memory search failure involves computational characteristics of neural mechanisms that accomplish memory search and memory representation. One example of this is the phenomenon of memory suppression - a topic to which we will return tomorrow, when I more fully explore this second aspect to memory search failure.

Note: This post is part 3 in a series of posts, in which I'll review and revise Schacter's "seven sins of memory" according to a new framework of memory failure, one that is both closer to neuroanatomy and wider in scope. Here is part 1, and here's part 2.

References:

Bauer, P. J. (2004). Getting explicit memory off the ground: Steps toward construction of a neuro-developmental account of changes in the first two years of life. Developmental Review, 347-373.

Morris, C. D., Bransford, J. D., & Franks, J. J. (1977). Levels of processing versus transfer-appropriate processing. Journal of Verbal Learning and Verbal Behavior, 16, 519-533.

Polyn, S. M., Natu, V. S., Cohen, J. D., & Norman, K. A. (2005). Category-specific cortical activity precedes retrieval during memory search. Science, 310, 1963-1966.

Related Posts:
The Seven Sins of Memory
The Transience of Memory
Overgrowth, Pruning and Infantile Amnesia
Neural Network Models of the Hippocampus

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