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

Errors: The ANOVA revealed a main effect of participant group [F(1,16) = 13.52, p <
.01], indicating that neglect patients committed reliably more errors than non-neglect controls
(7.9 % vs. 2.9 % errors). Additionally, the interaction of distance to the middle and size
relative to the middle was significant [F(1, 16) = 4.84, p < 05.]. This indicates that, for triplets
with a central number numerically close to the arithmetical middle, rejection was more errorprone
when the central number was smaller than the middle compared to when it was larger
than the middle (7.9 % vs. 4.8 % errors, respectively). However, this pattern was reversed for
triplets with a central number far from the middle (smaller: 2.5 % vs. larger: 6.5 %). There
were no further significant main effects for the manipulated item characteristics [all F < 2.43,
all p > .14] or interactions of group with any of them [all F < 2.35, all p > .15].
Bisected triplets (RT and error data):
Decade crossing: In line with the hypothesis, a marginally significant interaction of
decade crossing and participant group was observed for errors [F(1, 16) = 4.06, p = .06], but
not for response latencies [F(1, 16) < 1]. When directly testing the hypothesis of neglect
patients’ performance being more impaired for triplets crossing into the next decade, the
absolute effects of decade crossing were contrasted for neglect patients vs. non-neglect
controls. The t-test revealed [t(16) = 2.02, p < .05, one-sided] that the increase of errors in
triplets crossing a decade boundary was more pronounced in neglect patients (+ 18.4 % errors,
see Figure 1B) than in non-neglect controls (+ 3.3 % errors). However, one could argue that
neglect patients committed more errors as they were not able to correctly perceive and
identify the decade digits of the two-digit numbers constituting the numerical interval. To
evaluate this potential constraint a regression analysis on item RTs was conducted
incorporating problem size (operationalised by the arithmetic mean of a triplet) as predictor 1 .
It was observed that problem size reliably predicted item RT [b = .21, t(159) = 2.67, p < .01]
1 Please note that there were too many perfect scores to run a meaningful analysis on mean item error rates.
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