I just learned the physiology behind Matt Damon's losing his memories but still retaining all his ass kicking skills in the Bourne movies.
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His cerebellum is still intact, more specifically the lateral hemipsheres of the "little brain", which is where complex motor habits/skills are stored. So even if you lost your memory due to brain damage, as long as your cerebellum is intact, you might still have the ability to let's say, play the piano.