Coordinated multilaminar dynamics underlie multiplexed computation in motor cortex

Functional multiplexing is a signature of higher-order brain regions. Beyond single-unit mixed selectivity, the six-layered cortical microcircuit has been proposed as an optimal substrate for such parallel computations. Recurrent connections within and across layers allow cortical columns to retain incoming information for some time and integrate it into stable latent representations, thereby acting as functional units. The key question is whether the multiplexing capacity arises from specialized processing within individual layers or the collective coordination of multi-layer activity patterns. Here, we analyzed laminar recordings from the motor cortex of macaque monkeys performing a complex delayed match-to-sample task. We identified laminarly distributed, behaviorally specific subspaces that captured the encoding of distinct task-related variables. These subspaces, spanning the entire column and expressed as coordinated activity patterns, were functionally reused to encode the same variable over time and flexibly recycled to encode new ones. Subtle variations of laminar weights gave rise to multiple coexistent laminar coding subspaces, enabling multiplexing at the columnar level. Task-related information propagated across layers in temporally organized trajectories that transiently localized in superficial or deep layers at distinct trial epochs. These organized laminar trajectories were consistently observed across recording sites, but exhibited site-specific propagation patterns. The activity of the population on the other hand lacked structured dynamics. Thus, laminar trajectories of information emerged atop a background of spatially and temporally unspecific activity-fluctuations.