Abstract
Human alpha band activity (8-12 Hz) has been proposed to index the storage of arrays of representations in visual working memory, showing stronger suppression as the set size of to-be-remembered objects increased. However, the results of recent studies suggest that global alpha suppression might instead index attentional selection. How do we reconcile these competing explanations? If alpha suppression indexes attentional selection, then perhaps this signal is stronger when larger arrays are shown, because the focus of attention has to zoom out to select all of the objects in the array. When an array has fewer items, then the spatial extent is smaller. When the array has more items, then the spatial extent is larger. Here, we tested this hypothesis by varying the distance between multiple objects while keeping their numbers constant in low and high memory set size conditions. Our results show that the global alpha suppression was strongest when the distance separating the items was greatest, but was not modulated by memory set size. In contrast to this pattern of global alpha suppression, the neural index of visual working memory storage—the contralateral delay activity—became stronger when the memory set size increased, but was not modulated by the distance between objects. These results suggest that global alpha suppression during the visual working memory delay period reflects the spatial extent of attention and not visual working memory storage itself. This provides a more precise understanding of the attention mechanism of global alpha suppression during visual processing.