Chapter 2. Prehension

V F l p
PALM
Chapter 5 - Movement Before Contact 197
A B C
Figure 5.33 The behavior of the palm-focused model of setting up
opposition space for prehension. The dashed line along the long
axis of the object shows the opposition vector perceived in the
object. The solid lines show the hand configuration for pad
opposition, where VF1 is the thumb and VF2 is the one or more
fingers in opposition. Starting at an initial posture and hand
location (A), the palm is positioned and oriented and fingers
shaped into some ballpark of the desired location and posture,
shown in dashed lines (B). Then, while the arm makes necessary
fine adjustments to align the palm, the fingers close trying to go
into the object, but are stopped by actual contact (C).
mass, and distribution of mass. Since behavioral experiments define
the task as the environmentally defined task, we list tasks used in the
experiments, such as ‘aim’, ‘fit’, ’grasp’, ‘place’, etc. Observable
kinematic features of prehensile behaviors come from the transport
component and from the grasp component. From the transport
component, movement time, wrist velocity, acceleration, and even jerk
have been measured. For these, peaks and time to and after peaks
have been used to help identify potential control variables. In terms of
the grasp configuration, the aperture between thumb and index finger
has been a critical metric of prehension, while also joint angles have
also been measured.
Even from common everyday experiences, it can be seen that hands
preshape into some suitable posture as reaching to grasp objects
occurs. Teasing apart the reasons for how and why these unrestrained