The fast and accurate enumeration of a small set of objects, called subitizing, is thought to involve a different mechanism from other numerosity judgments, such as those based on estimation. The underlying mechanism for small-set enumeration was examined by using a novel “pointing” task that also obtains information about the perceived locations of enumerated objects. Observers were shown brief masked displays (50, 200, and 350 ms) of 2-9 small black discs randomly placed on a gray screen. The observers then placed markers on a blank screen to indicate the locations of these discs. The number of these markers provided an estimate of enumeration accuracy. Observers could “enumerate” accurately displays containing up to six items in contrast with the four-item limit typically found. Experiment 2 replicated this lower limit by asking observers to report numerosity using cardinal symbols. Here, enumeration performance was better in the pointing task than the numeral report task. Experiments 3-5 presented alternative control tasks in addition to the pointing task. Results from these experiments indicate overall better enumeration performance in the pointing task, which suggests a non-symbolic response method produces a higher subitizing range. To characterize the mechanism underlying the pointing task, we examined the errors in the memory for object locations. Localization error was measured as the distance between corresponding stimulus discs and response markers. These errors increased in magnitude with larger numerosities and shorter display durations. Responses also indicated a compression of distances around the center of mass. Additionally, stimulus displays with higher regularity in the spacing between discs produced better enumeration and localization performance. Overall, the localization results indicate spatial information can be represented with a rough accuracy that improves when increasing exposure time to the stimulus or reducing the number of items that must be remembered. In contrast, enumeration performance showed few errors in the same range (2-6 items). Together, these results support the possibility that the Visual Indexing mechanism assists small-set enumeration by individuating and selecting discrete objects without necessarily encoding detailed information about these objects.Ph.D.Includes bibliographical referencesIncludes vitaby Harry Haroutioun Haladjia
