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