Abstract
Despite a prevailing assumption in the developmental literature that changes in continuous quantities (i.e., surface area, duration) are easier to detect than changes in number, very little research has focused on the verity of this assumption. The few studies that have directly examined infants’ discriminations of continuous extent have revealed that infants discriminate the duration of a single event and the area of a single item with similar levels of precision (Brannon et al., 2006; vanMarle and Wynn, 2006). But what about when items are presented in arrays? Infants appear to be much worse at representing the cumulative surface area compared to the numerosity of an array (Cordes and Brannon, 2008a), however this may be due to a noisy accumulation process and not a general finding pertaining to representations of the extent within an array. The current study investigates how well infants detect changes in the size of individual elements when they are presented within an array. Our results indicate that infants are less sensitive to continuous properties of items when they are presented within a set than when presented in isolation. Specifically we demonstrate that infants required a fourfold change in item size to detect a change when items were presented within a set of homogeneous elements. Rather than providing redundant cues that aided discrimination, presenting a set of identical elements appeared to hamper an infant's ability to detect changes in a single element's size. In addition to providing some of the first evidence to suggest that the presence of multiple items may hinder extent representations, these results provide converging lines of evidence to support the claim that, contrary to popular belief, infants are better at tracking number than continuous properties of a set.
Highlights
What kind of quantitative information do infants represent when they see a set of objects such as a box of toys or a handful of cheerios? Do infants track the number of cheerios, the overall amount of cereal, or the size of each O? Much research has been dedicated to these questions, primarily focusing on the more specific question of whether infants are capable of representing something as abstract as number
Findings from studies stringently controlling for other array properties that tend to co-vary with number, such as surface area, density, and item size or duration have revealed that infant discriminations of number are ratio-dependent, such that 6-month-olds are capable of detecting a twofold change in number (e.g., 8 vs. 16) but not a 1.5-fold change (8 vs. 12; e.g., Xu and Spelke, 2000; Lipton and Spelke, 2003; Wood and Spelke, 2005)
The main effect of test trial type reflected longer looking times to the novel (M = 5.5 s) as compared to the familiar (M = 4.3 s) test trials [t(19) = 2.5, p < 0.03], indicating that infants detected the change in element size (16/20 infants looked longer to novel as compared to familiar, p < 0.01 Binomial statistics; Figure 4)
Summary
What kind of quantitative information do infants represent when they see a set of objects such as a box of toys or a handful of cheerios? Do infants track the number of cheerios, the overall amount of cereal, or the size of each O? Much research has been dedicated to these questions, primarily focusing on the more specific question of whether infants are capable of representing something as abstract as number. Findings from studies stringently controlling for other array properties that tend to co-vary with number, such as surface area, density, and item size (in simultaneous visual displays) or duration (in sequential presentations) have revealed that infant discriminations of number are ratio-dependent, such that 6-month-olds are capable of detecting a twofold change in number (e.g., 8 vs 16) but not a 1.5-fold change (8 vs 12; e.g., Xu and Spelke, 2000; Lipton and Spelke, 2003; Wood and Spelke, 2005). A prevailing belief, both in the developmental and animal cognition literature, has been that discriminating continuous quantities is trivial for both non-human animals (Davis and Memmott, 1983) and preverbal infants (Piaget, 1952; Mix et al, 2002a,b). While number was thought to be too abstract for a preverbal child or non-verbal animal, representations of surface area were considered perceptual and cognitively effortless
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