The Mental Number Line and the Human Angular Gyrus

To investigate the hemispheric organization of a language-independent spatial representation of number magnitude in the human brain we applied focal repetitive transcranial magnetic stimulation (rTMS) to the right or left angular gyrus while subjects performed a number comparison task with numbers between 31 and 99. Repetitive TMS over the angular gyrus disrupted performance of a visuospatial search task, and rTMS at the same site disrupted organization 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 parietal region, the supramarginal gyrus, in either the left or the right hemisphere.