Chapter 2. Prehension

150 THE PHASES OF PREHENSION
+60 1
L
Frorne Frorne
Figure 5.16 Thumb and index finger separating in order to grasp
dowel. Note how index finger opens more than the thumb. The left
side shows data from a 13 year old girl having a one degree of
freedom prosthetic hand directing her thumb towards the cylinder.
The same control algorithm is used by her normal hand, which is
shown on the right side (from Wing & Fraser, 1983; reprinted by
permission).
gested that with speeded movement, errors in transport could be com-
pensated for by a larger aperture, showing the spatial coupling.
It is interesting to note what happens in conditions where a pros-
thetic hand replaces the human hand. Wing and Fraser (1983) studied
a 13 year old girl who still moved the thumb less, even though the
prosthesis had 1 degree of freedom. This forced her to rotate her
forearm in order to maintain the same relationship between her pros-
thetic ‘thumb’ and the object (left side of Figure 5.16). Sometimes it
was even necessary to reverse the direction of the hand transport in
order to complement the effects of the wrist rotation.
Thus the speed demands of the task seem a key variable. Wallace
and Weeks (1988) factorially combined distance and movement
duration, suggesting that the maximum aperture was dependent on
movement time, rather than movement speed per se. Like Marteniuk,
ib