Date of Award


Degree Type


Degree Name

Doctor of Philosophy


Indexing of multiple locations in a visual display was examined in the context of a selective search task. Subjects searched for a conjunctively defined target among subsets of display items randomly distributed throughout the display, identified only by their abrupt onset relative to other items in the display. Experiment 1 indicates that search is faster when observers search selectively over a subset of three display items (among a total of fifteen) indicated as potential target positions. Moreover, this result cannot be due to selective attention to one of the indicated items only, because search times are influenced by characteristics of the set of indicated items (these same characteristics have no meaning for single items). In particular, search is faster when the selected subset includes only one type of distractor (thus, as a set, the items share only one feature with the target); in contrast, slower search is observed when the subset includes mixed distractors (thus, as a set, the selected items share both features with the target). Experiments 2 and 2b demonstrate that search times are not slowed when the spatial dispersion of the indexed items is increased, discounting hypotheses that one attentional locus is either expanded to include the indicated items, or moved in an analog fashion from item to item. According to the results of Experiments 3 and 3b, observers are able to select up to five items in a display, and the advantage for subsets including homogenous distractors increases with increases in the number of selected items. Taken together, the results of these experiments suggest that observers can select a small number of items in a display (up to four or five) and subsequently treat these items virtually as if they are the only items that appear. These results are discussed in the context of a theory of visual indexing (FINST theory), which assumes that the visual system uses a small number of indexes (FINSTs) to mediate the engagement of a single attentional mechanism.



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