Transfer of ballistic motor skill between bilateral and unilateral contexts 2 in young and older adults : neural adaptations and behavioural implications

28 Bilateral movement rehabilitation is gaining popularity as an approach not only to improve the recovery 29 of bimanual function, but also of unilateral motor tasks. While the neural mechanisms mediating the 30 transfer of bilateral training gains into unimanual contexts are not fully understood, converging evidence 31 from behavioural, neurophysiological and imaging studies suggests that bimanual movements are not 32 simply the superposition of unimanual tasks undertaken with both (upper) limbs. Here we investigated the 33 neural responses in both hemispheres to bilateral ballistic motor training, and the extent to which 34 performance improvements transferred to a unimanual task. Since aging influences interhemispheric 35 interactions during movement production, both young (n=9; mean age 19.4 years; 6 female) and older 36 (n=9; 66.3 years; 7 female) adults practiced a bilateral motor task requiring simultaneous ‘fast-as37 possible’ abductions of their left and right index fingers. Changes in bilateral and unilateral performance, 38 and in corticospinal excitability and intracortical inhibition, were assessed. Strong transfer was observed 39 between bimanual and unimanual contexts for both age groups. However, in contrast to previous reports 40 of substantial bilateral cortical adaptations following unilateral training, increases in corticospinal 41 excitability following bilateral training were not statistically reliable, and a release of intracortical 42 inhibition was only observed for older adults. The results indicate that the neural mechanisms of motor 43 learning for bilateral ballistic tasks differ from those that underlie unimanual ballistic performance 44 improvement, but that ageing results in a greater overlap of the neural mechanisms mediating bilateral 45 and unilateral ballistic motor performance. 46 47