Chapter 2. Prehension

Chapter 4. Planning of Prehension
“The introduction of motivation (drive or will) and corresponding
planning is one of the major points of the modern theory of
voluntary movement.”
--Kornhuber (1984, p. 165)
plan: v.t. 1. to make a plan of (a structure, piece of ground,
etc.). 2. to devise a scheme for doing, making, or arranging. 3. to
have in mind as a project or purpose. From the Latin planus,
plane, level (Webster’s New Twentieth Century Unabridged
Dictionary, 2nd Edition.)
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A subject sits in front of a table, with a hammer directly in front of
her midline. Her hand is resting on the table, slightly pronated in the
position of rest. Her eyes are closed, until a signal is given. She is
told that she will grasp the hammer to place it on the shelf. On the
signal, she will open her eyes and perform the task. Yet, even though
the signal to begin the task has been given, there is a delay before
actual movement towards the hammer begins. Why is there a delay
and what is happening during this time?
This chapter focuses on the preparatory processes related to the
organization and planning of the upcoming movement. The CNS
makes plans for moving the hand to some location in space near the
hammer (in a specific direction, for a specified distance), for orienting
the hand relative to the hammer (defining contact locations), and for
grasping it in accordance with the task of placing it on the shelf (with
functionally effective forces of a given direction and magnitude). In
choosing an opposition space useful for the task, this planning process
involves three aspects:
1) perceiving task-specific object properties,
2) selecting a grasp strategy, and
3) planning a hand location and orientation.
Choosing an opposition space useful for the task depends on in-
formation perceived about the object, such as its location and its in-
trinsic properties. The particular properties perceived are task-related.
For example, color is not particularly useful for grasping, other than
helping to distinguish features of the object. But perceived size,
weight, and shape are useful. In addition, through years of experi-