Visual working memory load persists during the comparison phase

Working memory is often measured by presenting arrays of visual items to be remembered over a short delay that must be compared with a test display. Extensive work has elucidated the neural mechanisms that support the encoding and maintenance periods of these tasks. Though, little is still understood about the comparison process itself. For example, the contralateral delay activity (CDA) is a sustained EEG component that provides a sensitive measure of the current working memory load during the retention period. However, it is not clear whether this activity continues to track the full working memory load during the comparison phase of the task or if it is reduced to just the one item from the array that is being tested. In Experiment 1, we used a change localization task with 2-item and 4-item arrays of colors. At test, subjects had to report which item changed. We observed that the response-phase CDA for set size 4 was significantly larger than the CDA for set size 2. In Experiment 2 we used a single-probe change detection design, in which only a single item from the original array was shown at test. Despite only a single item shown on the screen, we again observed that the response-phase CDA for set size 4 was still significantly larger than the CDA for set size 2. Our results suggest that the working memory load during test reflects the load from all of the items from the array that were stored.