Chapter 2. Prehension

146 THE PHASES OF PREHENSION
the object this opening must correspond to the length of the opposition
vector; in addition, the fingerpads must be aligned along the orienta-
tion of the opposition vector. As we will see, the grip aperture may be
a higher order control variable.
Considering the black box introduced in Figure 1.2, we can con-
sider all these kinematic measures of prehensile behavior as outputs.
We saw grasp types and opposition space parameters and outputs in
Chapter 2. Jeannerod introduced the important distinction between
intrinsic and extrinsic object properties accessible through vision; he
also addressed the issue of the role of visual information. In the fol-
lowing sections, we consider what has been learned about object
properties and task requirements, and the roles of sensory informa-
tion, through research analyzing the kinematics of movements, pri-
marily using pad opposition (or precision grip), but also using palm
opposition (or power grasp).
5.4.1 Task requirements and object properties
Further exploring the kinematics of grasping and aiming tasks,
Marteniuk, MacKenzie, Jeannerod, Athenes, and Dugas (1987) varied
precision requirements, showing how intention, context, and object
properties affect timing parameters of prehensile movements. Table
5.1 summarizes the experimental conditions and results. In the first
experiment, they varied the goal, asking subjects to either point at or
grasp medium sized disks. In the second experiment, they varied
object fragility, by asking subjects to either grasp a light bulb or a
tennis ball. In the third experiment, the subjects had to grasp a disk
and either fit it carefully or throw it, thus varying the movement intent.
Fitts’ Law predicts that MT increases with the precision requirements
of aiming or placing, but only addresses the effect of amplitude and
target width on movement time. Marteniuk et al. (1987) bring in task
influences (e.g., context, intent) as well as other object properties
(e.g., fragility). In addition to movement time, Marteniuk et al.
analyzed the velocity profile and separated the acceleration phase
(before peak velocity) from the deceleration phase (after peak
velocity). They observed (see Table 5.1) that the percentage of time in
the deceleration phase was longer for grasping than pointing, for
grasping the light bulb than grasping the tennis ball, and for fitting
rather than throwing. In arguing that all these effects could be due to a
‘task precision’ requirement, they demonstrated that the increased MT
is due to the disproportionately lengthened deceleration phase.
Marteniuk et al. suggested that the duration of the deceleration phase