Motor cortex is required for flexible but not automatic motor sequences

Abstract

How motor cortex contributes to motor sequence execution is much debated, with studies supporting disparate views. Here we probe the degree to which motor cortex's engagement depends on task demands, specifically whether its role differs for highly practiced, or 'automatic', sequences versus flexible sequences informed by external events. To test this, we trained rats to generate three-element motor sequences either by overtraining them on a single sequence or by having them follow instructive visual cues. Lesioning motor cortex revealed that it is necessary for flexible cue-driven motor sequences but dispensable for single automatic behaviors trained in isolation. However, when an automatic motor sequence was practiced alongside the flexible task, it became motor cortex-dependent, suggesting that subcortical consolidation of an automatic motor sequence is delayed or prevented when the same sequence is produced also in a flexible context. A simple neural network model recapitulated these results and accounted for the underlying circuit mechanisms. Our results critically delineate the role of motor cortex in motor sequence execution, describing the conditions under which it is engaged and the functions it fulfills, thus reconciling seemingly conflicting views about motor cortex's role in motor sequence generation.

The hippocampus pre-orders movements for skilled action sequences

“…Interestingly, this mapping breaks down when highly trained sequences share elements with other sequences the animal has been exposed to, e.g. lever presses appearing in a different order, with the skill becoming once-more cortically-dependent 64 . This is typical for many motor skills in humans, such as typing, handwriting and music production where elements are reused in different permutations, including highly overtrained sequences such as signing with one’s name or writing “Best wishes”.…”

An output-null signature of inertial load in motor cortex

“…Correlations between cortical neurons and the mapping between neural and muscle activity can change substantially between tasks 32,57 , though work in the cat suggests this mapping is preserved between voluntary gait modification and reaching 58 . In rats, lesions to motor cortex impair learning of an interval timing task, but do not affect performance if delivered after the task has been learned 59 , and the necessity of motor cortex for a task can depend on the preceding training regimen 60 . Meanwhile, studies of neural population dynamics in reaching primates have emphasized the significance of cortical dimensions that are decoupled from movement and contribute to internal computations during motor preparation 21,22 , initiation 49,61 , and learning 62,63 .…”