Chapter 2. Prehension

Appendix B - Taxonomies of Prehension 377
B.2 Prehensile Classifications for Infants
Descriptions of emergent prehesile behavior have been developed
for infants. For example, Halverson (1931) developed a taxonomy of
grasps after studying infants from 16 to 52 weeks of age. According
to Halverson, infants start making contact and primitive squeezes at
around 20 weeks old. The first time they exhibit an actual grasp, the
squeeze grasp, is at around 24 weeks old. It is in the palm grasp, at
28 weeks old, where infants first actively use their thumb in
opposition to the fingers.
Table B.2 has a limited summary of some of the research into
developmental prehension. Interested readers are encouraged to read
the literature cited in the table, as well as recent work by Forssberg,
Kinoshita, Eliasson, Johansson, Westling, & Gordon (1992) and von
Hofsten & Ronnqvist (1988).
B.3 Postures as Combinations of Oppositions
Figure B.l has summarized the postures developed in prehensile
classifications for the last century. It shows that prehensile postures
can be viewed as either pad, palm, or side opposition. In addition, it
shows that some postures are combinations of these oppositions, and
that a VF3 can be used to counteract task forces or torques or sense
the state of the object.
In Chapter 2, we discussed how palm, pad, and side oppositions
can be used in combinations to hold one or more objects. Using this
opposition space approach, prehensile postures can be described, as
shown in Table B.3. Shown are the opposition types and virtual
finger mappings for one, two, and three oppositions in grasps. There
are eight possible combinations of pad, palm, and/or side oppositions
(e.g., palm alone, pad alone, palm and pad, etc). For each
combination, there are differing possible virtual finger mappings.
For palm opposition, the palm is used as VF1. For pad opposition,
the thumb is used as VF1. For side opposition, the thumb is usually
used, however, in the adduction grasp, the index finger is used. For
VF2, one or more fingers can be used. Fingers not being used in VF1
and VF2 can be used in VF3, as seen in the last column of the table.
Fingers can be used alone as gravity dependent grasps or finger
touches.
Table B.3 can be examined for the symbolic postures listed in
Table B.l. For example, grasp 2 in Table B.3, consisting of one