Detection of task-relevant and task-irrelevant motion sequences: application to motor adaptation in goal-directed and whole-body movements

Abstract

1Motor variability is inevitable in our body movements and is discussed from several various perspectives 2 in motor neuroscience and biomechanics; it can originate from the variability of neural activities, it can 3 reflect a large degree of freedom inherent in our body movements, it can decrease muscle fatigue, or it 4 can facilitate motor learning. How to evaluate motor variability is thus a fundamental question in motor 5 neuroscience and biomechanics. Previous methods have quantified (at least) two striking features of motor 6 variability; the smaller variability in the task-relevant dimension than in the task-irrelevant dimension 7 and the low-dimensional structure that is often referred to as synergy or principal component. However, 8 those previous methods were not only unsuitable for quantifying those features simultaneously but also 9 applicable in some limited conditions (e.g., a method cannot consider motion sequence, and another 10 method cannot consider how each motion is relevant to performance). Here, we propose a flexible and 11 straightforward machine learning technique that can quantify task-relevant variability, task-irrelevant 12 variability, and the relevance of each principal component to task performance while considering the 13 motion sequence and the relevance of each motion sequence to task performance in a data-driven manner. 14 We validate our method by constructing a novel experimental setting to investigate goal-directed and 15 whole-body movements. Furthermore, our setting enables the induction of motor adaptation by using 16 perturbation and evaluating the modulation of task-relevant and task-irrelevant variabilities through 17 motor adaptation. Our method enables the identification of a novel property of motor variability; the 18 modulation of those variabilities differs depending on the perturbation schedule. Although a gradually 19 imposed perturbation does not increase both task-relevant and task-irrelevant variabilities, a constant 20 perturbation increases task-relevant variability. 21 In our daily life, we can repeatedly achieve desired movements, such as grasping a cup, throwing a ball, 23 and playing the piano. To achieve the desired movements, our motor system needs to resolve at least two 24 difficulties inherent in our body motion [1]. A difficulty is movement variability. Due to the variability 25 inherent in various stages such as sensing sensory information [2], neural activities in motor planning [3], 26 or muscle activities in motor execution [4], even sophisticated athletes and musicians cannot repeat the 27 same movements. Our motor systems somehow tame those variabilities to achieve the desired movements 28 [5]. Another difficulty is a large degree of freedom (DoF) inherent in our motor system [1, 6]. The number 29 of joints, muscles, and neurons are more than necessary to achieve the desired movements, resulting in 30 an infinite number of joint configurations, muscle activities, and neural activities that can correspond to 31 the desired move...