Are 4-month-olds able to discriminate ordinal sequences of non-numerical quantities?

To detect an ordered sequence of quantities refers to the ability to appreciate ‘greater than’ and ‘less than’ relations and it represents one essential component of numerical knowledge that develops early in infancy. Brannon (2002) argued that by 11 months infants possess this ability for numerical values, and that it follows a sudden onset between 9 and 11 months. In fact, by 9 months infants succeed at discriminating the ordinal direction of a single square varying in size, but fail for numerical sequences, suggesting that the ability to represent the ordinal information embedded in non-numerical quantities develops earlier than for numerical values. Our study aims to track the emergence of the ability to encode ordinal relations between continuous, non-numerical quantities. We tested 4-month-old infants in their ability to discriminate ascending vs. descending sequences of the size of a single square. In Exp. 1, infants saw increasing (N=10) or decreasing (N=10) sequences of a single square that varied in size across 3 steps following a 1:2 ratio. After habituation, all infants saw six new trials, alternating between increasing vs. decreasing sequences of the single square varying in size in a 1:2 ratio. The direction of the first sequence was counterbalanced across participants. We found that 4-month-old infants successfully discriminated the ordinal direction, Familiar vs. Novel, p=.001, but this ability was present only for infants habituated to the increasing sequence, who displayed a preference for the novel vs. the familiar sequence (M=9.3 s vs. 5.3 s; p=.002 ). In contrast, infants habituated to the decreasing sequence did not discriminate the familiar vs. novel sequence (M=4.6 s vs. 5.3 s; p=.20). These results suggest that by 4 months infants are sensitive to the ordinal information contained in a sequence that varies along a continuous dimension, i.e., the area of a single element. However, this ability seems to apply only to sequences that vary in an ascending order, i.e., ‘greater than’ relations, in line with studies showing that after being trained to respond to an ascending order, monkeys generalize the rule to new numerical values, but they fail for the descending order (Brannon and Terrace, 2000). However, because all squares in the sequences were located at the center of the screen, ascending sequences may generate the perceptual appearance of an approaching (looming) object, and descending sequences a retracting percept (zooming), suggesting that infants did not detect the ordinal information of the sequence. To test for this hypothesis, two additional experiments manipulated the spatial disposition of the elements within the ascending sequence. In Exp. 2, the position varied randomly within a region defined around the centre of the screen, and a masking was introduced in between changes of magnitude. Data collection is still in progress. In Exp. 3, the position of the element varied along the horizontal axis following a contiguous path, thus providing a spatial cue. Preliminary results indicate that infants fail to discriminate the ordinal direction of ascending sequences when spatial cues, in addition to temporal cues, are introduced.