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

egressions were directed to the second summand as compared to the first summand (0.37 vs.
0.10 regressions, respectively) as well as to the decade than to the unit digits (0.31 vs. 0.16
regressions, respectively) of both summands. Moreover, all possible two-way interactions
were statistically reliable but not the three-way interaction [F(1, 18) < 1]. The interaction of
problem element and carry [F(1, 18) = 21.27, p < .001] indicated that the increase of
regressions to the summands due to a carry was larger for the second summand than for the
first summand (+ 0.17 vs. ± 0.00 regressions). Additionally, the interaction of summand and
digit identity [F(1, 18) = 12.71, p < .01] meant that the effect of digit identity (i.e., more
regressions to the decade digit) was more pronounced on the second summand (+ 0.26 vs. +
0.03 regressions). Finally, the interaction of carry and digit identity [F(1, 18) = 4.50, p < .05]
implied that the increase of regressions towards the two summands due to a required carry
was stronger for regressions towards the decade as compared to the unit digit (+ 0.12 vs. +
0.05 regressions, respectively).
In summary, the present results indicated dissociable influences of the requirement of
a carry operation on the eye fixation patterns on the summands and the result in the addition
verification paradigm employed. On the one hand, the need of a carry operation specifically
increased total reading times on the unit digits of the summands (in particular on the unit digit
of the second summand), possibly reflecting processes related to evaluating whether of not a
carry is required. On the other hand, longer total reading times were observed on the decade
digit of the results in carry addition problems assumed to indicate processes of carry
execution. These results were further substantiated by the results of the analyses of origin and
target of the observed regressions.
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