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

the requirement of a carry is determined by unit-based calculations as soon as it becomes
apparent that the sum of the unit digits becomes a two-digit number. This interpretation is
corroborated by a number of further observations. First, when assuming that participants
performed the addition task from left to right (as implicated by the fixation sign on the left of
the first summand) the necessity of a carry can be recognized first when encoding the unit
digit of the second summand. Therefore, the increase of total reading times on the unit digits
of the summands should be more pronounced on the unit digit of the second summand than on
the unit digit of the first summand because no inference on a possibly required carry can be
drawn from encoding the unit digit of the first summand only. The eye fixation pattern
confirmed this hypothesis. The increase of total reading times on the unit digit of the second
summand was indeed more pronounced than the corresponding carry effect on the unit digit
of the first summand. Finally, additional evaluation of the number of regressions, in particular
from the second back to the first summand substantiated this interpretation. Not only that the
requirement of a carry led to a specific increase of regressions from the unit digit of the
second back to the first summand, it was also observed that upon the first summand the
majority of regressions was directed to the unit digits and particularly so when a carry was
required. This again corroborates the notion that (i) unit-based calculation processes may be
applied to evaluate whether a carry is required or not and (ii) the requirement of a carry
specifically increases processing demands as regards the unit digits of the summands. As the
former can only be decided while processing the unit digit of the second summand (before
any regressions occurred), this suggests that gaze duration on the unit digit of the second
summand (i.e., the sum of all fixation durations from first entering a region to first leaving it)
should be sensitive to the size of the unit sum (see Brysbeart, 1995 for the sensitivity of gaze
duration to magnitude processing). A correlation analysis confirmed this assumption. As the
sum of the unit digits increased, gaze duration on the unit digit increased as well on the
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