Chapter 2. Prehension

Chapter 2 - Prehension 39
ables that quantify a posture in both physical terms (e.g., amount and
orientation of force vectors, innervation density of grasping surface
patch) and abstracted terms (e.g., types of oppositions, virtual finger
mappings). In terms of state variables, an opposition space is defined
by the following state variables:
a) the number and type of oppositions (pad, palm, and/or side) being
used,
b) the virtual to real finger mapping (i.e., which fingers),
c) VF state variables for each VF in each opposition being used,
within given constraints on these state variables.
As noted earlier, postures consist of one or more oppositions.
Postures can also contain a VF3 which opposes a task-related force or
torque. Which real fingers are being used in each virtual finger in each
opposition is called the virtual to real finger mapping.
State variables for virtual fingers can be defined as follows
(Iberall, Torras & MacKenzie, 1990):
a) VF length (from the center of the grasping surface patch to the
joint where it connects to the palm)
b) VF orientation relative to the palm
c) VF width (number of real fingers mapped into the VF)
d) orientation of the grasping surface patch (the orientation of the
applied force)
e) amount of force available from the VF (mean, maximum, minimum)
f) amount of sensory information available at grasping surface patch
(innervation density of cutaneous mechanoreceptors)
Each VF is represented by the magnitude (1) and orientation ($) of a
vector relative to the palm (see Figures 2.7 and 2.9); the values change
as the fingers and thumb flex and extend. The VF grasping; surface
patch is the part of the palm or finger that comes in contact with the
object in order to apply the oppositional force. A virtual finger can
have a width, based on the number of real fingers mapped into the
virtual finger. A simple example of a VF is the index finger used in
pad opposition. Since it is being used in pad opposition, the grasping
surface patch is the distal finger pulp. The VF has a length (along the
straight line from the centroid of the grasping surface patch to the sec-
ond metacarpophalangeal joint) and an orientation (the angle that line
makes with the plane of the palm). It has a width of one because it is