Assessing the involvement of long-term memory in working memory

Complex span tasks are working memory (WM) tasks in which participants maintain series of items (e.g., letters) for further serial recall while performing a concurrent task (e.g., parity judgement on digits). It has been shown that even pushing the demand of this concurrent task at its individual limits strongly reduces, but does not totally abolish, memory performance. A small memory residual of about one item remains. The present study aimed at testing the hypothesis that this residual is retrieved from long-term memory (LTM). For this purpose, two experiments compared the size of memory residual either through immediate recall or after a 1-min delay filled with a backward counting task. If it is retrieved from LTM, a substantial part of this residual should still be accessible after the delay. Although this delay reduced the immediate memory residual, about two-thirds of this residual was still retrievable when the complex span task was performed under concurrent articulation. These findings confirmed that when processing almost entirely captures attention, memory residual mainly relies on LTM. However, the fact that forgetting rate during the complex span task was far larger than during the subsequent delay weakens WM theories suggesting that memory items are offloaded in activated LTM when attention is switched away. We suggest that our findings are more compatible with the short-term transient storage hypothesized by the synaptic theory of WM.