The Mental Number Line and the Human Angular Gyrus Silke Go ¨bel,* Vincent Walsh,* and Matthew F. S. Rushworth* , † *Department of Experimental Psychology, University of Oxford, South Parks Road, Oxford OX1 3UD, England; and †Oxford Centre for Functional Magnetic Resonance Imaging of the Brain, Department of Clinical Neurology, University of Oxford, John Radcliffe Hospital, Headley Way, Oxford OX3 9DU, England Received January 23, 2001; published online October 24, 2001 To investigate the hemispheric organization of a language-independent spatial representation of num- ber magnitude in the human brain we applied focal repetitive transcranial magnetic stimulation (rTMS) to the right or left angular gyrus while subjects per- formed a number comparison task with numbers be- tween 31 and 99. Repetitive TMS over the angular gyrus disrupted performance of a visuospatial search task, and rTMS at the same site disrupted organiza- tion of the putative “number line.” In some cases the pattern of disruption caused by angular gyrus rTMS suggested that this area normally mediates a spatial representation of number. The effect of angular gyrus rTMS on the number line task was specific. rTMS had no disruptive effect when delivered over another pa- rietal region, the supramarginal gyrus, in either the left or the right hemisphere. © 2001 Academic Press INTRODUCTION Our lives are influenced by numbers at every turn. Numbers mark events such as the beginning of every day on our alarm clocks, the amount of sugar in our morning coffee, the bus we take to work, the room we work in, and the money we get for working. Numbers also mark the beginning, duration, and end of our lives. “Numerals are always pictured by me in a straight line from left to right” (p. 254) said one of Galton’s subjects (Galton, 1880). As early as 1880 Galton found evidence for a spatial representation of numbers akin to a mental number line. He asked subjects to describe how they think of numbers and many described overt visual–spatial representations of numbers sometimes even with specific colors or characters associated with certain numbers. In most of these descriptions num- bers were represented on a line—a mental number line. A language-independent spatial representation for number magnitude (Seron et al., 1992; Dehaene and Cohen, 1995) is only one type of number representa- tion. There is evidence for a second representational format of numbers in human brains: for exact arith- metical knowledge we mainly use language-based rep- resentations (Dehaene et al., 1999). Over a hundred years after Galton’s observation, the idea of a language-independent spatial representation of numbers in the human brain is still useful (Butter- worth, 1999; Dehaene et al., 1993; Seron et al., 1992). This spatial representation may rely on visuospatial cerebral networks (Dehaene, 1999). As the parietal cortex plays a crucial role in these networks (Andersen et al., 1997; Colby and Goldberg, 1999; Nobre et al., 1987; Gitelman et al., 1999; Corbetta and Shulman, 1999; Corbetta et al., 2000) it is reasonable to propose the parietal cortex as a candidate area that may con- tribute to the representation of the number line. Within the human parietal cortex it is clear that the posterior-inferior regions, the angular gyrus and the adjacent intraparietal and superior temporal sulci, which bound the angular gyrus, are most closely in- volved in visuospatial attention (Nobre et al., 1997; Gitelman et al., 1999; Corbetta and Shulman, 1999; Corbetta et al., 2000; Rushworth et al., 2001b,c). The angular gyrus might therefore be expected to play a role in mediating a spatial representation of numbers. Recent neuropsychological and neuroimaging stud- ies strongly suggest that the left parietal cortex is important for the representation or estimation of num- ber (Stanescu-Cosson et al., 2000; Rickard et al., 2000; Pesenti et al., 2000; Zago et al., 2001; Gruber et al., 2001). Lesions in the posterior regions of the left hemi- sphere can cause acalculia; in many cases it is clear that the critical focus of the lesion is within the parietal cortex and the angular gyrus is often damaged (Hen- schen, 1919, 1920; Hecaen et al., 1961; Grafman et al., 1982; Jackson and Warrington, 1986; Warrington et al., 1986; Cipolotti et al., 1991; Mayer et al., 1999). Lesions in this region can also lead to a loss of number meaning (Delazer and Benke, 1997) or number magni- tude (Delazer and Butterworth, 1997). Although the percentage of subjects who are able to describe their number lines as colorfully and elegantly as Galton’s subjects might be low (Seron et al., 1992), NeuroImage 14, 1278 –1289 (2001) doi:10.1006/nimg.2001.0927, available online at http://www.idealibrary.com on 1278 1053-8119/01 $35.00 Copyright © 2001 by Academic Press All rights of reproduction in any form reserved.