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Your recent work compiles experimental data into a comprehensive model on how the body adapts to changing conditions. How would you describe the model?
SHADMEHR: We discovered that a Bayesian solution can explain almost all the patterns we observe with motor adaptation. Bayesian probability is a form of statistical analysis stating that prior belief can influence future decisions. Basically, your brain is not naïve when it moves; it sorts through its memory to develop a belief and uncertainty about each imminent move, then makes what it believes is the optimal movement. As we see with babies trying to walk, the brain isn’t always right, but it does get better with repetition.
Is this movement adaptation also relevant for adults?
SHADMEHR: Absolutely. Your body is constantly changing; you are not the same now as you were five minutes ago. Adult adaptations are more subtle, but you can sense them occasionally. Exercise on a bike or a treadmill for a long time and then stop your workout suddenly. That slight dizzy sensation in your head and leaden feeling as you take your first steps is due to your body being uncertain about your new state and thus recalibrating its movements.
Now you say the model can explain almost all the observed patterns?
SHADMEHR: Yes. One dilemma we still need to resolve is why our motor adaptations are asymmetrical in regard to gain or loss. For example, we make a lot of mistakes as we weaken, but mistakes when we get stronger are rare. Also, most of our work focuses on internal changes—aging, fatigue, disease—that can affect movement. In the real world, we need to account for the environment as well, so we want to expand our models to include external changes.
And these environmental changes lead to some intriguing questions, right?
SHADMEHR: I think this is a very exciting area because we start delving into issues of memories and awareness. For example, if you transfer a fish from water to a more viscous liquid, swimming becomes harder, and the fish wonders, Is this because of changes in my muscles, or changes in the environment? So this raises the possibility that our image-forming memories might have arisen from motor memories about external changes.
Reza Shadmehr on Rehabilitation and the Brain
Reza Shadmehr, Ph.D., is a professor of biomedical engineering at the Johns Hopkins University School of Medicine. He studies how the human brain perceives the world, learns and controls movements.