Chapter 2. Prehension

172 THE PHASES OF PREHENSION
T
150 T
-Ev
E 13ot
e,
L
P
70
i
I
50 1 0 2.5 4.0 7.0
-.- CNP
U-CP
-+- I
Dowel size (cm)
Figure 5.23 Peak aperture as a function of dowel size (2.5, 4.0,
and 7.0 cm in diameter) when grasping with pad opposition, using
independent fingers (I), with pad opposition using the four fingers
collectively in opposition to the thumb (CP) or with palm
opposition, using collective fingers and no precision (CNP). In
all cases, maximum aperture is greater when using the fingers col-
lectively in palm opposition than when using the fingers in pad
opposition, either independently or collectively. Note the
exceptional point is for the smallest dowel where the maximum
aperture is slightly greater in pad opposition with collective
fingers that in pad opposition between the thumb and index finger.
Linear regressions of maximum aperture (cm) as a function of
object size for each grasp type revealed: a) for I: Y = 6.48 + .85X
b) for CP: Y= 7.44 + .69X c) for CNP: Y = 10.3 + S8X. These
slope values, for dowels 11.5 cm high can be compared with
Marteniuk et al. (1990) who found Y = 4.89 + .77X for disks 2.5
cm high (Adapted from Sivak, 1989; reprinted by permission).
fingers (CP) was slightly higher than for pad opposition with inde-
pendent fingers (IP). This point led to a difference in the slopes com-
puted for the three functions plotted in Figure 5.23. Note the slope of
the function relating maximum aperture to cylinder diameter is the
steepest for the describing pad opposition with VF2 as one finger,
then pad opposition with VF2 as four fingers, and shallowest for the
function describing palm opposition.
The above findings provide strong support for the distinction be-