Bilateral field advantage in subitizing: Visual object selection is restricted to single items in each visual hemifield H Railo |
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Department of Psychology, University of Turku, Finland
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| Earlier studies suggest that object-based attention can only select one item at a time [Duncan, 1984, J Exp Psy: Gen, 113, 501-517], but participants can nevertheless individuate and access multiple objects simultaneously [Cavanagh & Alvarez, 2005, Psych Sci, 16, 637-643]. Such object individuation capacity has been shown to be split between hemifields. If the left and right visual hemifields have independent object individuation capacities, it should be reflected in subitizing, which refers to the effortless and errorless apprehension of small numbers of items (1–3). The present study shows that subitizing is faster and more accurate when items are presented bilaterally than unilaterally. Visual crowding cannot explain the results. In fact, the participants could report the number of two objects faster than the number of a single object, but only when the two objects were presented bilaterally. This speaks against both classical serial and parallel models of visual selection, and can be best explained by assuming independent attentional selection for the hemifields. The results support the view that the visual system can simultaneously select only one item per hemifield. The speed of subitizing is explained by object-based attentional selection, but the capacity of subitizing is explained by visual short-term memory. |
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