Chapter 2. Prehension

Chapter 6 - During Contact 229
assimilation, and control systems analysis. Our point here is that like
the eye, the sensory and motor functions of the hand are intertwined
and indivisible, as the hand is moving to grasp, feeling to grasp,
grasping to feel and grasping to move objects. This ability, called
active touch or haptics, is highly developed in the human sensorimotor
system.
Active touch, or touching was distinguished from passive touch,
or being touched by Gibson (1962), who noted that with active touch,
the impression on the skin is brought about by the perceiver. Active
touch, our concern here, is an active exploratory sense. Through
exploratory movements, tactile scanning can occur, providing
information about object properties.
6.2.1 Wielding a stick: feeling object length
In an extensive analysis of perception of length, weight, and
centre of mass in lifted rods, Hoisington (1920) showed that for non-
visual perception of length of a rod lifted or held in palm opposition,
the underlying essential condition is a ratio between two opposing
pressures, held together by directional and temporal relations; in
addition to the two opposed pressures are the absolute intensity of the
impressions, their temporal course, the pressure gradient formed and
the muscle and strain sensations from the hand and arm. Center of
mass of the object bears a constant direct relation to the ratio of
opposing pressures, but is removed from the experienced perception.
Perceptions of length reachable by hand-held rods, partial and
total rod lengths have been evaluated by Burton, Solomon, and
Turvey who suggested that the moments of inertia provided the kinetic
bases for judgements of rod length (Solomon & Turvey, 1988;
Solomon, Turvey & Burton, 1989; Burton & Turvey, 1991). To hold
stationary a rod held in the hand, the forces and torques that must be
applied are a function of the type of opposition (in this case, palm
opposition), virtual finger mapping, hand configuration, hand
placement on the rod, and the physical dimensions of the object.
Placing added weights and the hand at different positions along the
length of the rod altered perceived length in predictable ways
according to the center of mass, and moment arm length, consistent
with the earlier statements of Hoisington (1920) on ratios of opposing
pressures and intensities in the hand and arm. Burton and Turvey
(1991) address the questions of corresponding deformation patterns in
the hand and how prior intents are functionally equivalent to physical
constraints that harness dynamics to form task-specific mechanisms