As pianists, when we’re playing chamber music or an accompaniment, invariably we memorize the hard parts. Because we’ve practiced them the most, obviously.
I should like to add to this. Certainly we practice them the most, but why memorize? I bet there are a hundred things we’ve practiced a ton that we haven’t memorized.
The hardest parts in chamber music tend to be the most widely-spaced, uncomfortable or intricate passages. Large leaps in both hands, strange patterns, cross-hands, nested hands, that sort of thing. Acrobatics that we actually have to watch, to make sure we’re hitting the right notes. So we practice, and practice, and after a while we have “muscle memory,” and our hands “know” where to go. They can jump two octaves and a fourth in opposite directions and always land.
The problem is this: we may not notice it, but when we’re reading the music in those nasty passages, our eyes are constantly flicking toward the keyboard. We don’t even notice it unless we’re thinking about it, and it’s only for a fraction of a second to gain a point of reference. We always do this when reading music, but much more frequently in the hard passages.
So now, imagine your eyes flicking back and forth from your hands to the music three or four times a second. Practice that passage for half an hour. An hour. What happens?
You get dizzy.
Memorizing those passages is not only a result of practicing, it’s a necessity of the passage – to keep from losing balance and orientation at the keyboard. Once it’s memorized, we only have to look at our hands, and we’re much less likely to fall over.
(This happened twice today before I figured out what was going on, and realized the rapid flicking motion of my eyes over an hour and a half was probably simulating a seizure, and definitely throwing off my inner ear.)