Chapter 2. Prehension

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7.2
THE PHASES OF PREHENSION
Selection of a wrist position is done for comfort in the task, but
if two tasks are involved, sometimes the comfort is for the
second task (Rosenbaum et al., 1990).
Retinal information can be used for perceiving object properties
for planning a grasping strategy (Klatzky, McCloskey,
Doherty, Pellegrino & Smith 1987; Sivak & MacKenzie, 1992).
Planning is done in terms of ‘ballpark’ planning (Arbib et al.,
1985; Greene, 1972; Iberall, 1987a; Iberall & MacKenzie,
1988).
Object properties are perceived in hand-sized dimensions
(Newel1 et al., 1989).
Weight can be estimated from vision.
Multiple solutions exist for grasping objects for a given task.
The opposition vector is seen in the object and creates a task
coordinate frame.
The approach vector is in a palm-centered coordinate frame.
Force generation is computed in a palm-centered coodinate
frame, given the opposition vector.
During planning, vision is a dominant source of sensory
information.
Setting Up an Opposition Space
Given the perceived opposition vector, once hand locations and
postures are chosen, commands must be generated to set up the
opposition space and transport it towards the object. Figure 7.4
(introduced earlier as Figure 5.32) shows a palm-focused model of
Setting Up an Opposition Space, and includes a Reshape Schema,
and Orient Palm Schema, and a Drive Fingers Guarded Schema. In
this view, computations for finger control are in a palm-based
coordinate frame. Noting the interwoven action of wrist, forearm,
elbow, and shoulder muscles, we suggest that the palm is the likely
interface between the transport and shaping of the hand.
The relationship between the transport and grasping components
appears to be functionally specific to the high level goal of the task.
Research has suggested a coupling between these two components,
indicating that the way the arm behaves alone (as in pointing
movements) is different from the way the arm works when the hand is
involved (as in reaching and grasping movements). The way in which
the transport and shaping of the hand are coordinated is still an open
question. Some researchers show temporal invariance, and others
disagree, arguing that although the two components are not time-