Chapter 2. Prehension

188 THE PHASES OF PREHENSION
A +6 1
A-6 2
- . Initial Position Ballpark
OfA
Movefast Schema:
Move to some
location
near the target A
Moveslow Schema:
Move towards and
through A, and
expect another
schema to be
triggered
Figure 5.29 Arbib, Iberall, Lyons (1985) notion of ‘ballpark’. A)
Placing a mug on a table involves getting the mug to a location
above the target location, and then moving the mug through the
target location (i.e., through the table), resulting in sensory
contact triggering other schemas. B) Generalization of the ballpark
model showing how two Move Schemas work together.
For the fist phase of a movement, adding to the goal position A ,
would prevent accidental contact with the object; for the second phase
of a movement, subtracting & would ensure contact, as the goal of the
movement would actually be inside the object. In this way, uncer-
tainty can be dealt with in perceptual data.
These reaching schemas can be modelled using neural networks in
terms of ballpark modelling. Recalling the Coordinated Control
Program (Figure 5.1), the hand is moved by the Movefast Schema
from a given starting location and posture to be within the ballpark of a
desired location. The Reshape Schema shapes the virtual fingers into
a posture larger than the object at peak aperture. Then, the Moveslow
Arm Schema moves the wrist beyond the desired location. The
Enclose Schema brings the virtual fingers together, bringing the
fingers ‘into’ the object (of course, the object surfaces stop the fingers
at the appropriate time). The Approach Vector Selector model,
described in Chapter 4, provides one method for computing a goal
hand location (Iberall, 1987a). This ‘winner take all’ non-adaptive
framework computed a good location for contacting a 4 cm dowel with
two virtual fingers in pad opposition. A good location for the hand is
one where the VFs could reach the ends of the dowel, and where the
pads are aligned with each other as well as with the dowel. Following