Representational plasticity in Ml has been induced in a variety of animal models. Much of this work reconstructs movements of the fore-limb digits and wrist evoked by cortical electrical microstimulation of small neuronal assemblies to produce representational maps of movements. Studies of changing neuronal assembly representations have been combined with a search for the molecular, physiologic, and morphologic changes that accompany reorgani-zation.27,2 226 Experimental Case Studies 1-3 describes motor map plasticity in uninjured animals. Representational and morphologic changes that arise after a CNS injury are reviewed in Chapter 2.
Activity-dependent representational changes in human subjects parallel those in animal models. Motor learning induces adaptations in the cortical maps that represent movements during the training of skilled hand move-ments.227,228 A sequential finger opposition task practiced for a few minutes a day improves the speed and precision of performance, accompanied by a more extensive representation in Ml for the involved digits that evolves in stages. The cerebral activation by PET or fMRI is specific to the practiced fine motor coordination task and does not evolve with an untrained finger movement sequence. A different skilled finger sequence may overlap with elements of the same representation. Imaging also reveals fast and slow learning. An initial habituation-like decrease in Ml activation occurs during a session of practice. Several weeks of practice and the acquisition of a skilled series of movements leads to the emergence of an enlarged representation for the digits. More remarkable, the practice of just 120 synchronous movements of the thumb and foot induces representational changes in which TMS reveals a short-term change in which the cortical locus for stimulation of the thumb moves toward the foot region.229 This representational plasticity is consistent with the latent connnections of cortical motoneurons in M1 and the rather wide distribution of pyramidal projections to spinal motoneurons discussed earlier.
In normal subjects, TMS over M1, which selectively activates the corticospinal projections, has revealed representational plasticity during simple movements and with the acquisition of more complex skills. In a study of rapidly induced representational plasticity, TMS evoked either a flexor or extensor thumb movement in each subject at baseline. The subjects then practiced making repetitive thumb movements in the opposite direction. Within 5 to 30 minutes, thumb movements evoked by TMS changed to the direction of the practiced movement and this kinematic change persisted for up to 15 minutes.230 In another study, subjects practiced a five-finger piano exercise for 2 hours a day for 5 days. Transcranial magnetic stimulation-evoked movements showed an enlarging motor cortical area targeting the long finger flexors and extensors and a decreased threshold for activation as subjects learned the skill.231 In another experiment, cortical output maps to the muscles involved in a serial reaction time task, in which subjects had to learn from ongoing experience which buttons to push, enlarged as subjects learned the task.232 Up to that point, the subjects had implicit or
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