How does the visuo-spatial deficit impact basic numerical processing in Williams syndrome? The question of domain specificity.

It has been suggested that mathematics learning disabilities, including those of genetic origin, result from a basic impairment of quantitative representations. In Williams syndrome (WS) in particular, latest studies report a specific deficit in tasks requiring symbolic and non-symbolic numerical magnitude processing (Krajcsi et al., 2009 ; O’Hearn & Landau, 2007; Paterson et al., 2006). However, non-numerical quantitative processing has never been investigated. Moreover, as patients with WS were always tested in the visual modality, it remains unclear whether their deficit is specific to the processing of numerical magnitude or result from their basic visuo-spatial impairment, which is a main characteristic of the WS cognitive phenotype. Therefore, numerical and non-numerical acuity were assessed in a group of 20 patients with WS using quantitative comparison tasks with different visuo-spatial processing requirements. They were compared to 40 typically developing children, half of them matched on verbal mental age and the other half matched on nonverbal mental age. Participants were asked to compare: (1) the length of two sticks (spatial dimension) vs. the duration of two sounds (temporal dimension) to assess non numerical quantitative processing, (2) the numerosity of two visual arrays (spatial arrangement) versus two sequences of flashs (no spatial processing) to assess non-symbolic numerical processing, and (3) two Arabic numbers vs two Spoken verbal numerals to examine the access to symbolic number meaning. Compared to verbal matched participants, participants with WS are impaired in quantitative tasks requiring the processing of visuo-spatial dimension(s) (i.e. comparison of lengths or collections) but not in a visual task requiring processing numerosities with no spatial processing component (i.e. numerical comparison of sequences of flashs). They also present difficulties in accessing the meaning of numerical symbols whatever the format. Their performance corresponds to those of the non verbal typically developing children