TMS over right posterior parietal cortex induces neglect in a scene-based frame of reference

Although damage to right posterior parietal cortex (RPPC) produces bias in line bisection, Karnath et al. [Karnath, H.-O., Berger, M. F., Küker, W., & Rorden, C. (2004). The anatomy of spatial neglect based on voxelwise statistical analysis: A study of 140 patients. Cerebral Cortex, 14, 1164-1172] claim that it plays little role in spatial neglect, which is better measured by target cancellation. We used a detection task (approximating cancellation in requiring detection) to investigate this claim by compromising the parietal cortex with transcranial magnetic stimulation (TMS). Two outline shapes, one on each side of fixation, were briefly displayed before a mask. The target was a discontinuity in the left or right of the outline of one of these perceptual objects. Subjects indicated position or absence of target as fast as possible. Stimulus-mask onset asynchrony was adjusted individually to yield 75% detection. TMS was delivered over left posterior parietal cortex (LPPC), RPPC and Vertex, with Sham TMS over RPPC as a baseline control. Target detection was near ceiling and fastest at central positions and worst and slowest at the far right. Detection was significantly reduced at the far left position by TMS over RPPC. No other effects were obtained and latency was not affected by TMS. Disruption of RPPC by TMS does produce left neglect as measured by detection. Given the pattern of performance and since it was disrupted on one side of the display rather than on one side of each shape, attention and neglect were in a scene-based rather than object-based reference frame.