Chapter 2. Prehension

318 CONSTRAINTS AND PHASES
spent in the deceleration phase was predicted by ID as well or better
than MT. This was not the case for acceleration time. Only movement
amplitude affected the time to peak velocity. Thus, amplitude and
target size effects were dissociable in that the shape of the tangential
velocity profile was a function of target size (accuracy), and the peak
speed along the path of the trajectories was scaled according to
movement amplitude.
In grasping and aiming tasks (Marteniuk et al. 1987), precision
requirements were varied, showing how intention, context, and object
properties affect timing parameters of prehensile movements. The first
experiment varied goal (pointing to a target or grasping a disk); the
second one varied object properties (grasping compliant tennis ball or
fragile light bulb); the third one varied movement intent (placing or
throwing an object. Using Fitts’ Law to equalize the ID’S, it was
observed that the grasping took longer than pointing. 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 placing than throwing. They argued that all these effects could be
due to the requirement for ‘precision’. Fitts’ Law predicts that MT
increases with aiming precision requirements, but they demonstrate
this increase is due to the lengthening of the deceleration phase
disproportionately to the rest of the movement. As well, they show
other influences, such as object properties and task intent. Less
variability between conditions was seen in the early or acceleration
phase of the movement, and more variability during the deceleration
phase. They argued that the early part of the movement is more likely
to be directly influenced by central stereotyped movement planning or
programming, while the later part of the movement, during the
deceleration phase, uses more sensorimotor adjustments for
controlling the movement, causing more variability. Increasing
precision requirements of a task may induce subjects to use more
sensory information, particularly in the ‘homing in’ part of the task.
This relates to Jeannerod (1984), where it was argued that an initial
ballistic phase places the hand into the vicinity of the target, and a
second phase using feedback guides the hand to the target. It is
interesting to note that in pointing tasks, where subjects pointed to
imaginary targets in space, velocity profiles are symmetrical (Atkeson
& Hollerbach, 1985).
Another dimension to task requirements, in contrast to precision,
is the anticipated forces acting in the task. In Iberall et al. (1986), an
informal study is reported to observe the effect of anticipated forces on
the posture chosen to grasp a vertically standing dowel. Subjects were