Chapter 2. Prehension

38 WHAT IS PREHENSION?
and unwieldy. They provide no reliability measures. The goal of the
posture is lost amidst the highly detailed coding system. Finally, al-
though it is a small step, the system does not quantify the prehensile
postures per se10.
OMEIFN~ ~CMFIFN D2345 CMFIFRN+lOIMFIF 'OMEIFP? CMFIW? MFIFSIW
Figure 2.8 Examples from Jacobson and Sperling's coding sys-
tem. On the left, the thumb (1) opposes (0) with its MP joint
extended (ME) and IP joint flexed (IF); the index finger (2) adduces
(C) against the middle finger, with its MP and IP joints flexed
(MFIF). Both tips contact the object (N). In the middle figure, the
object is diagonally held (D) by the four fingers (2345) which are
adduced (C) with all joints flexed (MFIF) with the more proximal
surfaces making contact (R); the palm (V) makes contact;
indirectly (+), the thumb (1) opposes (0) with its joints flexed
(MFIF). On the right, the thumb (1) opposes (0) with its MP
joint extended (ME) and IP joint flexed (IF), using the pulp (P) to
make contact; the index finger (2) adduces (C) against the middle
finger, with its MP joint flexed (MF) and its PIP joint flexed and
DIP extended (I*F), with its pulp (P) making contact; the middle
finger (3) has all joints flexed (MFIF), with its side surface (S)
making contact; the web (W) of the thumb also makes contact
(from Jacobson and Sperling, 1976; reprinted by permission).
2.4.2 State variables for opposition spaces
A formal language can be used to describe oppositions and virtual
fingers. An opposition space provides values for a set of state vari-
OThe experiences with the coding scheme led to the development of a taxonomy
consisting of eight postures (Sollerman, 1980). As seen in Table 2.1, this included
4 finger grasps (pulp pinch, lateral pinch, tripod pinch, and five-fingered pinch) and
4 palmar grasps (diagonal volar, transverse volar, spherical volar, and extension
grip). These are similar to those already discussed.