Chapter 2. Prehension

Chapter 5 - Movement Before Contact 171
ger enclosure to capture the object into the palm of the hand did not require
precise finger placement; this type of grasp always had a wider
aperture, and did not show precise calibration to object size in the
same way as peak aperture with pad opposition.
The palm and pad oppositions described in the above studies differ
in at least two distinct and important ways: the parts of the hand used
in opposition (i.e., fingers opposed to the palm or thumb), and the
number of fingers used in the opposition. With palm opposition, typically
all (or most) of the fingers on the hand are flexed collectively to
apply force in opposition to the palm. In contrast, with pad opposition
(or precision grip), opposition is between the thumb, and the index
finger. Thus, for pad opposition in the above experiments, VF1 is
mapped onto the thumb, and VF2 is mapped onto the index finger; but
VF2 could be mapped also onto any number of other fingers in opposition
to the thumb. For example, VF2 could be index and middle
fingers or VF2 could be index, middle, ring and little fingers used in
opposition to VF1. Are the differences in these experiments due to the
use of independent vs collective fingers, or due to pad opposition requiring
precise placement of the fingertips vs palm opposition?
To answer this question, Sivak (1989, Experiment 7) suggested
that if precision requirements of finger placement could be equated
between collective and independent finger movements, one might
predict no differences in the transport component, or in maximum
aperture. Three grasp types were used: pad opposition with thumb
and index (called independent grasp), palm opposition (called collective
grasp, no precision),and pad opposition with the thumb opposed
to pads of the four fingers (called collective grasp with precision) and
three dowel sizes (2.5,4.0, and 7.0 cm in diameter). As in all previous
experiments, movements were 30 cm forward from the starting
position of the hand, in the midline sagittal plane. The results of
kinematic analyses of the transDort component showed that when pad
opposition was reauired. it didn’t matter whether there were one or
four fingers in oposition to the thumb. However, using the fingers
collectively in palm opposition was different from the two pad opposition
conditions. Using the fingers collectively in palm opposition had
the highest kinematic peaks, a shorter deceleration phase, and a shorter
per cent of time after peak deceleration. These robust, reliable effects
were shown by all six subjects. A similar grouping of the grasp types
was seen in the peak aperture data, shown in Figure 5.23. Note that
using palm opposition (CNP, using collective fingers with no precision)
had a larger maximum aperture in all cases. However, for the
2.5 cm dowel, the peak aperture for pad opposition with collective