Persistent neuronal density changes related to the establishment of a motor memory

Abstract Rats were trained in a lateralized reaching motor task during either an `early' (22–31 days old) or a `late' (62–71 days old) postnatal period. The `late' group showed significant neuronal density reduction in cortical layers II–III of the contralateral motor forelimb representation. The `early' group evidenced a similar localized contralateral effect that persisted after a subsequent period without training. Furthermore, in this group, a bilateral overall decrease in neuronal density was found throughout the motor cortex. This bilateral experience and age-dependent effect is conceivably related to a critical period of motor cortical development. The localized reduction of neuronal density strongly indicates a morphological expression of the motor engram. Our present study supports the concept that the acquisition and retention of motor learning involves the persistence of structural changes in the brain.