There is considerable controversy as to how the brain extracts numerosity informationudfrom a visual scene and as to how much attention is needed for this process. Traditionally, it hasudbeen assumed that visual enumeration is subserved by two functionally distinct mechanisms:udthe fast and accurate apprehension of 1 to about 4 items, a process termed "subitizing", and theudslow and error-prone enumeration of larger numerosities referred to as "counting". Further touda functional dichotomy between these two mechanisms, an attentional dichotomy has been proposed.udSubitizing has been thought of as a pre-attentive and parallel process, whereas countingudis supposed to require serial attention.udIn this work, the hypothesis of a parallel and pre-attentive subitizing mechanism wasudtested. To this aim, the amount of attention that could be allocated to an enumeration task wasudexperimentally manipulated. In Experiment 1, attentional set was manipulated such that attentionudcould either be drawn to the relevant of two subsets to enumerate or had to be distributedudto both subsets. Furthermore, the relationship of enumeration to perceptual grouping and itemuddiscrimination was explored. In Experiment 2, a dual-task approach was employed in whichudthe amount of attentional resources available to enumeration was systematically modulated byudimposing an additional task and by varying its attentional load. Experiment 3 investigatedudthe neural correlates of visual enumeration under attentional load using functional magneticudresonance imaging (fMRI).udResults indicated that (1) enumeration, particularly subitizing, was clearly compromisedudunder conditions of distributed or reduced attention. (2) Both the enumeration of smalludand large numerosities was a�ffected by such attentional manipulations. (3) Subitizing selectivelyudactivated brain areas associated with stimulus-driven attention. (4) Enumeration is contingentudon other potentially attention-demanding visual processes such as perceptual grouping. Theudevidence presented here seriously challenges the traditionally held claim of a parallel and preattentiveudsubitizing mechanism and suggests instead that small numerosity judgement requiresudvisual attention. This weakens the argument of an attentional as well as a functional dichotomyudand strengthens the idea that enumeration may be subserved by a single, continuous mechanism.
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