Typical and atypical development of visual estimation abilities.

Despite the fact that developmental impairments of number skills are common, they remain sparsely investigated. We explored low-level numerical representations and their developmental trajectory in a developmental disorder, Williams syndrome (WS). Groups of WS and typically developing (TD) individuals estimated rapidly-presented arrays of 5, 7, 9, and 11 dots. In comparison to the normal developmental trajectory, the ontogenesis of estimation skills in WS is both delayed and deviant. Whereas TD children's estimations became significantly more accurate and less variable over developmental time, only marginal developmental changes in estimation ability emerged across age in the WS groups. Our data highlight the importance of considering developmental changes in low-level components of numerical cognition in atypical development while at the same time emphasizing the importance of paying closer attention to quantitative changes and their functional role in typical development.