We don't think often about how we've learned to move to pick things and control our gestures. But now, neuroscientists have used a robotic joystick to discover how our brain controls movements. By combining experimental and computational approaches, they found that our brains contain two series of components which learn at different speeds. The first ones learn and forget fast while the second ones learn slowly but have a better 'memory.' These results could be used to rehabilitate "people who have lost motor abilities to brain injuries like strokes." Read more...
Let's start by looking at the experiments.
To test the need for time in mastering muscle control, the research team designed a simple and short task. Fourteen healthy human subjects were asked to hold onto a robot-controlled joystick and keep it from moving as the robot driver pushed repeatedly -- in quick pulses -- to one side. The joystick then pushed repeatedly in the opposite direction and again the subjects were asked to keep the joystick centered.
The research team found that after all this pushing in different directions the subjects still were inclined to push the joystick in the first direction, even when the joystick was perfectly centered and not moving. Somehow the brain and muscles in the arm had "learned" this simple movement over the course of the experiment, which took only a few minutes, according to the researchers, showing that sleep is not required for learning such simple movements.
Below is a picture showing how two learning processes (the slow state shown in blue and the fast one in green) "with distinct time courses contribute significantly to short-term motor skill acquisition" (Credit: PLoS Biology and the authors).
But this illustration tells only one part of the story. Below is another one showing how these learning states evolve with the number of trials (on the horizontal axis). The red curve -- which represents the multi-rate model -- shows a jump in performance "caused by adaptation rebound in the error-clamp phase" (Credit: PLoS Biology and the authors).
But how did the researchers reach their conclusions?
For example, by taking into account the number of repetitions it took for the subjects to push the joystick in the first direction to keep it centered and how long it took for the subjects to "forget" how hard to push the joystick, the predictions suggest that the brain learns muscle control using at least two different steps.
First, the computer programs were able to tease out that the brain picked up the control task quickly, but actually forgot the task quickly as well. But, at the same time, the brain also was learning the same task more slowly, and that was responsible for the subjects' being able to "remember" the initial joystick-pushing movement.
This research work was published by the PLoS Biology journal on May 23, 2006. Here is a link to a synopsis of the article named "A New Model of Short-Term Motor Adaptation" from which the first image above was extracted and which begins with the following introduction.
Starting at around three months old, children can finally reach for the countless toys their parents have been dangling before them since birth. These attempts often involve a good deal of flailing about, as motor skills, like anything else, require cultivation. Motor control depends on executing the proper musculoskeletal force to reach the desired object.
Prior learning facilitates motor control (all that flailing serves a purpose), which is aided by a fundamental feature of memory, called savings. When a novel response to a stimulus is learned in one set of trials, then “washed out” in an unlearning phase, subsequent relearning proceeds faster. Neuroscientists have been puzzled by savings and other features such as interference and rapid unlearning reported in adaptation studies because standard models of short-term motor adaptation couldn't explain them. But now Maurice Smith, Ali Ghazizadeh, and Reza Shadmehr have solved this puzzle.
If you're interested by this research, here is a link to the full paper named "Interacting Adaptive Processes with Different Timescales Underlie Short-Term Motor Learning" and from which the second illustration of this post was extracted.
And what will be the next step for these researchers? They want to discover which parts of the brain are responsible for slow-learning to "tailor therapy strategies to target slow-learning and increase recovery of muscle control after brain injuries."
Sources: Johns Hopkins Medical Institutions news release, May 23, 2006; and various web sites
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