ReviewCentral mechanisms of motor skill learning
Introduction
Neuroscience has evolved from the study of simple behaviors to examinations of complex behaviors. In particular, we are beginning to learn more about complex motor behaviors. We are usually unaware of how intricately our tongue moves during conversation and how elaborate our finger movements are during typing. Such awesome but implicit complexities had discouraged scientific approaches to skilled behaviors until recently.
A major breakthrough occurred when human imaging studies were developed. Recent imaging studies have addressed complex motor learning in human subjects. Their remarkable results have promoted neural theories of motor learning and have also renewed interest in studies of motor control on animal subjects.
In this review, we integrate diverse data obtained recently on the motor control of complex behaviors and provide a common ground for researchers working on motor skill learning. Due to space limitations, we leave out several important topics in motor learning, including visuomotor associations, sensorimotor adaptations, cellular mechanisms of neural plasticity, and motor learning in birds.
Section snippets
Multiple neural mechanisms for motor skill learning
A complex motor skill is often composed of a fixed sequence of movements 1., 2.. It has been suggested that the supplementary motor area (SMA) plays an important role in sequential movements [3]. By training monkeys to perform different movements in specific orders, Shima and Tanji [4•] found that many neurons in the SMA become active specifically at particular transitions, not in response to particular movements. Neurons in the presupplementary motor area (preSMA), a cortical area anterior to
Learning is optimized by the basal ganglia and the cerebellum
In addition to human functional imaging studies 22., 23., several lines of evidence suggest that both the basal ganglia (BG) and the cerebellum (CB) are involved in motor sequence learning. Several studies implicate the BG. Activity of monkey caudate neurons is related to spatial sequence [24]. Dopamine depletion disrupts skilful performance of sequential movements [25]. Population activity of striatal neurons changes with long-term motor learning [26]. Reversible blockade of the anterior
Rules, concepts, and models for motor learning
Having reviewed the literature on motor skill learning, we are struck by the diversity of brain structures and mechanisms that are supposedly responsible for motor skill learning. To understand the nature and mechanisms of motor skill learning, it is necessary to integrate such diversity of information into schemes or models 2., 33••., 41., 42., 43., 44., 45.. To make such attempts realistic, the concepts of coordinate transformation and loop circuits must be incorporated. For simple reaching
Conclusions and future directions
Motor skills emerge from our experience, not from knowledge, as they easily escape our consciousness. Naturally, we acquire many motor skills and execute them without awareness. Such ever-changing and hidden properties of motor skills have impeded analytical approaches. The discovery of synaptic plasticity in single neurons was revolutionary, but was far from sufficient to explain motor skills. Recent integrative and multidisciplinary approaches have begun to suggest that essential features of
Update
Lu et al. [66••] have recently found that many neurons in the supplementary eye field (SEF) were active in specific learned sequences of saccadic eye movements. These data, together with the preceding data on the SMA and preSMA, suggest that the medial frontal cortex represents learned sequences of eye–hand movements. They further suggest that the relationship between the eye and hand mechanisms is flexible, being either independent or well-coordinated, depending on the context or the level of
Acknowledgements
We thank Miya Kato Rand, Shigehiro Miyachi, Xiaofeng Lu, and Satoru Miyauchi for collaborative works and Johan Lauwereyns for helpful comments. This work was supported by Grant-in-Aid for Scientific Research on Priority Areas of the Ministry of Education, Culture, Sports, Science and Technology (MEXT) and the Japan Society for the Promotion of Science Research for the Future program.
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
• of special interest
•• of outstanding interest
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