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Development and plasticity of complex movement representations.

The mammalian motor cortex is topographically organized into representations of discrete body parts (motor maps). Studies in adult rats using long-duration intracortical microstimulation (LD-ICMS) reveal that forelimb motor cortex is functionally organized into several spatially distinct areas encoding complex, multi-joint movement sequences: elevate, advance, grasp, retract. The topographical arrangement of complex movements during development and the influence of skilled learning is unknown. Here we determined the emergence and topography of complex forelimb movement representations in rats between postnatal days (PND) 13 to 60. We further investigated the expression of the maps for complex movements under conditions of reduced cortical inhibition and whether skilled forelimb motor training could alter their developing topography. We report that simple forelimb movements are first evoked at PND 25 and are confined to the caudal forelimb area (CFA), whereas complex movements first reliably appear at PND 30 and are observed in both the caudal and rostral forelimb areas (RFA). During development, the topography of complex movement representations undergoes reorganization with 'grasp' and 'elevate' movements predominantly observed in the RFA and all four complex movements observed in CFA. Under reduced cortical inhibition, simple and complex movements were first observed in the CFA on PND 15 and 20 respectively, and the topography is altered relative to a saline control. Further, skilled motor learning was associated with increases in 'grasp' and 'retract' representations specific to the trained limb. Our results demonstrate that early-life motor experience during development can modify the topography of complex forelimb movement representations.