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Ryusuke Hayashi, Richard A. Andersen, Shinsuke Shimojo; Human parietal cortex remaps cue-priming effect across saccades: cortical location and dynamics assessed by transcranial magnetic stimulation.. Journal of Vision 2004;4(8):102. doi: 10.1167/4.8.102.
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Some coordinate transformation from the retinotopic (RET) frame to a non-retinotopic frame is necessary to keep our visuospatial perception constant across eye movements (we will group all non-RET frames as environmental (ENV)). Here, we applied dual-pulse transcranial magnetic stimulation (TMS) to the posterior parietal cortex (PPC) in healthy human observers to disrupt the coordinate transformation process during cue-primed tasks involving saccadic eye movements. We assessed the cue-priming effect by measuring reaction time (RT) to a target after external cue presentation. Observers were required to move their eyes in the interval between cue and target and to judge whether the same-colored target was left or right by pressing buttons. By comparing RTs between the two conditions, (1) the target presented at the ENV same/RET different location relative to the cue location, and (2) the target presented at the ENV different/RET same location, one can determine whether priming mainly affects either the RET or the ENV coordinate systems. The results of the behavior without TMS show that priming is yoked to the ENV frame rather than the RET frame. We then TMS the PPC during the same tasks, testing 9 parietal scalp sites, 2 saccade directions (left and right) and 3 TMS timings (0 ms, 100 ms and 200 ms after the saccade onset) to investigate the spatio-temporal aspects of the process. We found that TMS delivered to the right PPC at 100ms after the onset of a leftward saccade effectively causes the priming to shift reference frame from the ENV frame to the RET frame, indicating that the locus of priming is represented in the RET coordinate and updated with eye movements to fit with the ENV coordinate. The activity of a particular PPC area at a specific time is therefore vital for integrating the eye signal with the RET representation of primed location. The most effective TMS location to disrupt the remapping process may correspond to the human homolog of the monkey LIP.
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