A physiological signal that prevents motor skill improvements during consolidation.

Different memories follow different processing pathways. For example, some motor skill memories are enhanced over wakefulness, whereas others are instead enhanced over sleep. The processing pathway that a motor skill memory follows may be determined by functional changes within motor circuits. We tested this idea using transcranial magnetic stimulation to measure corticospinal excitability at 6, 21, 36, 96, and 126 min after participants learnt tasks that either were or were not enhanced over wakefulness. There was no change in corticospinal excitability after learning a motor skill that was subsequently enhanced; whereas, there was a substantial transient decrease in corticospinal excitability after learning a motor skill that was not enhanced. In subsequent experiments, we abolished the decrease in corticospinal excitability by applying theta burst stimulation to either the dorsolateral prefrontal or primary motor cortex, and induced motor skill improvements during consolidation. The motor skill improvements in each experiment were correlated with the corticospinal excitability after learning. Together, these experiments suggest that corticospinal excitability changes act as a physiological signal, which prevents improvements from developing over wakefulness, and so when this signal is abolished improvements are induced. Our observations show that the human brain can actively prevent the processing of memories, and provides insights into the mechanisms that control the fate of memories.