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

Rats were trained in a lateralized reaching motor task during either an 'ea rly' (22-31 days old) or a 'late' (62-71 days old) postnatal period. The 'l ate' group showed significant neuronal density reduction in cortical layers II-III of the contralateral motor forelimb representation. The 'early' gro up evidenced a similar localized contralateral effect that persisted after a subsequent period without training. Furthermore, in this group, a bilater al overall decrease in neuronal density was found throughout the motor cort ex. This bilateral experience and age-dependent effect is conceivably relat ed to a critical period of motor cortical development. The localized reduct ion of neuronal density strongly indicates a morphological expression of th e motor engram. Our present study supports the concept that the acquisition and retention of motor learning involves the persistence of structural cha nges in the brain. (C) 1999 Elsevier Science B.V. All rights reserved.