Multiple visual items can be simultaneously compared with target templates in memory

When we search for something, we often rely on both what we see and what we remember. This process can be divided into three stages: selecting items, identifying those items, and comparing them with what we are trying to find in our memory. It has been suggested that we select items one by one, and we can identify several items at once. In the present study, we tested whether we need to finish comparing a selected item in the visual display with one or more target templates in memory before we can move on to the next selected item. In Experiment 1, observers looked for either one or two target types in a rapid serially presented stimuli stream. The time interval between the presentation onset of successive items in the stream was varied to get a threshold. For search for one target, the threshold was 89 ms. When look for either of two targets, it was 192 ms. This threshold difference offered a baseline. In Experiment 2, observers looked for one or two types of target in a search array. If they compared each identified item separately, we should expect a jump in the slope of the RT × Set Size function, on the order of the baseline obtained in Experiment 1. However, the slope difference was only 13 ms/item, suggesting that several identified items can be compared at once with target templates in memory. Experiment 3 showed that this slope difference was not just a memory-load cost.