Chapter 2. Prehension

Chapter 5 - Movement Before Contact 137
Figure 5.11. Generalization capability learned after 15 trajectories.
The top-left trajectory contains a generalization of the joint
reversal on the shoulder. The rightmost trajectory in the second
row is a particular case of generalization in which the goal
location was on the limb endpoint. Although the network was not
explicity taught about the initial posture, it understood how the
limb is positioned at the beginning of each trajectory (from
Massone and Bizzi, 1989; reprinted by permission).
for some of the units, and between isolated pairs of flexors or between
isolated pairs of extensors in other units.
An interesting experiment had to do with changing the target loca-
tion after movement started, as seen in Figure 5.12. When the second
target appeared after two time steps were made by the limb, the trajec-
tory was generally straight; with three time steps, changes in the trajec-
tories were seen. Because each point is an equilibrium point between
the agonist-antagonists that had no history, the trajectories are not
smooth. One solution is to use dynamics to make the output also a
function of the past outputs.
The Massone and Bizzi model for muscle activation demonstrates a