Chapter 2. Prehension

152 THE PHASES OF PREHENSION
resultant velocity profiles were more asymmetrical, and a greater per-
cent of time was spent after peak deceleration than when the handle
was supported on the table. There was greater reposturing of the hand
after first contact when the handle was on the table; in contrast, the
longer deceleration phase when the handle was accessible may have
reflected more precise hand placement at contact. This interacted with
screwdriver size, such that the precision effects of handle accessibility
were most pronounced for the smallest screwdriver.
With respect to intrinsic object properties, one of the most reliable
and robust findings concerns the tightly calibrated relationship be-
tween object size (cylinder diameter) and maximum aperture. In
Chapter 4, we noted that even when subjects do not grasp an object,
they are able to match well with an unseen hand the required opening
to grasp seen objects of various sizes (Jeannerod & Decety, 1990).
Many investigators have studied the relationship between aperture
evolution and object size (e.g., Gentilucci et al., 1991; von Hofsten &
Ronnqvist, 1988; Jakobson & Goodale, 1991; Jeannerod, 198 1,
1984; Marteniuk et al., 1990; Wallace & Weeks, 1988; Wing &
Fraser, 1983; Wing, Turton & Fraser, 1986). Using 10 wooden
disks (2.54 cm high, and 1 - 10 cm in diameter), placed 30 cm for-
ward in the sagittal, midline plane, Marteniuk et al. (1990) had
subjects reach to grasp, lift and replace the disks on the table top.
They found that peak aperture increased by 0.77 cm for every 1 cm
increase in the diameter of the cylinders, with a correlation of .99.
The correlations between cyclinder diameter and peak aperture for
individual subjects ranged from .992 to .998! Consistent with the
findings of von Hofsten and Ronnqvist (1988) and Gentilucci et al.
(1991), Marteniuk et al. (1990) found that maximum aperture was
reached earlier (i.e., a greater time spent enclosing) when grasping a
small object, compared to a larger one?
What about the relative time spent in preshaping and enclosing?
Jeannerod (198 1, 1984) suggested that the time to maximum aperture
was about 75% of total movement time, compared to Wallace and
Weeks, who reported 60% of MT in preshaping. Others (e.g.,
Gentilucci et al., 1991; von Hofsten & Ronnqvist, 1988) have re-
ported that the relative timing of hand closure was related to object
size. While it has been demonstrated that preshaping relates to visu-
ally determined object size (Marteniuk et al, 1990), shape (Jeannerod,
l5 It is perhaps significant that infants 9-13 months old adjust their hands to target
size, but not the 5 - 6 month olds (von Hofsten and Ronnqvist, 1988). It is
usually between these times that pad opposition or precision grip appears.