Abstract
Contralateral delay activity (CDA) scales with the number of items stored in working memory (WM) and predicts WM capacity. Thus, the predominant view has been that CDA activity indexes the number of items held in memory. Recently, however, Berggren and Eimer (2016) challenged this interpretation with a study in which colour memoranda were sequentially presented in opposite hemifields during a change detection task. They found that CDA activity was primarily determined by the contents of the second array rather than the total storage load across both displays, in line with the hypothesis that the CDA tracks the current focus of attention rather than storage per se. We note, however, that in the Berggren and Eimer study, probe stimuli were not presented in the same positions as the initial memoranda. Thus matching the probe array with the combined memoranda required a challenging spatial transformation of the two arrays.. Our hypothesis is that this lack of perceptual correspondence between sample and probe displays may have encouraged subjects to offload the active representation of array 1 (and retrieving it at the time of probing) rather than storing both arrays concurrently. In support of this hypothesis, we show that when spatially compatible sample and probe arrays are used, CDA activity represents the total storage load across both arrays, in line with the view that CDA activity indexes the number of individuated representations in visual WM. By contrast, when precisely the same memory displays were paired with a spatially incompatible probe display, we replicated the findings of Berggren and Eimer. Thus, our findings reinforce the view that CDA activity track the total contents of WM. Moreover, these findings offer insight into the factors that encourage transitions between online and offline memory states, a collaboration between memory systems that is essential for virtually all complex tasks.