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

associated with actually calculating the correct result may not be finished completely before
participants processed the solution probe and decided whether it was the correct solution to
the problem at hand or not. Finally, and most importantly for the research question at hand,
the data suggested that it does not seem to be an either/or distinction between unit based
calculations determining that a carry is required and its processing. Instead, the present
observations on the increased processing demands associated with a carry operation indicated
that the carry effect in multi-digit addition seems to originate from both specific processing of
the unit digit of the summands to evaluate by calculation that a carry is needed as well as
particular processes reflecting the execution of the carry procedure (mostly associated with
the decade digit of the result).
CONCLUSIONS / PERSPECTIVES
Previous studies indicated that processes of unit-decade integration as required by a
carry operation pose particular difficulty on mental addition. In particular, response latencies
as well as error rates are increased for addition problems requiring a carry operation (e.g., 18
+ 27) compared to problems not requiring a carry (e.g., 13 + 32, e.g., Deschuyteneer et al.,
2005; Kong et al., 2005). However, the cognitive underpinnings of this carry effect remained
unclear. The current study aimed at pursuing this question by tracking participants’ eyemovements
during addition problem verification. Analyses of the eye-fixation data revealed a
differential influence of decade and unit digits on mental addition: On the one hand, the
requirement of a carry operation particularly increased total reading times on the unit digits of
the summands, whereas total reading times were particularly increased on the decade digit of
the result on the other hand. Furthermore, this dissociation was accompanied by a specific
increase of regressions from the units of the second to the units of the first summand as well
as from the tens of the result to the tens of the summands in carry addition problems
compared to non-carry problems. This implies that processing a carry is not a unitary process.
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