The influence of the place-value structure of the Arabic number ...

Finally, increased activation in the posterior superior parietal lobule was also observed
in the contrasts discussed above. Activation in this area is commonly attributed to the
activation of visuo-spatial representations, which may reflect mental navigation on a spatially
oriented number line (Dehaene et al., 2003).
Symbolic base-10 processing
Decade crossing also activated the fronto-parietal network, particularly the
intraparietal cortex bilaterally. When a decade boundary had to be crossed, determining the
correct mean of the interval not only seemed to activate the holistic magnitude representation
of the involved numbers, but also the decomposed magnitudes of tens and units. As range was
matched between triplets with and without decade crossing this effect cannot be attributed to
the activation of overall number magnitude, but corroborates the interpretation of decomposed
magnitude representations for tens and units (Wood et al., 2006). Recent fMRI studies have
shown that the magnitudes of tens and units of a two-digit number are represented separately
within the intraparietal cortex (Wood et al. 2006; Knops et al. 2006). The decomposed
magnitude representations of tens and units seem to rely heavily upon visuo-spatial
representations. For hemineglect patients Hoeckner et al. (2008) observed a larger impairment
in triplets with a decade crossing between the first and the central number than in triplets with
a decade crossing between the central and the third number in the NBT. This is equivalent to a
specific deficit in accessing the magnitude of relatively smaller decade digits represented
further to the left of the mental number line (Zorzi, Priftis, & Umiltà, 2002). Therefore, the
effect of decade crossing on fMRI signal may indicate a close interaction between the
intraparietal sulcus and the posterior superior parietal lobule, which was proposed to be
responsible for precise navigation on the mental number line (Dehaene et al. 2003).
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