Abstract
Voluntary movements are believed to undergo preparation before they are executed. Preparatory activity can benefit reaction time and the quality of planned movements, but the neural mechanisms at work during preparation are unclear. For example, there are no overt changes in muscle force during preparation. Here, using an instructed-delay manual task, we demonstrate a decrease in human muscle afferent activity (primary spindles) when preparing to reach targets in directions associated with stretch of the spindle-bearing muscle. This goal-dependent modulation of proprioceptors began early after target onset but was markedly stronger at the latter parts of the preparatory period. Moreover, whole-arm perturbations during reach preparation revealed a modulation of stretch reflex gains (shoulder and upper arm muscles) that reflected the observed changes in spindle activity. We suggest that one function of central preparatory activity is to tune muscle stiffness according to task goals via the independent control of muscle spindle sensors.
Highlights
A key mission in sensorimotor neuroscience is to understand the function and consequence of “preparatory activity”: the vigorous changes in neural activity that occur in multiple areas of the brain before a planned voluntary movement [1,2,3]
Our results demonstrate that movement preparation involves goal- directed tuning of muscle spindle receptors and stretch reflex gains
We suggest that one function of central preparatory activity is to adjust muscle mechanical compliance according to task goals
Summary
A key mission in sensorimotor neuroscience is to understand the function and consequence of “preparatory activity”: the vigorous changes in neural activity that occur in multiple areas of the brain before a planned voluntary movement [1,2,3]. A previous claim that preparatory activity represents a subthreshold version of movement-related cortical activity [2] has been contradicted more recently in support of the notion that preparation sets another initial dynamical state that promotes execution of the planned movement [7]. It is unclear what this initial state entails and by which neural mechanisms exactly the benefits of movement preparation are realized. Recent behavioral findings indicate that preparation is mechanistically independent from movement initiation, with a distinct neural basis [10]
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