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

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Two-digit <strong>number</strong> processing<br />

Basically, <strong>Arabic</strong> <strong>number</strong>s are <strong>structure</strong>d in a two-dimensional <strong>place</strong>-<strong>value</strong> system.<br />

First, <strong>the</strong> base dimension is reflected by <strong>the</strong> ten digits from 0 to 9, whereas, second, <strong>the</strong> power<br />

dimension is coded by <strong>the</strong> positions <strong>of</strong> <strong>the</strong> digits with a base ten. <strong>The</strong> position within a digit<br />

string defines <strong>the</strong> <strong>value</strong> <strong>of</strong> each digit on a power <strong>of</strong> ten dimension (cf. Zhang & Norman,<br />

1995; Chrisomalis, 2004). However, <strong>the</strong> way in which <strong>the</strong>se two dimensions <strong>of</strong> <strong>number</strong>s are<br />

represented psychologically on <strong>the</strong> mental <strong>number</strong> line is subject to controversial debates. At<br />

<strong>the</strong> moment three models accounting for <strong>the</strong> organization <strong>of</strong> this <strong>number</strong> line can be<br />

discerned.<br />

1. <strong>The</strong> holistic model<br />

According to <strong>the</strong> holistic model (e.g., Dehaene, Dupoux, & Mehler, 1990) <strong>number</strong>s<br />

are transformed into an integrated entity mapped onto a holistic magnitude representation<br />

before any magnitude comparison or numerical manipulation is performed. In <strong>the</strong> context <strong>of</strong><br />

<strong>number</strong> magnitude comparison this property implies that performance should be exclusively<br />

determined by <strong>the</strong> overall distance between <strong>the</strong> <strong>number</strong>s to-be-compared, <strong>the</strong>reby, overriding<br />

<strong>the</strong> base-10 <strong>structure</strong> <strong>of</strong> two-digit <strong>number</strong>s.<br />

However, to account for <strong>the</strong> problem size effect additional assumptions are required.<br />

In this context, ei<strong>the</strong>r linear coding <strong>of</strong> <strong>number</strong>s along <strong>the</strong> mental <strong>number</strong> line with scalar<br />

variability or logarithmic coding <strong>of</strong> <strong>number</strong>s with fixed variability have been suggested (for a<br />

recent discussion see Brannon, Wusth<strong>of</strong>f, Gallistel, & Gibbon, 2001; Dehaene, 2001). <strong>The</strong><br />

latter model implies that magnitudes <strong>of</strong> larger <strong>number</strong>s may be compared more slowly<br />

because magnitude is assumed to be represented along a logarithmically compressed mental<br />

<strong>number</strong> line. Consequently, <strong>the</strong> distance between <strong>the</strong> logarithmic magnitudes <strong>of</strong> two relatively<br />

larger <strong>number</strong>s is smaller than <strong>the</strong> distance between <strong>the</strong> logarithmic magnitudes <strong>of</strong> two<br />

relatively smaller <strong>number</strong>s. As variability is assumed to be constant for <strong>the</strong> representation <strong>of</strong><br />

234

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