Left parietal TMS disturbs priming between symbolic and non-symbolic number representations.

An amodal number representation activated by all types of numerical input, irrespective of the input notation, has often been proposed to be located in the left or right intraparietal sulcus (IPS). Two cross-notational priming experiments were carried out to test the existence of a notation-independent magnitude representation in the left or right parietal lobes. In Experiment 1, stimuli were Arabic digits and number words. Results revealed no significant effect of repetitive transcranial magnetic stimulation (rTMS) over left or right IPS during prime presentation. In contrast, in Experiment 2, digits and dot patterns were intermixed and here the priming distance effect (PDE) was reduced in the right TMS condition and absent for stimulation over left IPS. These findings suggest: (1) that TMS over left but not right IPS disrupts processes that are crucial for priming when symbolic and non-symbolic stimuli are intermixed, and (2) that disruption of the left IPS on its own is not sufficient to disrupt cross-notational priming when purely symbolic number notations are used. Our results point towards a crucial role of the left hemisphere for the mapping between small symbolic and non-symbolic numerosities.