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1 Nijmegen Institute for Cognition and Information, Radboud University Nijmegen, Nijmegen, The Netherlands; FC Donders Centre for Cognitive Neuroimaging, Radboud University Nijmegen, Nijmegen, The Netherlands; Canadian Institutes of Health Research Group on Action and Perception, London, ON, Canada
2 Imaging Research Labs, Robarts Research Institute, London, ON, Canada; Canadian Institutes of Health Research Group on Action and Perception, London, ON, Canada
3 Centre for Vision Research, York University, Toronto, ON, Canada; Canadian Institutes of Health Research Group on Action and Perception, London, ON, Canada
4 Physiology and Pharmacology, University of Western Ontario, London, ON, Canada; Canadian Institutes of Health Research Group on Action and Perception, London, ON, Canada
* To whom correspondence should be addressed. E-mail: p.medendorp{at}nici.kun.nl.
Recently, using event-related fMRI, we located a bilateral region in the human posterior parietal cortex (retIPS) that topographically represents and updates targets for saccades and pointing movements in eye-centered coordinates. To generate movements, this spatial information must be integrated with the selected effector. We now tested whether the activation in retIPS is dependent upon the hand selected. Using 4-T fMRI, we compared the activation produced by movements, using either eyes or the left or right hand, to targets presented either leftward or rightward of central fixation. The majority of the regions activated during saccades were also activated during pointing movements, including occipital, posterior parietal and premotor cortex. The topographic retIPS region was activated more strongly for saccades than for pointing. The activation associated with pointing was significantly greater when pointing with the unseen hand to targets ipsilateral to the hand. For example, although there was activation in the left retIPS when pointing to targets on the right with the left hand, the activation was significantly greater when using the right hand. The mirror symmetric effect was observed in the right retIPS. Similar hand preferences were observed in a nearby anterior occipital region. This effector specificity is consistent with previous clinical and behavioral studies showing that each hand is more effective in directing movements to targets in ipsilateral visual space. We conclude that not only do these regions code target location, but they also appear to integrate target selection with effector selection.
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