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

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Apparatus: Eye fixation behaviour was recorded online using an EyeLink 1000 eyetracking<br />

device (SR Research, Mississauga, Ontario, Canada) and stored for <strong>of</strong>fline analysis.<br />

<strong>The</strong> EyeLink 1000 provides a spatial resolution <strong>of</strong> less than 0.5 degrees <strong>of</strong> visual angle at a<br />

sampling rate <strong>of</strong> 1000 Hz. Stimuli were presented on a 21’’ monitor with resolution set to<br />

1024 x 768 pixels and driven at a refresh rate <strong>of</strong> 120 Hz. Participants’ heads were <strong>place</strong>d in a<br />

chin rest throughout <strong>the</strong> experiment to keep viewing distance (50 cm) and viewing angle<br />

constant.<br />

Task, Stimuli, and Design: Participants had to evaluate whe<strong>the</strong>r displayed addition<br />

problems were solved correctly by simultaneously presented probes or not. <strong>The</strong> addition<br />

problems consisted <strong>of</strong> one- and two-digit <strong>number</strong>s with a sum not exceeding 99. <strong>The</strong> stimulus<br />

set comprised 96 addition problems. In a factorial 2 x 2 design problem size (small vs. large,<br />

i.e. sum < 40 vs. sum > 40) and <strong>the</strong> requirement <strong>of</strong> a carry operation (required vs. not<br />

required) were manipulated orthogonally. Each problem was presented twice once toge<strong>the</strong>r<br />

with <strong>the</strong> correct result and once with an incorrect probe. Incorrect probes deviated from <strong>the</strong><br />

correct result by ei<strong>the</strong>r 2 or 10 to prevent parity based solution strategies with average split<br />

kept constant (i.e., zero) across item groups (for an overview <strong>of</strong> stimulus properties see <strong>the</strong><br />

Appendix). Addition problems were presented in <strong>the</strong> form xx + xx = xx at a position<br />

slightly to <strong>the</strong> right <strong>of</strong> <strong>the</strong> centre <strong>of</strong> <strong>the</strong> screen. To ensure that all participants started to encode<br />

<strong>the</strong> addition problem at <strong>the</strong> first summand a fixation point to <strong>the</strong> left <strong>of</strong> this summand was<br />

used (x/y coordinates: 112/384). Digits and arithmetic symbols were shown in white against a<br />

black background using <strong>the</strong> unproportional font New Courier (size: 48; style: bold). At this<br />

size each digit subtended 1.4 degrees <strong>of</strong> visual angle in width and 1.9 degrees in height. <strong>The</strong><br />

distance between <strong>the</strong> both summands as well as between <strong>the</strong> second summand and <strong>the</strong> result<br />

equalled 10.4 degrees <strong>of</strong> visual angle to keep <strong>influence</strong>s <strong>of</strong> parafoveal preprocessing at a<br />

minimum.<br />

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