Chapter 2. Prehension

268 THE PHASES OF PREHENSION
system has a different set of mechanical and functional properties.
Once grasped, the object is lifted, and it may be held (as in lifting a
mug to wipe a countertop), transported (to place the mug on a shelf),
manipulated (to hand to someone, to drink from, or to explore its
features) and replaced (on the countertop).
In considering manipulative hand movements, a profound
distinction was made by Kunesch, Binkofski and Freund (1989).
They analyzed the temporal frequency characteristics of manipulative,
serial hand movements, for tasks including tactile discrimination,
pencil shading, handwriting, typewriting, and repetititive tapping.
They report a bimodal result: two separate classes of natural preferred
frequencies (see Figure 6.21). At 1.56 Hz cluster exploratory finger
movements in which the hand is used in active touch, to determine
object properties like shape and texture. They suggest the slower
range reflects ‘focal sensory control’, i.e., the temporal requirements
of the sequential sampling from the mechanoreceptor population.
These movements use motion to collect detailed somatosensory
information. At 4 - 7 Hz cluster handwriting, and other cyclic, learned
hand movements, with frequencies close to those of fast tapping.
Frequencies for handwriting and shading were almost the same,
regardless of whether the fingertip or pencil was used. They
suggested the higher frequency movements are under different sensory
control: the high frequency movements are not performed entirely
open-loop, but monitored by ‘preattentive sensory processes’. These
movements use proprioceptive and somatosensory inputs to monitor
motion. This distinction between the two types of movements,
consistent with Gibson’s (1962) ‘exploratory’ and ‘performatory’
hand movements, is important for several reasons. First, the role of
sensory information in motor control is different in the two classes of
movements. Second, the task requirements for manipulation are key
inputs for control, and manipulation to extract object properties using
active touch is fundamentally different from manipulation in using
touch for action with objects.
It is important to distinguish between a static grasp for holding
and transporting an object and a dynamic grasp for manipulation. In
the first instance, an object is held firmly in a stable grasp with
constant contacts, and object motion is achieved via the wrist, elbow,
shoulder, or even through locomotion. In contrast, coordinated
movements of the digits can manipulate an object within the hand.
With respect to humans transporting a grasped object, it was
observed in Chapter 5 that the kinematics of pointing with a loaded
limb (Atkeson and Hollerbach, 1985) are scaled from the trajectory of