Sleep consolidates the effector-independent representation of a motor skill

During off-line consolidation a motor skill becomes less vulnerable to interference (stabilisation) and improves in performance (enhancement). Here we examined whether off-line consolidation contributes to the process of generalisation in the extrinsic and intrinsic coordinate frame in the motor domain. Participants trained with the left hand a sequential finger tapping task that has proved sensitive to off-line consolidation. Generalisation was tested by the ability to transfer the original sequence (extrinsic transformation) or the mirror sequence (intrinsic transformation) to the right hand, and this was compared with performance on a new sequence not learned before. To determine acute effects on generalisation, transfer was assessed immediately after training of the left hand. To study the effects of off-line consolidation participants were tested after an interval of daytime (training at 8 am and retrieval test on the transfer sequences at 8 pm) or after an interval of night-time sleep (training at 8 pm and retrieval test on the transfer sequences at 8 am). Acutely, training of the left hand induced significant transfer effects to the right hand for the extrinsic transformation of the sequence, but there was no advantage for the intrinsic transformation. After a period of daytime wakefulness the transfer from the left to the right hand for the extrinsic sequence transformation had vanished and, again, there was no transfer effect for the intrinsic transformation of the sequence. By contrast, nocturnal sleep saved the initial transfer effect for the extrinsic sequence transformation. The intrinsic sequence transformation was not affected by sleep. Our results show that an effector-independent representation in an extrinsic co-ordinate frame of a skill develops soon after initial training. Sleep has the capacity to consolidate this transfer and, in this way, contributes to the generalisation of a motor skill.