Chapter 2. Prehension

Chapter 8 - Constraints on Human Prehension 311
Other object properties have an effect on force generation as well.
Cochran and Riley (1986) found that handle shape affects the force
exerted: higher forces were seen with irregularly shaped handles
(rectangular and triangular) than with uniformly shaped ones
(circular). Westling and Johansson (1984) looked at precision
grasping of objects with different textures and different weights. If
the object's surface texture is sandpaper, then there is a higher
fictional component to the interactive forces than if the surface texture
is suede or silk. They looked at the ratio between the grip force (force
normal to the object's surface) and the load force (force tangential to
the object surface in line with gravitational force). The value for this
ratio below which the object will slip out of the fingers is called the
slip ratio. Johansson and Westling found evidence that subjects
maintain a safety margin above this ratio, and even use one of two
strategies for maintaining a safety margin: either the safety margin is a
constant fraction or else a constant absolute value of the grip force.
While the slip ratio is a physical constraint, maintaining such a safety
margin above it is a functional constraint, imposed by some goal of the
performer (in fact, for Johansson and Westling to measure the slip
ratio, they had to request that their subjects 'slowly separate the thumb
and index finger until the object drops', which is a difficult task to
perform).
How the CNS actually obtains task relevant object information is
a function of the interaction with the object. Klatzky and Lederman
(1990) have shown that subjects use their hands in particular ways to
extract the sensory information needed in a task. For example, lateral
motion is used to extract texture information and unsupported holding
is used for extracting weight information. Even though a procedure is
specialized for just one dimension, it is still informative along others.
For example, contour following is used specifically to extract exact
shape and volume, but it can also extract sensory information for all
the object properties they measured. While a slower process than the
more specialized ones, contour following nevertheless is a procedure
typically observed in prehension. In an informal mug grasping study
(Arbib, Iberall, & Lyons, 1985), the subject's fingers were seen to
move along the inside of the handle while capturing the object into the
grasp, thus perhaps providing to the CNS additional knowledge about
the texture, hardness, temperature, and weight of the mug.
In summary, prehensile behaviors are subject to the constraints
imposed by the laws of physics. A dextrous, multi-fingered hand is
not constrained at this level of analysis to unique prehensile postures