Chapter 2. Prehension

Chapter 5 - Movement Before Contact 155
by covariations in size, by using visibly similar cylindrical dowels of
constant size (10.3 cm high and 2.5 cm in diameter), but varying in
weight (from 20 to 410 grams). From a fixed starting position, and
relaxed hand posture, with the thumb and index finger in contact over
the table top, the task was to grasp and lift a dowel placed 30 cm in
front of the body midline. Subjects grasped the dowels under blocked
(weight known, since a group of trials had all the same dowel weight)
and random conditions (weight unknown, with random variation of
the different weighted dowels over trials). Markers were placed on the
wrist, index finger and thumb. Using the WATSMART system
(sampling at 200 Hz, reconstruction error of about 2.1 mm, filtering at
4 Hz using a second order Butterworth filter with dual pass), kinemat-
ics were analyzed from the time of hand lift to dowel lift.
Results revealed that, as expected, maximum aperture did not vary
across the conditions, since the dowels were all of the same diameter.
However, subjects spent a longer time after peak aperture and peak
deceleration for the 410 gram dowel than the lighter dowels. Further,
more time was spent in the deceleration portion of the movement on
trials when the weight was unknown than when the weight was
known. Visual examination of the wrist velocity and the aperture
profiles suggested this longer time reflected ‘tails’ or unchanging ve-
locity/aperture values at the end of movement, during the time when
the subject was in contact with the dowel prior to lift (see Weir et al.,
1991 for details). In a second experiment therefore, the time in contact
with the dowel prior to lift was measured directly. A metal contact
breaking system defined the times of hand lift, contact of the thumb
and index finger with the dowel, and dowel lift. The results con-
fmed that the timing and kinematics of the movement prior to contact-
ing the dowel were unaffected by the dowel weight or weight uncer-
tainty. We were surprised to discover that, even for blocked trials
when subjects knew the weight of the dowel, there were no anticipa-
tory changes in the kinematics prior to contacting the dowel. Since the
dowels were visibly similar, this emphasizes the dominance of visual
information prior to contacting the object, and the influence of tactile
and kinesthetic information once contact is made. All the timing and
kinematic effects occurred after contacting the dowel, prior to lift off.
The time spent in contact with the dowel prior to lift increased sys-
tematically with dowel weight. More time is spent in contact with the
dowel prior to lift when the weight is unknown than when the weight
is predictable based on previous trials. This increased time spent in
contact with the dowel is consistent with research showing that the
functions for grip and load force application over time had an in-