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

(applicable only for non-bisected triplets), and (iii) when triplets were not part of the
multiplication table (applicable only for bisected triplets).
Increased activation in the bilateral intraparietal cortex was found for triplets spanning
a large range. This reflects more demanding magnitude processing in these triplets as
magnitude representation becomes less accurate in larger numerical intervals (i.e. according to
the Weber-Fechner law, see Dehaene et al., 2003). Activation in the bilateral intraparietal
cortex also increased when distance between the middle number and the correct mean was
small. So, difficulty in rejecting the middle number was inversely proportional to the distance
between the central number and the mean of the two outer numbers. This result can be
interpreted as a distance effect for the magnitude comparison between a standard (i.e. the
correct mean of the interval) and a probe (i.e. the actual central number of the triplet; Goebel
et al., 2004; Pinel et al., 2001, Wood et al., 2006).
Moreover, when triplets were not part of a multiplication table stronger activation of
the fronto-parietal network was observed. Again, this increase in activation indicates deeper
magnitude processing in these triplets. A trivial item difficulty explanation cannot account for
stronger frontoparietal activation in non-multiplicative triplets, since overall magnitude, range
and problem size were matched between multiplicative and non-multiplicative triplets.
Therefore, when evaluating non-multiplicative triplets, participants rely more on the
magnitude representation than in multiplicative triplets. This result is in line with an
interpretation for the changes in neuroanatomical regions activated before and after training
arithmetical problems (Delazer et al., 2003; 2005; Ischebeck et al., 2006; Ischebeck,
Zamariaan, Egger, Scjocke, & Delazer, 2007). In their training studies, participants practiced
arithmetical problems for some days. Without practice, brain activation was stronger in the
intraparietal cortex. Whereas, after practice the main locus of activation for trained items was
in the left angular gyrus. The authors accounted for this change in activation by the formation
of lexical representations for items repeatedly presented during training.
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