Chapter 2. Prehension

190 THE PHASES OF PREHENSION
activa,tion of reaching and grasping
r"""""""""'"""""""""
I I Object size
Object distance ~~
1 I
I
I
I I I
Y
Transport
Arm
Schema
Time
needed
# Time-
Based
Coordination
uration
Duration
I
Time
needed
Duration
L
u
Preshape
Schema
Figure 5.30 Hoff and Arbib updated schema model for preshaping
and enclosing. Extrinsic object properties (distance) are sent to
the Transport Arm Schema, while intrinsic ones are sent to the
Preshape Schema. Both compute the length of time needed for the
action. The Time-Based Coordination Schema determines which is
maximum and sends the planned duration back to both. Enclose
Time is subtracted from the value sent to the Preshape Schema.
The hand will then be preshaped prior to arrival of the hand at the
object with enough time to enclose the fingers around the object
(from Hoff & Arbib, in press; adapted by permission).
ceives the object size, computing how much time it, along with an
added constant enclose time, needs as well. The Time-based
Coordinator receives these Time Needed values and compares them,
finding the maximum. This value is then sent back to the Transport
and Preshape Schemas as planning duration, subtracting the Enclose
Time from the value it sends to the Preshape Schema. This scales the
preshaping and transport in time. The hand will always reach its peak
aperture prior to arrival of the hand at the object, with just enough time
to enclose the hand. Computer simulations of this model were per-
formed, with results mirroring the experimental data of Jeannerod
(198 1) and of Paulignan, Jeannerod, MacKenzie, and Marteniuk