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 in particular, recent 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, to our knowledge, the integrity of non numerical quantitative processing has never been investigated. Moreover, in the visual modality, it is difficult to disentangle the influence of the massive visuo-spatial impairment, which is a main characteristic of the cognitive phenotype of Williams syndrome, from the numerical magnitude processing deficit. Therefore, numerical and non numerical acuity were assessed in a group of patients with Williams syndrome using tasks with different visuo-spatial processing requirements. To assess the processing of non numerical continuous quantities, participants were asked to compare the length of two sticks (spatial dimension) vs. the duration of two sounds (temporal dimension). Their ability to process non-symbolic numerical magnitudes was explored in tasks requiring the comparison of two arrays of elements (spatial arrangement) versus two sequences of dots flashed in a single location (no spatial processing). Finally, their access to symbolic number meaning was assessed through the numerical comparison of two Arabic numbers vs two Spoken verbal numerals. In each task, the ratio between the quantities was manipulated in order to measure the entire psychophysic curve and to measure the acuity of their numerical and non numerical magnitude representations in non-symbolic tasks.