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Edward Ester, Trafton Drew, Edward Vogel, Edward Awh; Neural measures reveal a fixed item limit in subitizing.. Journal of Vision 2012;12(9):945. doi: 10.1167/12.9.945.
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© ARVO (1962-2015); The Authors (2016-present)
Humans are endowed with an extraordinary ability to rapidly and accurately enumerate small sets of items, a process known as subitizing. Despite over a century of research, there is still active debate regarding the mechanisms that mediate this ability. For example, some have argued that subitizing reflects the operation of a limited-capacity individuation mechanism that enables concurrent access to a small number of items. However, others have argued that subitizing reflects the operation of a continuous numerical estimation mechanism whose precision varies with numerosity in a manner consistent with Weber’s law. Critically, quantitative models based on either of these views can provide a reasonable description of performance on a subitizing task, making it difficult to discriminate between these alternatives solely on the basis of subjects’ behavior. Here, we attempted to discriminate between limited-capacity and continuous estimation models of subitizing using neural measures. In two experiments, we recorded EEG while subjects performed a demanding subitizing task and examined set-size dependent changes in a neurophysiological marker of visual selection (the N2pc ERP component) evoked by an array of to-be-enumerated items. We reasoned that if subitizing reflects the operation of a limited-capacity individuation mechanism, then N2pc amplitudes evoked by a subitizing array should reach an asymptotic limit at or near behavioral estimates of subitizing span. In both experiments, N2pc amplitudes increased monotonically within the subitizing range before reaching an asymptotic limit at around 3 items. Moreover, intersubject differences in the location of this asymptote were strongly correlated with behavioral estimates of subitizing span. Thus, neural activity linked with subitizing ability shows evidence of an early and discrete limit in the number of items that can be concurrently apprehended.
Meeting abstract presented at VSS 2012
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