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

(see Rayner, 1998 for a review). Transferred to the addition task this implies longer total
reading times in carry as compared to non-carry problems.
However, as already outlined above, the goal of the current study was not a mere
replication of RT data. Rather, following up on above considerations on two possible origins
of difficulty in carry addition problems, more specific hypotheses for the eye fixation
behaviour can be derived:
(i)
When the difficulty associated with a required carry operation arises at the unit
calculation stage then longer total reading times should be observed on the unit
digits of the summands for carry addition problems than for non-carry problems.
Additionally, regressions from the second to the first summand should specifically
involve the unit digits. This should be the case as specific evaluation of the units
(i.e., the unit sum being equal or larger than 10) determines whether or not a carry is
required.
(ii) When difficulty is associated with the execution of the carry procedure itself, then
longer total reading times on the decade digit of the result should be found as the
sum of the decade digits of the summands has to be updated by the carry to obtain
the correct result (e.g., adding 1 to the sum of the decade digits). Accordingly,
regressions from the result back to the summands should be prominently focused on
the decade digits.
The present study pursued these questions.
METHOD
Participants: 20 students (15 female) of the Paris-Lodron University of Salzburg
participated in the study as partial fulfilment of course requirements. Mean age was 22.7 years
with a standard deviation (SD) of 2.4 years (range: 20 - 31 years). All participants reported
normal or corrected to normal vision.
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