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

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a view was corroborated by a variety <strong>of</strong> neuro-imaging studies in which processing<br />

carry addition problems as compared to non-carry problems was associated with<br />

increased frontal activation indicating increased demands on e.g., working memory<br />

(e.g., Ashcraft & Kirk, 2001; Fürst & Hitch, 2000), cognitive control (e.g., Miller,<br />

2000) or . At <strong>the</strong> same time no specific increase in parietal activation around <strong>the</strong> IPS<br />

(known to subserve <strong>the</strong> processing <strong>of</strong> magnitude information) was found. However,<br />

recently, <strong>the</strong> data <strong>of</strong> Klein and Co-workers (under revision) suggested a special role <strong>of</strong><br />

<strong>the</strong> unit digits <strong>of</strong> <strong>the</strong> summands in two-digit addition verification. In several regression<br />

analyses <strong>the</strong> authors could show that <strong>the</strong> carry effect did not seem to be purely<br />

categorical. Instead, <strong>the</strong> continuous predictor reflecting <strong>the</strong> sum <strong>of</strong> <strong>the</strong> unit digits <strong>of</strong><br />

<strong>the</strong> summands turned out to be a reliable predictor <strong>of</strong> task performance. <strong>The</strong> specific<br />

<strong>influence</strong> <strong>of</strong> <strong>the</strong> unit digits only again indicated decomposed processing <strong>of</strong> two-digit<br />

<strong>number</strong>s even in <strong>the</strong> case <strong>of</strong> basic arithmetic in <strong>the</strong> way that tens and units <strong>of</strong> two-digit<br />

<strong>number</strong>s are seemingly processed separately ra<strong>the</strong>r than holistically as <strong>the</strong>ir <strong>influence</strong>s<br />

on performance can be dissociated. However, <strong>the</strong> question remains whe<strong>the</strong>r it is <strong>the</strong><br />

recognition <strong>of</strong> a carry that makes carry addition problems more difficult (only if <strong>the</strong><br />

sum <strong>of</strong> <strong>the</strong> unit digits <strong>of</strong> <strong>the</strong> summands is equal or larger than 10 a carry is required) or<br />

ra<strong>the</strong>r <strong>the</strong> execution <strong>of</strong> <strong>the</strong> carry procedure involving processes <strong>of</strong> <strong>place</strong>-<strong>value</strong>, i.e.,<br />

unit-decade integration as indicated by specific processing <strong>of</strong> ei<strong>the</strong>r tens or units. <strong>The</strong><br />

present study addressed this issue by recording participants’ eye fixation behaviour in<br />

a two-digit addition verification paradigm. According to <strong>the</strong> eye-mind and <strong>the</strong><br />

immediacy assumption on eye fixation behaviour above mentioned processes <strong>of</strong><br />

recognizing vs. executing <strong>the</strong> carry should be distinguishable from each o<strong>the</strong>r. While<br />

<strong>the</strong> former should be associated with a specific increase <strong>of</strong> fixations on <strong>the</strong> unit digits<br />

29

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