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Abstract:
Humans are capable of elegantly controlled visually-guided
grasping movements, and the posterior parietal cortex is believed
to play an important role in this fundamental skill. Damage to the
right parietal cortex often results in hemispatial neglect, a
failure to attend to or report information appearing on the left
side of space. The current study examined the effects of
hemispatial neglect on shape perception and grasping. Twelve
different wooden shapes (two of each) were used to compare the
ability of patients with hemispatial neglect to discriminate
between shapes and use shape information to control grasping. The
shapes have smoothly bounded contours and lack clear symmetry; the
determination of stable grasp points therefore requires an analysis
of the entire contour envelope of the shape. As deficit severity
increased, patients had increased difficulty distinguishing one
object from another on the basis of their shape. In addition, the
variance of their grasping points increased from the center of mass
of the shape and shifted further to the relative right side of the
presented object, indicating that the patients were unable to form
a complete representation of the object. These results fit well
with the finding that manipulation cells found in the posterior
parietal cortex of monkeys appear to be tied to the properties of
the goal object as well as to the distal movements that are
required for grasping that object.
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