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Tomonari Shimomura, Takatsune Kumada; Spatial working memory load affects counting but not subitizing in enumeration. Journal of Vision 2011;11(11):1252. doi: 10.1167/11.11.1252.
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© ARVO (1962-2015); The Authors (2016-present)
It is known that the reaction time for deciding how many objects comprise a scene typically remains roughly constant up to approximately four items but over which then increases linearly with the number of items. The enumeration consists of two components: subitizing as efficient enumeration for the small number (up to four items) and counting as the time-consuming enumeration for larger number. An outstanding question in studying enumeration processes is what determines the subitizing span. Present research investigated whether spatial or non-spatial visual working memory capacity is responsible for the subitizing span by manipulating memory load with a dual-task procedure. We compared the enumeration performances under memory load with those under no-load. Under the dual-task condition the dot-enumeration task intervened during 5,000 msec retention interval of a secondary memory task. Participants reported whether the memory item and the probe were identical in terms of location (Experiment 1) or shape (Experiment 2) after the retention interval. In the enumeration task, participants made a speeded response when they apprehend the number of dot and then reported the number at the end of the trial. Under the control single-task condition the memory task was omitted. The results showed that neither remembering locations nor shapes affected subitizing efficiency. Only counting efficiency was impeded when spatial memory load was imposed. Non-spatial memory load did not impair the counting efficiency. The subitizing span estimated by fitting a bilinear function was not decreased by both types of the memory load. Subsidiary experiments in which the amount of memory load was varied revealed that subitizing efficiency and span were unaffected by the increase of memory set size. These results suggest that subitizing span reflects neither spatial nor non-spatial working memory capacity. Rather, independent limitations are involved in subitizing and visual working memory.
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