Chapter 2. Prehension

154 THE PHASES OF PREHENSION
the distal pulps into contact even though a great deal of variability was
seen in the kinematic features. On the left of Figure 5.17, the posi-
tions of the finger and thumb tips are identified. The figure shows the
covariable relation between thumb and finger paths. The goal of the
task, to produce a required force, was consistently achieved although
there were a wide variety of joint configurations and spatial paths in
the thumb and index fiiger. On the right of the figure, Cole and Abbs
showed hypothetical and observed configurations. The top figure
shows an observed configuration. In the middle figure, hypothetical
joint positions are shown, demonstrating that the distal pads will not
make contact when the thumb IP joint extends but only the finger MP
joint flexes. In the lower figure, Cole and Abbs showed actual ob-
served actions; the pads are brought into contact from reciprocal ad-
justments of the PIP and MP joint in response to the thumb joint flex-
ion. The precision effect found by Marteniuk et al. (1987) is relevant to
the difference between preshaping and enclosing. They argued that the
early part of the movement is more likely to be directly influenced by
central stereotyped movement planning, while the later part of the
movement, during the deceleration phase, is controlled by feedback.
Increasing the precision requirements of a task may induce subjects to
use more sensory information, particularly in the ‘homing in’ part of
the task, where the enclosing of the hand is occurring.
Jeannerod (1981, 1984) reported that object size affected only the
grasp component, not the transport component. This finding was
replicated by Wallace and Weeks (1988) in their examination of tem-
poral constraints when grasping .3 cm or 2.5 cm dowels. In contrast,
and for reasons which are still not clear, Marteniuk et al. (1990) re-
ported an invariant time to peak deceleration, but a lengthening of the
time after peak deceleration to object contact for the smallest object,
consistent with the effects of target size on pointing. Contrary to in-
dependent channels for intrinsic and extrinsic properties, Jakobson &
Goodale (1991) reported also that object size and object distance af-
fected kinematic landmarks for both the transport and grasping com-
ponents.
To now, we have considered only those intrinsic properties which
can be assessed by vision, and were identified in Chapter 4. In all of
the above studies of object size, object weight covaried with object
size (Jeannerod, 1984; von Hofsten 8z Ronnqvist, 1988; Marteniuk et
al, 1990), that is, the larger objects were always heavier. A set of ex-
periments was designed (Weir, MacKenzie, Marteniuk, Cargoe &
Frazer, 1991) to examine the effects of object weight uncontaminated