61, t(20) = 3.60, p = .0020] and Inhibition [β = .35, t(20) = 2.18, p = .0421] were individually

significant predictors. Subitizing slope remained a non-significant predictor when it was entered into the regression with only the Inhibition ability measure [R2 = .368, F(21,2) = 6.13, p = .0080; Subitizing: β = −.19, p = .34; Inhibition: β = .48, p = .0297]. We have contrasted five theories of DD using several measures of the MR theory and alternatives. We found robust evidence for impaired visuo-spatial WM and STM in DD and also found evidence for impaired inhibition function in DD. Data did not support the MR theory of DD. In contrast, verbal STM/WM were intact including both digit and word span. Several studies reported

similar dissociation between ATM/ATR assay spatial and verbal STM/WM in DD (McLean and Hitch, 1999, Andersson and Ostergren, 2013, Schuchardt et al., 2008, Ashkenazi et al., 2012 and Passolunghi and Mammarella, 2010). Other studies reported impaired verbal STM/WM in DD (e.g., Geary et al., 1991 and Geary et al., 2012). A potential dissociating feature seems to be that studies not reporting verbal WM differences noted that they attempted to match DD and control groups on reading and/or verbal performance (McLean and Hitch, 1999, van der Sluis et al., 2005, Schuchardt et al., 2008, Andersson and Ostergren, NVP-BGJ398 order 2013, Ashkenazi et al., 2012 and Passolunghi and Mammarella, 2010). Our DD group also only included children with pure DD with no dyslexia and with normal reading/verbal IQ. This probably explains the lack of verbal memory differences. In fact, Schuchardt et al. (2008) tested both visual and spatial STM in DD, dyslexic, DD + dyslexic and normal populations and found only visual STM impairment in DD and only verbal STM impairment in dyslexics. Hence, it seems that when reading and verbal

function is preserved, that is, in pure DD, a crucial impairment concerns visuo-spatial WM and/or STM. At least three neuro-imaging studies provide supporting evidence to our findings. Rotzer et al. (2009) demonstrated weaker IPS activation in a spatial WM task in DD than in controls. Rykhlevskaia et al. (2009) reported reduced Olopatadine gray matter density in DD not only in the IPS but also in the fusiform, lingual, parahippocampal gyri and in the hippocampus, areas which may be related to encoding complex visual stimuli. Davis et al. (2009) did not find any IPS differences between DD and controls in an approximate calculation task but reported differences in various brain regions associated with WM and cognitive control functions. Visuo-spatial memory probably provides a mental workspace for various transformations and operations crucial for mathematics. Visuo-spatial strategies and heuristics can be used even in seemingly non-visual tasks, e.g., when adding or subtracting numbers, operations and operands can be imagined/conceptualized along a number line.