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

INTRODUCTION
Patients suffering from neglect usually fail to attend to or respond to objects and/or
people in the hemispace contralateral to their lesion site (Kerkhoff, 2000; Robertson &
Halligan, 1999). When asked to bisect a physical line, neglect patients misallocate the true
midpoint of the line and place it towards the right (Marshall & Halligan, 1989). Furthermore,
neglect not only impairs the perception of visual objects but also affects representational
space in general (Bisiach & Luzzatti, 1978).
While such symptoms of neglect concerning external space have been studied
extensively, the impact of neglect on the representation of number magnitude as well as its
processing has only more recently been focussed on (Goebel, Calabria, Farnè, & Rossetti,
2006; Priftis, Zorzi, Meneghello, Marenzi, & Umilà, 2006; Rossetti et al., 2004; Vuilleumier,
Ortigue, & Brugger, 2004 and Zorzi, Priftis, & Umiltà, 2002; Zorzi, Priftis, Meneghello,
Marenzi, & Umiltà, 2006). Number magnitude is assumed to be represented along a
continuous (Priftis et al., 2006; Rossetti et al., 2004) and spatially organized Mental Number
Line (MNL; Dehaene & Cohen, 1995; Restle, 1970), upon which smaller numbers are
represented on the left and larger numbers are associated with the right (Dehaene, Bossini, &
Gireaux, 1993; Gevers, Verguts, Reynvoet, Caessens, & Fias, 2006; Zorzi et al., 2006; Zorzi
et al., 2002; Priftis et al., this issue, but see Vuilleumier et al., 2004).
In a seminal study, Zorzi et al. (2002) orally presented neglect patients with two
numbers (e.g. “one” and “nine”) that defined a numerical interval and asked them to bisect
this interval by naming the arithmetical middle (e.g. “five”) without doing any calculations.
The size of the numerical interval was varied much as the line length is in the commonly used
line bisection task. The authors observed that error rates increased as a function of the interval
size as did the relative magnitude of the erroneous responses. For instance, all neglect patients
produced errors such as falsely naming “6”, “7”, or “8” as being the midway between 1 and 9.
On the other hand errors such as “2”, “3”, or “4” did not occur. So, neglect patients
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