RM: I found VITE and DIRECT. They look like output-generation models to me. If they are, then it would be easy to show the superiority of the PCT model; just apply gentle and slow disturbances to the movements made by all the models (and the human subjects) doing the A-H task. Those disturbances should have little or no effect on the shape of the velocity profiles produced by the humans and the PCT model but should be fully effective on the shape of the profiles produced by VITE, DIRECT and all the other models. Remember, PCT is a model of controlled input , not of caused output .
It is exactly opposite. It velocity profile is the controlled variable in humans, then disturbances will not have an effect. If the velocity profile is not controlled, then disturbances will have an effect on the velocity profile. If temperature in the room is controlled, then opening the window will not have much of an effect, because the AC will work stronger, and vice-versa.
VITE, DIRECT, NOCH, REACH, etc. are interesting and wonderful models to me, in the sense that they show different ways of producing hand trajectories, they look into biological structures, human behavior, etc. The have some strange feedback structures, I would not call them “caused output”, but they do use inverse kinematics and dynamics which might be overly complicated. There is also learning and adaptation, etc, which is very interesting if someone is into those kinds of things.
Some of them do have explicit trajectory sensing and control, rejecting disturbances to the trajectory, etc.
All of those researchers are trying to find good models of human hand and arm control, and they have varying degrees of success, just like the people in the PCT community. Some things they might be doing good, some things maybe less good, but so am I, and so are we.
AM: The weakness of the Little Man is that there is no “trajectory control”.
RM:I’m not sure that’s true. Our work with the Little Man suggests that it accounts for the invariant velocity profiles without the addition of trajectory control.
It sure does not require trajectory control to make nice bell-shaped velocity profiles, but neither does a damped mass on a spring. Lots of systems show bell-shaped velocity profiles. It is not too impressive or convincing is what I’m saying.
What is missing in the broader sense of growing the model toward biological realism, is a way of producing different trajectories seen in humans, such as drawing letters, or producing rhythmic movement, etc. It is not clear what are controlled variables in those behaviors, Bill hints it might be paths, for example, but he never programmed that level into any of the models.