Foraging through multiple target categories reveals the flexibility of visual working memory

A key assumption in the literature on visual attention is that templates, actively maintained in visual working memory (VWM), guide visual attention. An important question therefore involves the nature and capacity of VWM. According to load theories, more than one search template can be active at the same time and capacity is determined by the total load rather than a precise number of templates. By an alternative account only one search template can be active within visual working memory at any given time, while other templates are in an accessory state - but do not affect visual selection. We addressed this question by varying the number of targets and distractors in a visual foraging task for 40 targets among 40 distractors in two ways: 1) Fixed-distractor-number, involving two distractor types while target categories varied from one to four. 2) Fixed-color-number (7), so that if the target types were two, distractors types were five, while if target number increased to three, distractor types were four (etc.). The two accounts make differing predictions. Under the single-template account, we should expect large switch costs as target types increase to two, but switch-costs should not increase much as target types increase beyond two. Load accounts predict an approximately linear increase in switch costs with increased target type number. The results were that switch costs increased roughly linearly in both conditions, in line with load accounts. The results are discussed in light of recent proposals that working memory reflects lingering neural activity at various sites that operate on the stimuli in each case and findings showing neurally silent working memory representations.