The effect of different spatial working memory loads on visual search

"Working memory (WM) has been shown to be an important factor in visual search. For instance, there is evidence that both spatial and visual WM load lead to a decrease in search performance, resulting in a longer time to complete a search. However, the findings regarding search efficiency, i.e., search time as a function of display size, are less clear. This measure has been reported to be affected by spatial but not visual WM load. In three experiments, with approximately 20 participants each, we tested how two different types of spatial WM load affect visual search in terms of search performance and efficiency. In all experiments, participants were asked to memorize the spatial locations of two (low load) or four items (high load) presented either serially (Experiment 1) or simultaneously (Experiments 2 and 3). After that, they had to search for a target letter in a display of 5, 10 or 15 letters. In Experiment 3, participants additionally performed a verbal WM task. A control condition with no memory load (search only) was also included in each experiment. The results showed that, compared to the search-only condition, search times increased when spatial load was added. This was regardless of the type of spatial WM load. No search-time differences were found between the low and high-load condition. The additional verbal WM task had no effect on search performance. Furthermore, and in contrast to previous findings, search efficiency was not affected by either type of spatial WM load. These results suggest that visual search performance, but not search efficiency, is affected by spatial WM load."