The experiment involves a subject guiding a spot across a transparent screen by moving a robot arm. The goal is to move from one place on the screen to another. In response to the subject's movement, the computer produces a patter of forces that has to be learned. In about an hour's time, subjects get much better at controlling the motion.
During the learning process, Shadmehr programs in occasional radical changes in the forces, totally upsetting reality.
The brain is forced to alter the primitives, readjusting to the reality rather than the perception of reality --enlightenment right out of Plato's parable of the cave, where reality was depicted as shadows on the wall. By mathematically analyzing how these sudden changes affect the subsequent movements, the report infers the shape and size of the primitives used to construct the model of reality in the brain.
"We think the shapes are related to the neural firing behavior of cells in the brain, the cerebellum," Shadmehr reports in the Nature article. It is possible that it is in this region of the brain that the internal model is constructed.
Then, Shadmehr programs in a radical change, totally upsetting reality.
For an un-Platonic analogy, think of a milk bottle, solid white and apparently full of milk, standing on a shelf. In order to pick it up, the brain sends a signal to the arm and hand to lift the bottle, making presumptions about how much the bottle weighs so sufficient force can be utilized. If the presumptions are correct, the procedure works flawlessly. But what if the bottle is only painted white, is actually empty? The hand, expecting it to weigh more, uses too much force and the bottle goes flying.
This shows that humans use internal models of reality to guide movements.
The change in the program is the equivalent of secretly replacing the full
'"/>
Contact: Gary Stephenson
gstephenson@jhmi.edu
410-955-5384
Johns Hopkins Medical Institutions
11-Oct-2000