Chapter 2. Prehension

Chapter 5 - Movement Before Contact 169
front of the start position, along a midline sagittal plane. Note an im-
portant difference from Castiello et al. (1992): in Sivak’s experiment,
the intrinsic and extrinsic object properties remained constant, ie., the
retinal image about intrinsic and extrinsic dowel properties remained
invariant. Only grasp type changed. In blocked trials, subjects used
pad opposition, making contact with the thumb and index finger pads,
or palm opposition, making first contact with the palm of the hand
while the fingers and thumb enclose the dowel. Replicating Castiello
et a1.(1992) she showed that movement time and the proportion of
time spent in the deceleration phase of movement was longer when
reaching for dowels using pad opposition. For pad opposition,
subjects spent 40% of MT or 385 ms after peak deceleration; in
contrast, they spent 21% or 184 ms after peak deceleration when
using palm opposition (see Figure 5.21). For the grasp component,
even though the dowel size remained constant, the hand opened wider
with palm opposition (107 mm), than with pad opposition (79 mm).
Sivak noted that with pad opposition, for all subjects, peak aperture
occurred after peak deceleration, but peak aperture between the thumb
and index finger occurred before peak deceleration in palm opposition
trials (for 5 out of 6 subjects). The relative time of maximum aperture
did not change between the two grasp types (about 34% of time spent
enclosing after peak aperture, with both grasp types), but the relative
time spent in deceleration was longer for pad than palm opposition.
These findings led Sivak (1989) to suggest that the neural processing
in organizing the size of the aperture may be independent from the
neural processing organizing the timing of maximum aperture. Sivak
suggested that more time is needed in the final phase for precision
tasks like pad opposition. With pad opposition, precise placement of
the fingers was required; in contrast, with palm opposition, the object
was acquired after initial contact with the palm. Related precision
requirements were seen also in Marteniuk et al. (1987, 1990), with
different tasks and different size disks.
What is the relationship of pad and palm opposition to dowel size?
Castiello et al. (1992) stressed the importance of a natural mapping
between object size and type of grasp. Small cylindrical objects are
often grasped with pad opposition and as object size increases, aper-
ture increases, more fingers are added to oppose the thumb, and
eventually a palm opposition emerges. Sivak (1989, Experiment 6)
investigated the sensitivity of palm and pad opposition to object size,
reasoning that palm opposition (grasps using the fingers collectively)
may not show the same robust effects of object size as pad opposition
demonstrated previously by von Hofsten and Ronnqvist (1988) and