Roi Cohen Kadosh, Kathrin Cohen Kadosh, Teresa Schuhmann, Amanda Kaas, Rainer Goebel, Avishai Henik, and Alexander T Sack (2007)
Virtual dyscalculia induced by parietal-lobe TMS impairs automatic magnitude processing.
Curr Biol, 17(8):689-93.
People suffering from developmental dyscalculia encounter difficulties in automatically accessing numerical magnitudes [1-3]. For example, when instructedto attend to the physical size of a number while ignoring its numerical value, dyscalculic subjects, unlike healthy participants, fail to process the irrelevant dimension automatically and subsequently show a smaller size-congruity effect (difference in reaction time between incongruent [e.g., a physically large 2 anda physically small 4] and congruent [e.g., a physically small 2 and a physicallylarge 4] conditions), and no facilitation (neutral [e.g., a physically small 2 and a physically large 2] versus congruent) . Previous imaging studies determined the intraparietal sulcus (IPS) as a central area for numerical processing [4-11]. A few studies tried to identify the brain dysfunction underlying developmental dyscalculia but yielded mixed results regarding the involvement of the left  or the right  IPS. Here we applied fMRI-guided TMS neuronavigation to disrupt left- or right-IPS activation clusters in order to induce dyscalculic-like behavioral deficits in healthy volunteers. Automatic magnitude processing was impaired only during disruption of right-IPS activity. When using the identical paradigm with dyscalculic participants, we reproduced aresult pattern similar to that obtained with nondyscalculic volunteers during right-IPS disruption. These findings provide direct evidence for the functional role of right IPS in automatic magnitude processing.