Previous studies have implicated the human parietal lobes in the on-line guidance of action. However, no study to date has examined at what stage in the on-line adjustment process do the parietal lobes play their most critical role. Repetitive transcranial magnetic stimulation (rTMS) was applied over the left intraparietal sulcus as participants reached to grasp a small or large illuminated cylinder. On some trials, the illumination could suddenly switch from the small to large cylinder, or vice-versa. Small–Large switches were associated with relatively early grip aperture adjustments, whereas Large–Small switches were associated with relatively late grip aperture adjustments.
When rTMS was applied early in the movement, it disrupted on-line adjustments to Small–Large target switches, but not to Large–Small switches. Conversely, when rTMS was applied late in the movement, it disrupted adjustments to Large–Small target switches but not to Small–Large switches. The timing of the disruption by rTMS appeared linked to the initiation of the adjustment. It was concluded that the left parietal lobe plays a critical role in initiating an on-line adjustment to a change in target size, but not in executing that adjustment. The implications of these results for current views of on-line control are discussed.