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Test

FIGURE 6.8 Sample habituation and test stimuli used by Xu and Spelke (2000) to test 6-month-old infants in an 8 vs. 16 discrimination. (From Xu, F. and Spelke, E.S., Cognition, 74, B1-B11, 2000.)

1987). Therefore, more definitive evidence that infants form numerical representations that are modality independent is still needed.

6.4.4 Continuous Dimensions Confound

In experiments with animals it is possible to use large sets of very diverse stimuli to ensure that only numerical discriminations would allow the animal to be successful. With human infants, it is often the case that sessions are limited to 10 to 15 min, and each infant generally participates in one or two sessions, each of which contains a handful of stimuli rather than the endless trials that can be conducted with nonhuman animals. For these reasons, it is a more challenging enterprise to design studies that conclusively demonstrate numerical competence in infants.

Recent studies suggest that under some circumstances infants may keep track of continuous dimensions such as total contour length rather than number (Clearfield and Mix, 1999; Feigenson et al., 2002b; Mix et al., 2002; Newcombe, 2002). For example, Clearfield and Mix (1999) habituated 6- to 8-month-old infants to stimuli with a constant number (2 or 3) and contour length and then tested the infants with stimuli of a new number with an unchanged contour length or a new contour length with an unchanged number. Infants looked longer to the test stimuli that changed in contour length, compared to the last few habituation trials, and did not show a comparable elevation in looking time to the test stimuli that changed in number. Such results at the very least suggest that continuous dimensions must be more carefully controlled in studies of numerical cognition. These studies also suggest a need for psychophysical studies of the perception of both number and continuous dimensions in infancy. However, it is important to remember that there are two distinct questions that should not be confused. One question is whether infants are capable of purely numerical discriminations and computations. The second question is whether continuous dimensions are more salient than number in infancy. The Clearfield and Mix study is more pertinent to the second question.

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