Contribution of the right intraparietal sulcus to numerosity and length processing : an fMRI-guided TMS study

The critical involvement of the left and right intraparietal sulci (IPS) in numerosity and length processing was tested with neuro-navigated repetitive transcranial magnetic stimulation (rTMS). Participants had to categorize linear arrays of dots as containing "few" or "many" dots, and filled rectangles as "short" or "long". The stimulation sites were determined for each individual participant by identifying the IPS areas showing peak activity during a number comparison task in earlier functional magnetic resonance imaging (fMRI) studies. Compared to the stimulation of the vertex chosen as a control site, rTMS over the right IPS increased the error rate in both tasks. This result indicates that the integrity of the right IPS is a necessary condition not only for discriminating numerosities but also for performing accurate judgements of lengths. We propose that these two processes rely on a common representation and/or mechanism in the right IPS.