Chapter 2. Prehension

60 THE PHASES OF PREHENSION
isting evidence from experimental and computational models, will
further our understanding of prehension.
3.2 Derivation of Criteria for Defining a Phase
At certain times throughout a prehensile task, unique events take
place. Each one may be viewed as a phase. The criteria for defining a
phase are crucial. Evidence for the existence of different phases can
be derived from:
1) Motor characteristics: Analysis of motor characteristics involves
various kinematic and kinetic features and invariances,
movement precision and outcome, and electromyographic
(EMG) recordings. For example, movements can be identified
as being ballistic, corrective, or tracking. Is the function of these
movements to get in the ‘right ballpark’ or precisely ‘home in’?
Where possible, relationships among kine tics, kinematic
features/invariances, EMG, and outcomes with respect to task
related goals are examined. Important issues to consider include
the coordinate frame in which the movement occurs and whether
feedback or feedforward control might be at work. In addition,
the level of the motor command must be considered (e.g.,
muscle level, movement level).
2) Sensory information: Sensory information is analyzed by type of
information available, by its modal characteristics, its frame of
reference, and by the time needed to influence motor charac-
teristics. If multiple modalities are available, how do they in-
teract? Possible influences of sensory information are examined
in terms of lag time to elaborate models of sensorimotor
integration. The influence of motor on sensory systems and vice
versa is acknowledged for goal-directed movement. We
emphasize the importance of vision prior to contacting the object.
Once contact is made with the object, the role of touch becomes
critical. Important to the initiation of a phase are the events that
possibly trigger it, and important to control issues is the nature
of sensory feedback.
3) Intent: Intent determines movement and apparent anticipatory
behaviors, which is why it is suggested here that the movements
of grasping are teleological. This is related to the individual’s
understanding of task and goals. In terms of control, in certain
phases, a subgoal may dominate a goal, e.g., maintain stability if
an object is slipping from grasp. Subgoal dominance probably