Chapter 2. Prehension

336 CONSTRAINTS AND PHASES
less precise sensory information, and only the arm and wrist are
available to provide grosser motions. Side opposition is a compromise
between these, a link or bridge between them. It occurs between
the thumb pad (or ulnar side of fingers) and the radial side of the
fingers along an axis generally transverse to the palm. This posture
offers a medium range of forces while still offering some availability
of sensory information due to the thumb pad being in contact with the
object and also some ability to impart motions to the object (as in
turning a key). Importantly, combinations of these three oppositions
create postures that bring to bear the sensorimotor features of the hand
in order to match the task requirements, exactly as Napier (1956)
argued.
Besides the two virtual fingers used to create an opposition, a third
virtual finger can be used to apply an opposition against a task related
force or torque, as in holding a suitcase in a hook grip. A virtual finger
three (VF3) can apply a force against gravity, can impart motion
mostly with the arm or wrist, and can gather sensory information
using sensors in the hand surfaces to determine the state of the object.
Extending a finger (usually the index) as an antenna is an excellent
way to enhance the availability of sensory information (due to the large
number of mechanoreceptors in the finger pads and interphalangeal
joints) and apply a force in a specific direction. Although it reduces the
number of fingers available for applying forces, the extended finger in
itself can apply a limited force, provide direction and impart some
motion as necessary for a VF3. Importantly, it can be used in
combined grasps, as seen in extending the index finger on a
screwdriver or knife, or placing the thumb on the side of a beer mug.
The sensorimotor features of the hand integrate into active touch in
order to extract object properties. Specific exploratory movements of
the hand that have been studied include wielding (Hoisington, 1920;
Solomon, Turvey, & Burton, 1989), jiggling (Brodie & Ross, 1985;
Victor Raj, Ingty & Devanandan, 1985), holding, lateral motion,
pressure, contact, enclosing, and contour following (Klatzky &
Lederman, 1987). Human movements of this type have been
observed to have a natural frequency of 1.56 Hz (Kunesch, Binkofski
& Freund, 1989). But the sensorimotor features of the hand integrate
into active manipulation using sensory information to monitor and
guide motion. Manipulative actions, or performatory movements, of
the fingers have been listed as: squeezing, twiddling, rolling, rocking,
sliding, and stepping, while the dynamic tripod posture allows
oscillations of the object (Elliott & Connolly, 1984). These
movements occur along the same coordinate frame that oppositions