As a musician for whom memorising music is important, I'm fascinated by memory. As I've also realised, however, memorising isn't just for music.
Although my visual memory isn't very good, I have a highly developed
memory for touch. I use this (muscle memory) to memorise music, but it can also take on a different and rather bizarre form. I often form a memory of
a particular sensation, such as the pain of a badly bruised toe or the
ache from the prick of a needle, and this can be randomly
triggered so I actually experience that sensation again, long after it
originally ocurred. I can't make this happen at will and I can't figure
out what causes it, but it's quite fascinating.
A short while ago I decided to learn Russian, because I love Russian literature, music and so forth and thought it would interesting to learn the language too. I've previously dabbled in Latin and Greek (a long time ago!) and, more recently, French. What I've discovered is that there are lots of parallels between music and language.
Learning the Russian Cyrillic script (which consists of 33 letters!) is rather like learning to read written music; learning grammar is like learning theory - harmony, counterpoint and structure; and learning vocabulary is basically memorising. However, I'm a lot better at memorising music than I am at memorising words, and the reason is that there is a physical, spacial dimension to learning a piece of music.
I know that different pianists have very different techniques of memorising, and I can really only speak for myself when I say that I rely almost entirely on muscle memory. That is, I form a memory of the spatial relationships between intervals, the way the keyboard feels under my hands, how much I have to stretch my fingers or reach with my arms to hit the right notes, and so forth. Of course I also make conscious decisions - 'here I have to play this note and that will lead me on to this section' - that sort of thing, but these are only momentary, and help me get through 'change-over points' in the musical structure. Once I've memorised a piece, I don't have to think about it - it just plays itself, allowing me to put as much feeling and expression into it as I like. In fact, as soon as I start thinking about what I'm doing, I'm likely to forget. My fingers know where to go, and thinking is a distraction.
I've heard a lot of people say that muscle memory is unreliable, and shouldn't be used for long pieces. Some people seem to be able to visualise the score in their head, but I have a very poor visual memory and although I could easily sing any of the pieces I'm learning at the moment, knowing what a piece sounds like does not equate to knowing where to put your fingers.
I can't consciously memorise an extensive section of music, either: I do, however, do this for very small sections that I'm having trouble remembering because they signal an important harmonic change in an otherwise uniform pattern that could be repeated endlessly without ever moving on. I have been known to start 'looping' a section of music like this (a notable example being Chopin's Etude Op. 25 No. 12, which can pretty much go on forever if you keep taking the 'wrong turn'!)
I often find it helpful to say to myself 'I have to play a B flat here' or 'This progression is a series of arpeggiated diminished 7th chords' to memorise an elusive passage. Once I've got it right enough times, I develop a spacial memory of it anyway and no longer need to give myself clues.
Memorising Russian vocabulary is a completely different matter! Although I've switched my Facebook and Twitter accounts to Russian and stuck a picture of a Cyrillic computer keyboard to my laptop, I'm not really using Russian on a regular basis so I have to make an effort to ensure I can remember the words I've already learnt. A technique I've found useful is to think of an English word or set of words that sounds a bit like a Russian word I'm trying to memorise (the results are often hilarious.) Unfortunately this doesn't guarantee I'll remember what the word is meant to mean!
I'd be interested to know what techniques other people use to memorise things, whether it be music, a language, or something else entirely! And I'm particularly interested in what pianists think about muscle memory and whether it's a good thing or not.