Chapter 2. Prehension

158 THE PHASES OF PREHENSION
between the thumb and index. Of interest in these studies was the ob-
served invariance of the contact point between the thumb and index
finger. In all conditions, the subjects maintained the contact on the
finger pads, the location of specialized cutaneous mechanoreceptors.
Cole and Abbs further argued that the goal of contact was in sensory
terms; i.e., maintaining the same cutaneous input and achieving the re-
quired force. In terms of enclosing the hand during prehension, per-
haps the goal is to bring the chosen pads (for pad opposition) into a
specific contact with the object. It is obvious from these data that the
CNS knows the direction that the pads are pointing.
n
150
v
5
a
UJ
5 100
I=
2
cd
I 50
A
. 7
-
n
E
70
E 60
W
z
2
a
2
g50
-
-
-
2 - 40
B
-. ..
.\cc. 30 Start
L
+p :.:< ..
Figure 5.18 A single trial in which a mechanical perturbation
pulled the arm backwards, and both transport and aperture reversals
were observed. A. Shows hand transport (solid line, left axis) and
grip aperture (dashed line, right axis). The acceleration is double
differentiated from the hand transport (long dashed lines, axis f 1
cm/s/s). The solid horizontal bar represents a pull perturbation of
15 N. B. For the same trial as A, a spatial plot of hand aperture
against hand transport. The perturbing force began at +P, the
transport reversal began at TR, the hand aperture reversal at AR
and the perturbing force offset at -P. Perturbed trial shows a
distinct loop at reversals; the control trial is a dashed line (from
Haggard 8z Wing, 1991; reprinted by permission ).
I