Numerosity estimates for attended and unattended items in visual search

The goal of this research was to examine memories created for the number of items during a visual search task. Participants performed a visual search task for a target defined by a single feature (Experiment 1A), by a conjunction of features (Experiment 1B), or by a specific spatial configuration of features (Experiment 1C). On some trials following the search task, subjects were asked to recall the total number of items in the previous display. In all search types, participants underestimated the total number of items, but the severity of the underestimation varied depending on the efficiency of the search. In three follow-up studies (Experiments 2A, 2B, and 2C) using the same visual stimuli, the participants' only task was to estimate the number of items on each screen. Participants still underestimated the numerosity of the items, although the degree of underestimation was smaller than in the search tasks and did not depend on the type of visual stimuli. In Experiment 3, participants were asked to recall the number of items in a display only once. Subjects still displayed a tendency to underestimate, indicating that the underestimation effects seen in Experiments 1A-1C were not attributable to knowledge of the estimation task. The degree of underestimation depends on the efficiency of the search task, with more severe underestimation in efficient search tasks. This suggests that the lower attentional demands of very efficient searches leads to less encoding of numerosity of the distractor set.