Children usually spill if trying to drink from a full cup, but adults rarely do. How we learn to almost automatically complete complex movements -- like how to lift a cup and tip it so the liquid is right at the edge when we're ready to drink -- is one of our brain's mysterious abilities. Now, by conducting experiments with robots and humans, scientists at Johns Hopkins have solved part of this mystery and created a new computer model that accurately reflects how the brain uses experience to improve motor control. "Now we have a much better idea of how the brain uses information from a variety of sources to create a model of the world around us, and how errors modify that model and change subsequent movements," says Reza Shadmehr, Ph.D., associate professor of biomedical engineering at The Johns Hopkins University School of Medicine. "We don't just know how to control objects around us, we have to learn how."The researchers' work is described in the November issue of PLoS Biology, a new peer-reviewed journal launched by the Public Library of Science. In the researchers' experiments, volunteers grasped the end of a robot arm that precisely tracked their attempts to overcome resistance to reach a target, a stopping point 10 centimeters (about four inches) away. While real-life resistance might be a paperweight or a full mug, in these experiments the researchers programmed forces that would hinder movement of the robot arm in predictable ways. To reach the target in the allotted time (half a second), volunteers had to learn to balance those forces. To provide the spatial information necessary for the brain to create a model, or map, of forces expected in the "world" of the experiment, subjects started from one of three positions -- left, center or right. For different groups of subjects, starting positions were separated by as little as half a centimeter (less than a quarter inch) up to 12 centimeters (about four and three-quarter inches). In initial trials without resistance, subjects moved the robot arm in a straight line toward the target from each of the starting positions. In the next set of trials, subjects had to overcome resistance when beginning from the left and right starting positions, but not from the center.
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