Chapter 2. Prehension

Chapter 4 - Planning of Prehension 105
of these movements, and for perturbations of object orientation is pro-
vided in the next chapter.
4.7 Summary of Planning an Opposition Space
In order to perform the simple task of lifting an object such as a
hammer, preparatory processes related to the organization and plan-
ning of the upcoming movement occur in the CNS, coordinating
highly complex activity. A task plan, constructed as a coordinated
control program or as something close to such a program, is needed to
tie together the serialization of multiple sub-tasks, such as transporting
the hand to the correct location and orientation, and shaping the hand
into a suitable posture.
In this chapter, task plans from a variety of fields have been exam-
ined. A robotic task plan makes distinct the phases of a task. In addi-
tion, it puts forth a language, complete with a hand coordinate frame
for interfacing to the robot arm. Finally, it suggests a task plan can be
constructed as a skeleton plan that is filled in with more details for a
feedback controller. The Arbib coordinated control program is a task
plan that models the brain as a distributed controller. A hierarchical
task plan such as TOTE, points out that a task plan must have contin-
gencies for error. Finally, a neural task plan suggests how plans such
as these could be mapped out across regions of cortical and sub-corti-
cal areas in the CNS. This fits in with brain studies which show pre-
movement activity in parietal, frontal and subcortical regions.
The more complex the movement, the longer the reaction time.
Here, complexity includes both the number of things that must be
done, as well as the time it will take to perform the task. However, in
terms of planning, it is hard to say how far movements are planned in
advance; some complex movements seem to be only partially planned
prior to movement. This means that further movements can be
planned during initial movement.
In order to accomplish a task plan, three issues have been ad-
dressed: perceiving task-specific object properties (including the =
position vector based on knowledge about task requirements and task
mechanics), selecting a grasp strategy, planning a hand location and
orientation. Experimental evidence suggests that intrinsic object
properties, such as size and shape, and extrinsic object properties,
such as object location and orientation, are perceived prior to move-
ment. The object properties perceived are specific to the task
demands. Importantly, knowledge about the function and behavior of
objects is stored over time, and our ability to anticipate simplifies the