Chapter 2. Prehension

Chapter 8. Constraints on Human Prehension
"Our hands become extensions of the intellect, for by hand
movements the dumb converse, with the specialized fingertips
the blind read; and through the written word we learn from the
past and transmit to the future.''
--Sterling Bunnell(l944)
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The dextrous and versatile performance of human prehension can
be viewed as emerging from a large multidimensional constraint space.
Roboticists and experimentalists seem to be at odds currently in terms
of the important variables examined in the quantification of motor
control. It has been argued (Nelson 1983; MacKenzie & Marteniuk
1985; Marteniuk et al. 1987) that interactions are occurring among
multiple performance constraints. Nelson (1983) argued that dynamic
models, such as that of Hollerbach (1980), are inadequate for
explaining human movement unless they also include both the
performance constraints and the objectives affecting the neural and
neuromuscular inputs. MacKenzie and Marteniuk (1985) described
two types of constraints: those variables that constrain the use of
feedback and those due to structural limitations that affect the
preparation and execution of goal directed movement. Under the first
category are included issues such as how fast feedback available, the
types of feedback that can be used, and the experience of the
performer in learning sensorimotor manipulations. In the second
category are limitations from anatomical and neurological structure,
from the structure of communicated information, the structure of
environmental information, and constraints due to context and
intentions. Marteniuk et al. (1987) emphasized the interaction between
knowledge (which includes a person's experience and movement
objectives) and the biological structure. Rewording the list of
MacKenzie and Marteniuk (1985), Marteniuk et al. (1987) made even
more explicit the notions of task constraints and conditions of speed
and accuracy. Taking a task-specific view of movement planning and
control, a task is defined as the interaction of the performer with the
environment under given movement goals. This view predicts that
movement is optimized and specific to the unique requirements that
arise from an individual interacting with the environment.
In order to study the complex interaction between movement
goals, object properties, environmental characteristics, and the