Chapter 2. Prehension

78 THE PHASES OF PREHENSION
Figure 4.6 Interpolated spaces for pinching vs clenching a two
dimensional object. Pinch is shown on the left and clench on the
right. The percentage of subjects naming a given hand shape in
response to an object cue is shown on the z axis as a function of
the object’s depth and picture-plane area (from Klatzky, et al.,
1987; adapted by permission).
least to effect movement changes based on changes in the object
property. By observing the time it takes subjects to make adjustments
in the hand posture, one is provided with some clues into CNS
prehensile planning. Using a box with a mirror in it and lights
strategically placed, Jeannerod (198 1) asked subjects to reach out and
grasp an object. Initially, the subjects see a spherical shaped object;
but as soon as they start moving their hand, an image of an oval-
shaped object is superimposed on the sphere. In reviewing the video
of the hand as it preshaped for the sphere, Jeannerod noted that it took
about 500 ms after the perturbation for the hand posture to change and
reflect the more oval-shaped object. No EMG studies were done, but
separating out the isometric tension time for the extensor digitorum
communis of about 100 ms, this would leave about 400 ms for the
subject to visually observe the change, replan a new posture, and
generate the new motor commands. Other studies, detailed in Chapter
5 indicate a minimum time of at least 300 ms to effect changes in
response to perturbations of intrinsic object properties (e.g.,
Paulignan, Jeannerod, MacKenzie, & Marteniuk, 199 1).