Transcranial magnetic stimulation (TMS) of the supramarginal gyrus: a window to perception of upright

Although the pull of gravity, primarily detected by the labyrinth, is the fundamental input for our sense of upright, vision and proprioception must also be integrated with vestibular information into a coherent perception of spatial orientation. Here, we used transcranial magnetic stimulation (TMS) to probe the role of the cortex at the temporal parietal junction (TPJ) of the right cerebral hemisphere in the perception of upright. We measured the perceived vertical orientation of a visual line; that is, the subjective visual vertical (SVV), after a short period of continuous theta burst stimulation (cTBS) with the head upright. cTBS over the posterior aspect of the supramarginal gyrus (SMGp) in 8 right-handed subjects consistently tilted the perception of upright when tested with the head tilted 20° to either shoulder (right: 3.6°, left: 2.7°). The tilt of SVV was always in the direction opposite to the head tilt. On the other hand, there was no significant tilt after sham stimulation or after cTBS of nearby areas. These findings suggest that a small area of cerebral cortex--SMGp--has a role in processing information from different sensory modalities into an accurate perception of upright.