Abstract
Pronounced activity is observed in both hemispheres of the motor cortex during preparation and execution of unimanual movements. The organizational principles of bi-hemispheric signals and the functions they serve throughout motor planning remain unclear. Using an instructed-delay reaching task in monkeys, we identified two components in population responses spanning PMd and M1. A "dedicated" component, which segregated activity at the level of individual units, emerged in PMd during preparation. It was most prominent following movement when M1 became strongly engaged, and principally involved the contralateral hemisphere. In contrast to recent reports, these dedicated signals solely accounted for divergence of arm-specific neural subspaces. The other "distributed" component mixed signals for each arm within units, and the subspace containing it did not discriminate between arms at any stage. The statistics of the population response suggest two functional aspects of the cortical network: one that spans both hemispheres for supporting preparatory and ongoing processes, and another that is predominantly housed in the contralateral hemisphere and specifies unilateral output.
Highlights
In the primate cortex, direct control of arm movement is primarily mediated by contralateral descending projections [1,2,3]
We analyzed the responses of individual neurons in the motor cortex and modeled their patterns of coactivity across the population to characterize the changes that distinguish left and right arm use
Across preparation and execution phases of the task, we found that signals
Summary
Direct control of arm movement is primarily mediated by contralateral descending projections [1,2,3]. Units in primary motor cortex (M1) mainly become active during movement itself and show a pronounced contralateral bias [8]. This suggests a transition from abstract planning to explicit specification of motor output parameters in the signals of individual neurons. A similar transition has been shown in the activation of different cell-types within rodent motor areas [17,18] These studies have found that neurons with intracortical projections show little lateral bias, during pre-movement phases. Neurons with descending output display much stronger laterality, especially just before and during movement This adds yet another level of granularity in the discussion of lateralized motor function. These single-unit studies support a notion that there exist two distinct components within the motor cortex: one that is bilateral and likely involved in abstract processing, and another that is dedicated to a single side of the body for execution
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