Chapter 2. Prehension

234 THE PHASES OF PREHENSION
more accessible to touch). Any kind of movement will encode texture;
therefore, the EPs enclosing and contour following will also provide
texture information. Statistically, static contact between the palmar
surface of the hand and object will work as well. Hardness is a
feature better found haptically. In order to apply pressure, the object
must be stabilized either by the environment, the other hand, or by
other fingers. Pressure can be applied while performing lateral motion,
contour following, or enclosure. Subjects appear to encode hardness
along with texture. Hardness is measued by pressure, which is
quickly assessed and easily executed on a homogeneous object. It
also seems to be the most specialized procedure. Temperature is a
feature better found haptically, and can be determined statistically as
well by enclosure and by contour following. Weight tends to covary
with size (large objects tend to be heavier). Weight is a feature better
found haptically than visually. These researchers suggest statistically
all EPs will extract weight.
In terms of visually-determined features, volume is a feature
better found visually. Enclosure is quick but only conveys gross size
information. However, it seems to be the least specialized. Contour
following may occur for larger objects. As well, statistically static
contact and unsupported holding will convey volume. Shape is a
feature better found by the visual system, which is good at spatial
analysis of patterns. Contour following does not occur on a
homogenous surface; it is a relatively slow procedure and subject to
error (due to memory and integration). Enclosure is quick but only
conveys low level information. Global shape can be determined by
static contact, contour following, and unsupported holding.
6.3 Analytic Model of Grasping
An object has a weight, acting at its center of mass (see Figure
6.10). The forces applied by the virtual fingers to the object’s
surfaces must counterbalance this weight sufficiently in order to
overcome gravity and lift the object. These applied forces, supplied
by the muscles or tendons acting on the joints, are transmitted through
the fingerpads (in pad opposition) against the object’s surfaces.
Identifying human hand postures and muscle commands that will
solve this problem is similar to problems in the field of robotics,
where researchers develop algorithms to determine whether a
particular robotic grip imposes sufficient constraint on a grasped
object, Aspects of this analysis involve how the forces are transmitted
through the fingertips, how the fingertip forces relate to joint torques,