Chapter 2. Prehension

Chapter 5 - Movement Before Contact 177
provides a hypothesis that lesions in parietal Area 7 could disconnect
visual and somatosensory information critical for patterning of visually
goal-directed movements. It is assumed that this disconnection of vi-
sual and somatosensory information, and the associated distortions in
sensorimotor transformations prevent alignment of the visual and pro-
prioceptive maps of opposition space.
Somatosensory processing through the dorsal roots of the spinal
cord is essential for grip formation, as has been demonstrated with
deafferentations of the arm. Experimental monkeys with dorsal root
sections show a corresponding loss of grip formation (Liu &
Chambers, 1971; cited by Jeannerod, 1986). Dorsal column input has
been implicated as essential for proprioceptive guidance of movements
to achieve ‘tactile foveation’ in pad opposition. Glendinning and col-
leagues examined stumptail macaques (Macaca artoides) before and
after lesions of the fasciculus cuneatus (Glendinning, Cooper, Vierck,
& Leonard, 1992). For the first two weeks following surgery, mon-
keys neglected the affected hand, and did not use it for climbing, lo-
comotion, support, foraging, scratching or grooming. Gross arm and
finger movement returned gradually over the next few months, but the
monkeys retained a ‘wrist-drop’ posture, with abduction and hypoto-
nia of the fingers. They were unable to perform pad opposition in the
same way postoperatively. prior to surgery, the monkey approached
small food objects with a highly consistent grip formation. The fin-
gertips formed a small grip aperture during approach, and pad opposi-
tion between the index and thumb was obtained through a multiarticu-
lar pattern whereby the proximal joint flexed and the distal interpha-
langeal (DIP) joint of the index finger extended to align with the op-
position vector, directly through the object into the thumb pad.
Videotape analysis of kinematics, and grasp outcome postoperatively
revealed alterations in grip formation: either excessive or not enough
finger opening, with flexion about all joints of the fingers, a marked
contrast to the DIP extension at contact preoperatively. All the mon-
keys used the table surface to help grasp items with fingers in opposi-
tion to the palm because they were unable to oppose the thumb and
forefinger successfully. Vierck (1975, 1982) had earlier revealed
permanent deficits in orientation of the hand during approach and
grasp of an object using pad opposition, but had not captured the
kinematic changes in grip formation. These results implicate kines-
thetic sensory information in the planning and execution of grip for-
mation, prior to tactile contact with the object.
For the interested reader, excellent reviews of the neural
mechanisms in reaching and grasping are provided in the following