Two orienting mechanisms in posterior parietal lobule: an rTMS study of the Simon and SNARC effects

The aim of the present study was to investigate, with repetitive transcranial magnetic stimulation (rTMS), the involvement of posterior parietal lobule (PPL) circuits in producing the Simon effect and the spatial-numerical association of response codes (SNARC) effect. The Simon effect is indexed by faster responses to left-side stimuli with left- than with right-side key-presses and faster responses to right-side stimuli with right- than with left-side key-presses. The SNARC effect is indexed by faster responses to smaller numbers with left- than with right-side key-presses and faster responses to larger numbers with right- than with left-side key-presses. Participants were required to perform a parity judgement on numbers ranging from 1 to 9 (without 5), by pressing a left or a right response key. The numbers were presented to either the left or the right side of fixation. Two bilateral PPL areas lying along the intraparietal sulcus (IPS) were stimulated with rTMS. Results suggested a causal role of the anterior portion of PPL of both hemispheres in the Simon effect and of the posterior portion of PPL of both hemispheres in the Simon effect and the SNARC effect.