Chapter 2. Prehension

156 THE PHASES OF PREHENSION
creased slope and longer duration prior to lift for heavier objects
(Johansson & Westling, 1988b), to be discussed in Chapter 6.
A similar experiment was conducted to examine the effects of ob-
ject texture, which can be partially assessed by vision (Weir, Mac-
Kenzie, Marteniuk, Cargoe & Fraser, 1991). Subjects reached,
grasped and lifted slippery (coated with Vaseline), neutral (polished
metal) or coarse (covered in sandpaper) dowels in blocked conditions.
Results indicated that a longer time was spent in contact with the
slippery dowel, prior to lift, compared with the other two dowels.
They suggested that even though the texture information was available
during vision, the kinematics prior to contact revealed no anticipation
appropriate for the coefficient of friction at contact. Replicating and
extending this study, Fikes, Klatzky and Lederman (1993) showed
that under randomized conditions, reaching for a slippery dowel,
individuals spent a greater time from movement onset to contact. They
discussed these results in terms of the greater geometric and dynamic
precision required for lifting a slippery object. Given the lower
coefficient of friction, subjects might adapt to the frictional
requirements in two ways: post contact (as demonstrated by Johan-
sson and Westling, 1984b) or precontact. Fikes et al. suggest that they
may prepare appropriate grip geometry precontact, and the associated
precision for placement anticipating slip might take more time. This is
an important suggestion, because such placement precision demands
should be quantifiable in kinematic landmarks prior to contact.
In this section, we have seen how intrinsic object properties like
size and weight, and extrinsic properties like distance, direction, object
motion and surface texture affect kinematic landmarks of the reaching
and grasping components. In examining task requirements, the evi-
dence seems to indicate that reaching with the arm to point is very dif-
ferent from reaching to grasp, and further that the requirements for the
hand are ‘driving the arm’. The kinematics of reaching and grasping
reflect the setting up of opposition space parameters, and the anticipa-
tory preparation for task demands once the object is grasped. The arm
and the hand seem to be functionally coupled in their control.
5.4.2 Visual and mechanical perturbations to grasping
Additional evidence for the view that the arm and hand are func-
tionally coupled to align the grasping surface patches on the hand with
the opposition vector is derived from perturbation studies. We con-
sider mechanical perturbation studies, where unexpected loads are
applied to oppose or interfere with the ongoing movement (as in being