Chapter 2. Prehension

Chapter 5 - Movement Before Contact 199
movements occur is a complex problem. The constraints acting on
this behavior, ranging from the availability and use of sensory in-
formation, the way movement can be generated by our muscles, the
activation and subsequent use of force generating muscles for
compliant motion, all combine to produce complex interactions
between the behavior of the transport component and the grasping
component of prehension. 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). For example, Jeannerod (1984) proposed a temporal
coupling, while Wing et al. (1986) argued for a spatial coupling.
Movements that involve an interaction with the environment seem
to be different than movements that don’t. Fitts’ law, one of the few
lawful relationships in motor control, demonstrates a speed-accuracy
tradeoff. Extending Fitts’ Law, MacKenzie et al. (1987) showed that
asymmetric velocity profiles point to the possibility of a two-phase
controller for complex movements where interaction with the envi-
ronment is required. Questions have been raised regarding the rela-
tionship between movement duration, distance, velocity, and accelera-
tion. Some invariances have been identified. In terms of how people
interact with objects, the environmentally-defined goal seems to affect
the motor control as well, suggesting that a precision effect might be in
play when performers interact with the environment. This precision
effect, as described by Marteniuk et al. (1987), shows up in the see
ond part of the movement where sensory information is needed for
changing from an unrestrained movement to a guarded and compliant
movement. An important point is the question dealing with the type of
sensory information needed for visually-guiding the arm. As Wing et
al. (1986) suggested, very little sensory information may be needed, if
the correct information is available (e.g., if vision of the thumb or fin-
ger is not occluded at a crucial time in its trajectory). Finally, an im-
portant question is the differential influence of intrinsic and extrinsic
object properties on the arm (the transport component) and the hand
(the grasping or effecting component). Ultimately, answers to this
question will relate back to Fitts’ Law and the use and need of sensory
information in a motor task.
In terms of visual information needed in a task, movement compo-
nents are affected when no visual information is available. Lacking
visual feedback, accuracy is decreased and movements are more con-
servative (e.g., grip aperture opens wider), as shown by Wing et al.
(1986), among others. While the notion of two-phase movements