Chapter 2. Prehension

184 THE PHASES OF PREHENSION
(1982b), this information appears to selectively contribute to the plan-
ning and control of transport of the hand to the object, not to the for-
mation of the grip. Under natural viewing, central vision provides
important information about object size (and possibly other intrinsic
object characteristics), for accurate grip calibration and hand posturing
in opposition space18. This objective object size information con-
tributes to the organization and control of both the grasp and transport
components. Without the high resolution provided by central vision,
subjects adopt a tactile control strategy, increasing the length of time
they are in contact with the object, in order to successfully complete
the grasp.
Accurate distance information appears to be provided by binocular
vision; with only monocular viewing, subjects appeared to underesti-
mate the distance to objects, as well as their size (Servos, Goodale &
Jakobson, 1992). Combining three oblong object sizes with three
movement distances, they found that when viewing the objects
monocularly (with the preferred eye), movement times were slower,
peak velocities and accelerations were lower than with binocular
vision. As well, greater time was spent in the deceleration phase after
peak velocity with only monocular vision (583 ms or 69%), compared
to binocular vision (390 ms or 63%). These viewing effects interacted
with object distance on kinematic peaks, e.g., the effects of distance
on peak velocity replicated earlier work but were attenuated in the
monocular viewing condition. Subjects always had a smaller peak
aperture under monocular viewing conditions (84 mm) compared to
binocular ones (90 mm). Thus, it appeared that subjects were under-
estimating both the distance and the size of the objects. With both
monocular and binocular viewing, the maximum aperture showed a
similar calibration to visually observed intrinsic properties (i.e., slopes
of the function relating maximum aperture to object size were not
provided, but appear similar, even though with monocular vision,
maximum aperture was consistently smaller). Although not
mentioned, it is assumed that under all conditions, subjects accurately
18 Sivak & MacKenzie (1992) noted with interest neurodevelopmental differences
between central and peripheral regions of the retina. The retina is fully developed at
birth except for the foveal area which is not completely functional until about 6
months of age. This is about the same time at which pad opposition emerges in
infancy (see von Hofsten & Ronnqvist, 1988). Neural, behavioral and functional
parallels in foveal development for foveal grasping and the development of the
somesthetic macula of the finger pads for pad opposition is an inmguing area for
further exploration.