Chapter 2. Prehension

56 THE PHASES OF PREHENSION
Executive controller
using feedforward
control
Other ‘low-level’
control system
using feedback to
make small corrections
Table, recipes, rules of thumb:
generate standard set
Figure 3.5 Greene (1972) notion of ‘ballpark’. The executive
selects an appropriate combination that is ‘in the ballpark’ of the
correct movement; the lower centers refine this approximation
closer to the desired movement.
and Paillard is a hierarchical system, where central, higher centers set
up some motor commands that are then transformed by lower centers
(Brooks, 1979; Keele, 1981). Greene (1972, 1982) suggested that
the nervous system has a few degrees of freedom, but it governs sub-
systems having many degrees of freedom, much like a puppet master
with a marionette. To do this, using a hierarchical organization, an
executive brings together combinations of movements that are con-
strained by lower-levels. Shown in Figure 3.5, the executive selects
an appropriate combination that is ‘in the ballpark’l of the correct
movement; lower centers refine this approximation closer to the de-
sired movement. In effect, by locking some joints, the executive is
‘assembling a virtual arm’. Greene made an interesting analogy to an
The ballpark analogy is derived from the American game of baseball, referring to
the notion of being within the area of the baseball field. Similarly, one might
think of a soccer field as defining the soccer workspace.