Arm selectivity during reach planning in macaque posterior parietal cortex


Steve W. Chang, Anthony R. Dickinson & Lawrence H. Snyder

Washington University School of Medicine
St. Louis, MO


The specificity of arm-selection for reach planning in the posterior parietal cortex (PPC) remains unknown.  In the present study, we examined whether PPC participates in the formation of a reach plan that is independent of which arm will be used. 


We tested 109 cells in the parietal reach region (PRR) while monkeys prepared a memory-guided reach with either the contralateral or ipsilateral arm.  We considered only those cells with brisk visual responses and memory activity during the task.  Fifty-three percent of these cells were active independent of which arm was to be used for reaching, while another 35% were much more active when the contralateral arm was to be used.  An index formulated to compare contramanual and ipsimanual preference was bimodally distributed, supporting the existence of distinct bimanual and contramanual populations.  Moreover, while bimanual and weak contramanual cells were found throughout the posterior end of the medial bank of the IPS (close to the junction of MIP and V6a), strong contramanual cells were clustered together at one particular location. 


We then asked if the preferred direction of a PRR cell would change depending on which particular arm was to be used (16 direction center-out task).  The majority of cells showed identical tuning, suggesting that the arm selectivity in unimanual cells was not due to tuning differences.  In addition, we tested whether there was a correlation between arm selectivity and receptive field location.  We found no correlation, which suggests that receptive field location does not determine whether a cell is more active for reaches planned with one arm or the other.


Our results indicate that both a generic reach plan (bimanual) and a specific reach plan (unimanual) are represented in PPC, and suggest that arm-selection for reach preparation occurs early in visuomotor processing.


Support Contributed By: NEI; McDonnell & EJLB Foundations