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

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eye fixation behaviour for <strong>the</strong> evaluation <strong>of</strong> basic numerical processes has come from studies<br />

on basic <strong>number</strong> related tasks such as <strong>number</strong> reading (Brysbaert, 1995) and magnitude<br />

comparison (Moeller, Fischer, Nuerk, & Willmes, 2009a) as well as more complex tasks such<br />

as <strong>the</strong> <strong>number</strong> bisection task (Moeller, Fischer, Nuerk, & Willmes, 2009b) . As regards<br />

addition, a first study evaluating eye fixation behaviour has been reported by Green et al.<br />

(2007). Yet, <strong>the</strong> authors employed eye-tracking to validate <strong>the</strong> use <strong>of</strong> strategies, which <strong>the</strong><br />

participants were instructed to apply, but did not use <strong>the</strong> eye fixation data to investigate <strong>the</strong><br />

nature <strong>of</strong> <strong>the</strong> cognitive mechanisms underlying <strong>the</strong> difficulty <strong>of</strong> carry operations in multi-digit<br />

addition. This will be <strong>the</strong> main focus <strong>of</strong> <strong>the</strong> current study.<br />

In summary, eye movement measures seem to be well-suited to <strong>of</strong>fer new insights into<br />

<strong>the</strong> sequence and <strong>the</strong> nature <strong>of</strong> cognitive processes employed in both ra<strong>the</strong>r basic and more<br />

complex numerical tasks. Based on this <strong>the</strong> current study aimed at dissociating two different<br />

processes associated with solving carry addition problems. <strong>The</strong>se processes may help to better<br />

understand what makes carry addition problems more difficult than <strong>the</strong>ir non-carry<br />

counterparts.<br />

Objectives <strong>of</strong> <strong>the</strong> current study<br />

Despite <strong>the</strong> fact that it is widely agreed that <strong>the</strong> requirement <strong>of</strong> a carry is a crucial<br />

predictor <strong>of</strong> difficulty in multi-digit addition, <strong>the</strong> question what exactly causes <strong>the</strong> difficulty<br />

associated with <strong>the</strong> carry operation is not yet resolved. We suggest that at least two processes<br />

involved in processing a carry in addition can be differentiated which may drive <strong>the</strong> increased<br />

difficulty <strong>of</strong> <strong>the</strong>se problems.<br />

(i) Before any carry procedure can be executed it has to be recognized that a carry is<br />

needed to compute <strong>the</strong> correct result. One way to determine whe<strong>the</strong>r a carry is needed<br />

or not is to keep track <strong>of</strong> <strong>the</strong> sum <strong>of</strong> <strong>the</strong> to-be-added unit digits: a carry is required<br />

whenever <strong>the</strong> unit sum is equal or larger than 10. In this case, <strong>the</strong> decade digit <strong>of</strong> <strong>the</strong><br />

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