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

impossible to bisect by an integer should specifically activate regions in prefrontal cortex
which are involved in cognitive set changes. Whereas for triplets possible to bisect increased
activation of the intraparietal cortex would indicate reliance upon magnitude manipulations to
derive the correct answer.
METHOD
Participants: Seventeen male right-handed volunteers (mean age=24.2 years, range
21-30 years) took part in the study after having given their written consent in accord with the
protocol of the local Ethics Committee of the Medical Faculty.
Experimental task: 360 one- and two-digit number triplets including integer numbers
ranging from 1 to 99 were presented in a 2 x 2 within participant design in Arabic notation for
both bisected (e.g. 23_26_29) and non-bisected triplets (e.g. 23_25_29). For bisected triplets,
multiplicativity (number triplets part/not part of a multiplication table, e.g. 21_24_27 vs.
22_25_28) and bisection range (the distance between the outer numbers; small: 4-8, e.g.
23_26_29 vs. large 12-18, e.g. 22_30_38) were varied. For non-bisected triplets, we
manipulated bisection possibility (whether the mean of the interval is an integer, e.g.
21_24_29, mean = 25, or not e.g. 21_24_28, mean = 24.5) and distance of the second number
to the mean of the interval (far: 2-8, e.g. 6_7_18 vs. near: 0.5 – 1.5, e.g. 6_11_18).
Problem size, overall magnitude, average parity, parity homogeneity, decade crossing and
inclusion of multiples of ten were matched between the respective stimulus groups. In
addition to these variables, bisection range and size of the central number relative to the
correct mean (i.e. larger/smaller) were matched for non-bisected triplets.
Procedure: Participants were lying in the fMRI scanner while responding to the
stimuli. The stimuli were rear-projected on a screen via a mirror attached to the head coil of
the scanner at a distance of 12 cm from the participants. Participants were instructed to decide
as quickly and accurately as possible whether the interval between the two outer numbers was
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