Target selection during "snapshot" foraging.

While previous foraging studies have identified key variables that determine attentional selection, they are affected by the global statistics of the tasks. In most studies, targets are selected one at a time without replacement while distractor numbers remain constant, steadily reducing the ratios of targets to distractors with every selection. We designed a foraging task with a sequence of local "snapshots" of foraging displays, with each snapshot requiring a target selection. This enabled tighter control of local target and distractor type ratios while maintaining the flavor of a sequential, multiple-target foraging task. Observers saw only six items for each target selection during a "snapshot" containing varying numbers of two target types and two distractor types. After each selection, a new six-item array (the following snapshot) immediately appeared, centered on the locus of the last selected target. We contrasted feature-based and conjunction-based foraging and analyzed the data by the proportion of different target types in each trial. We found that target type proportion affected selection, with longer response times during conjunction foraging when the number of the alternate target types was greater than the repeated target types. In addition, the choice of target in each snapshot was influenced by the relative positions of selected targets and distractors during preceding snapshots. Importantly, this shows to what degree previous findings on foraging can be attributed to changing global statistics of the foraging array. We propose that "snapshot foraging" can increase experimental control in understanding how people choose targets during continuous attentional orienting.