Abstract
Behavioral, neuropsychological, and neuroimaging data support the idea that numbers are represented along a mental number line (MNL), an analogical, visuospatial representation of number magnitude. The MNL is left-to-right oriented in Western cultures, with small numbers on the left and larger numbers on the right. Left neglect patients are impaired in the mental bisection of numerical intervals, with a bias toward larger numbers that are relatively to the right on the MNL. In the present study we investigated the effects of optokinetic stimulation (OKS) – a technique inducing visuospatial attention shifts by means of activation of the optokinetic nystagmus – on number interval bisection. One patient with left neglect following right-hemisphere stroke (BG) and four control patients with right-hemisphere damage, but without neglect, performed the number interval bisection task in three conditions of OKS: static, leftward, and rightward. In the static condition, BG misbisected to the right of the true midpoint. BG misbisected to the left following leftward OKS, and again to the right of the midpoint following rightward OKS. Moreover, the variability of BG’s performance was smaller following both leftward and rightward OKS, suggesting that the attentional bias induced by OKS reduced the “indifference zone” that is thought to underlie the length effect reported in bisection tasks. We argue that shifts of visuospatial attention, induced by OKS, may affect number interval bisection, thereby revealing an interaction between the processing of the perceptual space and the processing of the number space.
Highlights
Number processing is a fundamental skill for everyday living
In the present study we investigated the effects of optokinetic stimulation (OKS) – a technique inducing visuospatial attention shifts by means of activation of the optokinetic nystagmus – on number interval bisection
The variability of BG’s performance was smaller following both leftward and rightward OKS, suggesting that the attentional bias induced by OKS reduced the “indifference zone” that is thought to underlie the length effect reported in bisection tasks
Summary
Number processing is a fundamental skill for everyday living. A brain without numbers could make little sense of its internal and external environment, given that various and important everyday activities involving processing of numbers should be compromised (see Dehaene, 1997; Butterworth, 1999, for comprehensive reviews). An essential question is how knowledge of number is represented in the brain. For example, can we decide quickly and effortlessly which of two numerals expresses the greater magnitude or which of two sets contains the smaller number of elements? Recent research on numerical processing in animals and humans converge on the view that knowledge of numbers constitutes a domainspecific cognitive ability, with a specific neural substrate located in the left and right inferior parietal cortices (see Dehaene et al, 2003)
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