Selection Efficiency in Updating Working Memory

In Vogel, McCollough, and Machizawa's fascinating 2005 Nature paper, they describe how individual differences in short-term memory span appear to relate to the efficiency with which individuals can select the items to be maintained in memory. If this is truly the source of individual differences in working memory, then current "span measures" may be somewhat off the mark in their focus on capacity differences per se; likewise, even the relatively new "period measures" advocated by Towse et al., may only indirectly index the true source of memory differences.

Vogel et al. established the idea that selectivity is a primary source of differences in short-term memory with a relatively simple set of tasks. First, they had subjects view a display of colored and randomly rotated rectangles, and directed them to remember only the red rectangles in one particular half of that display. On any given trial, the items in the to-be-remembered half of the display consisted of either two red items, four red items, or two red and two blue items.

Previously, Vogel and Machizawa had identified a specific wave of electrical contralateral delay activity (CDA) which can be used as an index of an individual's working memory capacity. Specifically, this wave increases until it "maxes out" at an individual's capacity limit; the amount of increase in this wave between situations in which the subject is asked to remember 2 items, and when the subject is asked to remember 4 items, can be used as an index of WM capacity.

In the current study, Vogel et al. were able to show that those subjects with high capacity were more likely to be able to ignore the blue items than the subjects with low capacity. In other words, the low-capacity subjects showed a CDA that increased to around four items when viewing a display that contained only two red and two blue items - suggesting they mistakenly updated WM representations with the distractor items. In contrast, high-capacity individuals showed no change in CDA between the two-red-and-two-blue display and the simple two-red display, suggesting they were able to more efficiently select the representations with which to update their short term memories.

In an extension to this work, the authors also showed that low-capacity individuals were somewhat better at selectively updating WM representations when the distractor items differed in terms of location, rather than simply color. This is to be expected, given that it intuitively seems easier to ignore items based on their location than on their color - you can simply avoid looking in their direction! Nonetheless, it underscores the importance of updating as one of the executive functions that subserves working memory.

If these results are to be believed, then it seems plausible that working memory span differences occur partly because of differences in capacity, but also partly because of differences in how well low-capacity individuals are able to use that capacity. But why should these traits be correlated? In other words, if these are truly separable functions, it seems likely that some individuals with a high-capacity could be impaired at updating. Likewise, it seems natural to expect that low-capacity individuals would optimize their WM updating, so as to make the most of what little capacity they have.

This mystery is still up for grabs - it's possible that a third variable influences both (such as genetics), but it's also possible that these two factors are causally related in a more direct way - for example, a by-product of increased selection efficiency may be more focused gating of representations, which could lead to a higher CDA asymptote. For example, consider the case where neural oscillations in the thalamocortical circuit allow information to enter working memory. For individuals with high-selection efficiency, these oscillations may "slosh around" in this circuit in a more precise way than they do in individuals with low-selection efficiency. If we assume that increasing the focus of the oscillations increases the amount of neural activity directed at a particular representation, then it becomes clear that more highly-activated representations are more likely to cause an increase in the CDA wave, and thereby be maintained in working memory.