Background: Accumulating evidence suggests anatomical and functional differences in connectivity between the anterior and posterior parts of the inferior-parietal lobule (IPL) and the frontal motor areas.

Objective/ Hypothesis: This study investigates whether different intra-hemispheric parietal-motor interactions can be observed along the anterior-posterior axis of the IPL in the resting human brain.

Methods: We use a twin coil transcranial magnetic stimulation technique to test intra-hemispheric interactions between three points adjacent to the intra-parietal sulcus (anterior, central, posterior) and the ipsilateral primary motor cortex (M1) at rest in both hemispheres.

Results: We found that stimulation of the anterior IPL resulted in an inhibition of the ipsilateral M1 in both hemispheres. Stimulation of the central and posterior IPL resulted in a facilitatory effect on ipsilateral M1 in the left but not for the right hemisphere. Additionally we show that there is considerable inter-subject variability concerning the optimal parietal facilitatory and inhibitory position.

Conclusions: The IPL has distinct inhibitory and facilitatory connections to the ipsilateral M1. Whereas inhibitory connections are observed in both hemispheres, facilitatory connections are asymmetric. These parietal-motor networks may represent the basis for the functional differences between these regions in reaching and grasping tasks and mirror the functional asymmetry observed in the motor system. From a practical point of view, we note that the inter-subject variability means that future TMS studies of the parietal area might consider a hot-spot localization similar to the procedures commonly used for M1.