9/22/2006

Attentional Control and Active Maintenance

Yesterday I described how some attention control mechanisms might interact with working memory. For example, momentary failures of goal maintenance could result from stochastic dopamine fluctuations, and thus result in errors or extremely long reaction times on tasks involving interference. On the other hand, the primary (unskewed) distribution of reaction times on such tasks can be explained by a time-consuming process of "competition resolution," which might also differ in efficiency between individuals. In summary, measures of working memory capacity may reflect not only 'capacity' but also individual ability a) to maintain representations (of goals or specific stimuli) in a highly active state, as well as b) to suppress representations that might interfere with the to-be-remembered items.

In a 2005 Memory and Cognition paper, Hester and Garavan approach working memory in a different way: they find that WM loads impair both switching and inhibitory ability in a material-specific way.

In their first experiment, subjects studied a list of 2, 5, or 8 letters, and were then presented with a series of trials in which they had to judge whether a given letter was a member of the previously studied list. Some of the letters presented for this judgment were colored, indicating that subjects had to perform a secondary task (make a vowel/consonant judgment or judge whether the letter's color was red or green).

Not surprisingly, all trial types (memory judgments, vowel/consonant judgments, and red/green judgments) were harder when subjects were remembering 8 letters as opposed to 2 (as reflected in both reaction times and, to a lesser extent, accuracy). Paradoxically, it took subjects longer to switch from the primary to the secondary task if the colored letter was part of the studied list, and this difference increased with the number of items that had been studied. Likewise, it took subjects longer to switch back to the primary task if the next stimulus was a member of the studied list, and this difference also increased with memory load. The authors suggest that subjects may have suppressed these memorized items in order to perform the secondary task.

In a second experiment, Hester & Garavan had subjects study lists of 1, 3 or 5 items, and then presented them with a series of letters which may or may not have been members of that studied set. Subjects were required to give a response for each stimulus that was not a member of the studied list (which was the case for over 90% of the trials); however, for presented items that had been studied, subjects were required to withhold a response. As in the previous experiment, accuracy decreased as memory load increased. However, the reaction time on incorrect trials (where subjects should have withheld response) was much faster than that on correct trials.

A third experiment differed from the second only insofar as subjects were also required to withold a response to any item that had also been presented on the previous trial. As in previous experiments, accuracy decreased as memory load increased - but this time, only for items that had been maintained in memory. In other words, performance on the "repeat" items was unaffected by memory load.

In summary, Hester & Garavan found that it takes longer to switch away from, and to switch back to, items that are maintained in memory for another task, and that this problem is exacerbated by increasing the number of items maintained in memory. Likewise, they showed that this same trend applies to inhibitory control, in that it is generally more difficult to inhibit responding to items maintained in memory than to other stimuli, and that this is even more the case for high memory loads. This work suggests that working memory and executive control can interact in the sense that it is more difficult to control competing responses with WM load (and particularly responses that are related to items maintained in WM). Additionally, both these problems are made substantially worse by increasing the number of items maintained in WM.

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