Motor learning transiently changes cortical somatotopy.

Learning a complex motor skill is associated with changes in motor cortex representations of trained body parts. It has been suggested that representation changes reflect the storage of a skill, i.e., the motor memory trace. If a reflection of the trace, such modifications should persist after training is stopped for as long as the skill is retained. The objective here was to test the persistence of learning-related changes in the representation of the forelimb of the rat after learning a reaching task using repeated epidural stimulation mapping of primary motor cortex. It is shown that the forelimb representations enlarge after 8 days of training (n=8) but contract while performing arm movements without learning (n=7, p=0.006); hindlimb representations remain unchanged. Enlargement correlated with learning success (r=0.82; p=0.012). Subsequently, after 8 days without training, representation size reverted to baseline while the motor skill was retained. Somatotopy remained unaltered by a second training phase in which performance did not improve further (n=5). These findings suggest that successful acquisition but not storage of a motor skill depends on cortical map changes. The motor memory trace in rats may require changes in motor cortex organization other than those detected by stimulation mapping.