beefing up vs. slimming down

While learning to juggle appears to increase grey matter density in the part of the brain that handles visual motion, a new study suggests that training to be a professional ballet dancer appears to decrease grey matter density in brain regions controlling motor activity.

The distinction appears to be that while acute training beefs up relevant brain regions, long-term training can slim then down, by making grey matter more efficient (so that you need less of it); also, long-term practice of repetitive motor actions (like learning to ride a bicycle!) makes things less conscious and more automatic — you use your cortex less, because you're using other resources (basal ganglia; cerebellum; spinal cord) more.

The suggestion that training can beef up short-term and slim down long-term was apparently first made in 1998.