Subitizing reflects visuo-spatial object individuation capacity.

Center for Mind/Brain Sciences, University of Trento, Italy.
Cognition (Impact Factor: 3.63). 06/2011; 121(1):147-53. DOI: 10.1016/j.cognition.2011.05.007
Source: PubMed

ABSTRACT Subitizing is the immediate apprehension of the exact number of items in small sets. Despite more than a 100years of research around this phenomenon, its nature and origin are still unknown. One view posits that it reflects a number estimation process common for small and large sets, which precision decreases as the number of items increases, according to Weber's law. Another view proposes that it reflects a non-numerical mechanism of visual indexing of multiple objects in parallel that is limited in capacity. In a previous research we have gathered evidence against the Weberian estimation hypothesis. Here we provide first direct evidence for the alternative object indexing hypothesis, and show that subitizing reflects a domain general mechanism shared with other tasks that require multiple object individuation.

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    ABSTRACT: Subitizing refers to people’s ability to enumerate small sets of items fast and accurately. The present study examined if the speed and scope of subitizing is improved when the items to be enumerated are presented bilaterally across hemifields rather than unilaterally in a single hemifield. Such an effect, known as the bilateral field advantage, has been observed in a number of other visual tasks. A second aim was to examine whether the speed of subitizing could be explained by the speed it takes to detect the items to be enumerated, as simple reaction times to multiple stimuli are known to be faster than responses to individual items (known as the redundant target effect, RTE). The results revealed a bilateral field advantage even for enumerating two items. Moreover, the two item condition was the optimal subitizing condition – even enumerating one single item took longer – but this effect was not due to the redundant target effect. In fact, the speed of simply detecting the stimuli (RTE) negatively correlated with the speed of enumerating the same stimuli.
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