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

correctly bisected by the second number (right hand key) or not (left hand key). Each triplet
was presented for 3 seconds, followed by an ISI of 1.5 seconds in which only the fixation
marks ( _ _ ) were visible. RTs longer than 3 seconds were not recorded. After every eighth
trial, there was a longer ISI of 5.6 seconds. All stimuli were presented in Courier New 150.
360 triplets were presented in 5 blocks, each lasting approximately 6 minutes. Between
blocks, there was a break of approximately 3 minutes. Trial order was pseudo-randomized so
that each stimulus condition appeared about equally often in each block. The experiment was
preceded by 30 warm-up trials.
fMRI acquisition: Five functional imaging runs sensitive to blood oxigenation leveldependent
(BOLD) contrast were recorded for each participant in a Philips 1.5T Gyroscan
MRI system (T2*-weighted echo-planar sequence, TR = 2500 ms; TE = 50 ms; flip angle =
90°; FOV = 220 mm, 64 x 64 matrix; 30 slices, voxel size = 3.4 x 3.4 x 4 mm). Five runs of
146 scans each were acquired. In a rapid event-related design, 360 trials were presented in a
pseudo-randomized sequence at a rate of 4.5 seconds in 45 small blocks of 8 trials interleaved
with null events of 5.6 seconds. The fMRI time series was corrected for movement artifacts
and unwarped using SPM2. Images were resampled every 3 mm to a standard template using
sinc interpolation to interpret them in Talairach coordinates (Talairach and Tournoux, 1988),
and smoothed with a 6 mm Gaussian kernel.
We convolved brain activation separately for bisected and non-bisected triplets with
the canonical hemodynamic response function (HRF) and estimated the effects of parametric
regressors. For bisected triplets regressors coding for multiplicativity, decade crossing,
bisection range, and problem size were entered as parametric predictors of activation. For
non-bisected triplets, regressors coding bisection possibility, decade crossing, bisection range,
problem size and distance between the central number and the mean of the interval were
entered as parametric predictors of activation. The effects of different regressors on the fMRI
signal were estimated using the first-order term of the polynomial expansion of the typical
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