Chapter 2. Prehension

Chapter 5 - Movement Before Contact 113
Figure 5.2 Topographical organization of motor cortex. Note
large representation of hand (from Kandel & Schwartz, 1985;
reprinted by permission).
the reaching schemas, and the target object size to the grasping
schemas. A fundamental question is how the CNS represents
information and converts it into a motor command. For example, if the
task is to place your wrist at a location close to an object in order to
grasp and lift the object, then six coordinates in an extrinsically-
defined coordinate frame will uniquely describe the position (x,y,z)
and orientation (a,b,g) of that three-dimensional location. However,
the arm itself has many degrees of freedom, and there are also many
potential levels of motor control. At the joint level (as seen in
Appendix A), the arm has about 11 degrees of freedom (3 at the
pectoral girdle, 3 at the shoulder, 2 at the elbow, 1 at the radioulnar
joint, 2 at the wrist). Since the task has six degrees of freedom and
the arm has 11 degrees of freedom, five extra degrees of freedom are