On the Encoding Capacity of Human Motor Adaptation

[ Information ].

Journal of Neurophysiology, vol. 126, no. 1, pp. 123-139, 2021.

[ Authors ]

Seungyeon Kim, Jaewoon Kwon, Jin-Min Kim, Frank Chongwoo Park, and Sang-Hoon Yeo

[ Abstract ]

Primitive-based models of motor learning suggest that adaptation occurs by tuning the responses of motor primitives. Based onthis idea, we consider motor learning as an information encoding procedure, that is, a procedure of encoding a motor skill intoprimitives. The capacity of encoding is determined by the number of recruited primitives, which depends on how many primitivesare“visited”by the movement, and this leads to a rather counterintuitive prediction that faster movement, where a larger num-ber of motor primitives are involved, allows learning more complicated motor skills. Here, we provide a set of experimentalresults that support this hypothesis. First, we show that learning occurs only with movement, that is, only with nonzero encodingcapacity. When participants were asked to counteract a rotating force applied to a robotic handle, they were unable to do sowhen maintaining a static posture but were able to adapt when making small circular movements. Our second experiment fur-ther investigated how adaptation is affected by movement speed. When adapting to a simple (low-information-content) forcefield, fast (high-capacity) movement did not have an advantage over slow (low-capacity) movement. However, for a complex(high-information-content) forcefield, the fast movement showed a significant advantage over slow movement. Ourfinal experi-ment confirmed that the observed benefit of high-speed movement is only weakly affected by mechanical factors. Taken to-gether, our results suggest that the encoding capacity is a genuine limiting factor of human motor adaptation.