In 1995 Alvaro Pascual-Leone had a group of volounteers practice a five-finger piano exercise, and a comparable group merely think about practicing. They focused on each finger movement in turn, essentially playing the simple piece in their heads, one note at a time.
 
Actual physical practice produced changes in each volounteers motor cortex, as expected. But so did mere mental rehearsal, and to the same degree as that brought on by physical practice. Motor circuits become active during pure mental imagery. Like actual, tphysical movements, imagined movement trigger synaptic change at the cortical level. Merely thinking about movement produced brain change comparable to those triggered by actual movement.
 
Journal of Neurophysiology, 74,  pp 1037 - 1045