Here is a recent fascinating paper (original study and a nice comment). Briefly, the study shows that absolute pitch — the ability to identify a note when played on its own — is not really that absolute. In one of the experiments, subjects with absolute pitch listened to a piece of music that was slightly detuned. For listeners with absolute pitch this detuned music established a new reference point. After listening, their internal map of pitches shifted, and they identified notes in accord with the detuned reference they had just heard.
John Lienhard pointed me to his related radio episode. He points out that even people without absolute pitch will sing a familiar song in the original key. However, the experiment above points to how flexible our minds can be. Our memories, and the internal categories we establish are not absolute. They can be shifted to adjust to the environment – and this will happen without us being aware of these internal changes.
Perhaps this is even more impressive than muscle memory. We do many things mechanically and unconsciously. But our unconscious brain is not dumb robot. It is flexible, and self-correcting. We are under the illusion that our ever changing mind is stable.