Valid Dimensions of Memory: Strength, Endurance, and Capacity?

Working memory is a central concept in cognitive sciences, and is typically assessed through "span measures," which are frequently used as indices of individual differences in working memory. However, aspects other than capacity might also underlie individual differences in working memory, such as the endurance, or strength of mental representations. In the most extreme, one can even imagine issues of strength or endurance to be orthogonal to issues of capacity.

Authors Towse, Hitch, Hamilton, Peacock and Hutton describe the dominant measures of working memory (i.e., span measures) as reflecting the "suitcase metaphor" of working memory: individuals differ in how much they can pack into these mental suitcases, a function of both "packing efficiency" (perhaps related to chunking?) and the size of the suitcases to begin with.

Towse et al. contrast this view of working memory with what they light-heartedly call a "thermos flask metaphor," in which individuals differ not only in flask size and "packing efficiency" but also in the degree to which they can maintain contents in their original state. Thus, they advocate investigation of what they call "working memory period," which is the longest interval over which information can be maintained during concurrent processing.

For example, in one of their tasks ("operation period") subjects had to remember the answers to a series of mathematical formulae. The number of formulae in any testing block always remained the same, but the length of the formulae changed. (Note that in the traditional measure of operation span, all the formulae are of the same length and blocks differ in the number of formulae).

In a series of three experiments, the authors administered peroid tasks (along with standard span versions of each task, and various measures of scholastic aptitude) to 60 8-year-old children. The results showed that period measures show similar test-retest reliability and that in some cases period measures appear to be a more isolated or controlled measure of one aspect of span tasks. For example, operation span did not account for differences in one measure of scholastic aptitude after controlling for operation period and processing speed.

The authors conclude that period measures may be a useful index of working memory abilities, and that their results tentatively support a task-switching view of working memory function, in which processing and maintenance occur sequentially, and in which additional processing increases the possibility that previously maintained items would decay.

Other theorists discuss concepts such as memory "strength," in the context of a perspective that memory consists of mental representations that are graded in their strength. Memory strength is assessed through measures like simple reaction time to a memory task, or Braver's (2001) working memory context index, based on the AX-CPT task. Memory strength may ultimately reflect the coherence of neural firing, and is hypothesized to positively correlate with task-switching abilities. However, one basic and as yet unanswered question is how these strength measures might correlate with the peroid measures above.

Which of these dimensions (strength, endurance, and capacity) account for unique variance in memory function, and what is the source of their uniqueness? Resource-sharing models of working memory are challenged by the results presented by Towse et al., in that processing efficiency did not decrease as a function of how many items were being maintained. In contrast, a task-switching view is both more compatible with these results, and more compatible with the idea that strength, endurance, and capacity might all be valid characteristics of the mechanisms subserving working memory. Furthermore, several working memory subfunctions have been proposed, and a task-switching view of WM is also more compatible with these theoretical advances.

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