Before voluntary movement, population activity within motor cortex establishes a preparatory state that is proposed to seed movement-generating dynamics. Yet this view derives largely from discrete movements; real-world movements are often executed within longer sequences. In such cases, does motor cortex simply prepare and execute each component movement sequentially? Or are movements prepared and executed holistically, as a single ‘chunk’?
To distinguish between these potential strategies, we trained a monkey to perform single and compound reaches, recorded from individual neurons in motor cortex, and isolated population-level preparatory activity. While behavior and muscle activity supported the holistic strategy, we found clear evidence for the sequential strategy within motor cortex.
These results demonstrate a surprising feature of motor cortex: even when performing simple, well-learned motor sequences, each component movement is prepared and executed sequentially. Rapid sequencing was not achieved through a holistic strategy but rather by preparation for subsequent movements occurring simultaneously with the execution of current reaches.
Laureline Logiaco, Postdoc (Abbott lab)
Flexible, robust and efficient motor sequencing through thalamic control of cortical dynamics
The mechanisms by which the brain can learn an extensible library of motor motifs and flexibly string them into arbitrary sequence orders are unclear. To investigate this issue, we develop an anatomically constrained model in which inhibitory basal ganglia (BG) output neurons project to thalamic units that are themselves bidirectionally connected to a recurrent cortical network. During movement sequences, BG neurons show sustained activity that switches at the boundaries between sequence components. Assuming that these BG signals are many thalamic neurons' dominant source of input, we find that the remaining thalamic neurons that freely interact with cortex can act as a powerful controller of cortical dynamics, hence giving thalamocortical loops the ability to direct robust and flexible motor sequences.