Virtuosos Do It
Famous composers, performers and virtuosos have all been know to do it; they regularly practice away from their instrument.
Virtuosos have been known to practice their entire performance while traveling or in a hotel room. They picture themselves sitting at the piano. They see all of the keys and their hands. They see and hear every note that they're going to play. It's like they're there in the practice hall but it's all happening in their mind. It's not only performers who do this. Dancers and all sorts of physical performers regularly go through their entire routine without ever leaving their chair.
Fantasy vs Reality
There's a famous experiment where a group of ordinary people are tested on their ability to shoot free throws. There are three groups of people, all inexperienced at the game of basketball. They are all tested at the beginning of the experiment to see how many free throws they can get. They are then separated into 3 groups. Group A is set up to practice shooting hoops for a half an hour everyday. Group B is not allowed to practice at all. And, Group C is instructed to 'imagine' shooting hoops for half an hour everyday. The results were surprising. Group A scored the same or worst. Group B scored slightly better than their initial score. Most surprising was the fact that Group C had the greatest increase of shots scored . Remember this group had only practiced the exercise in their mind. They, like Group A, hadn't touched a basketball for over a month.
No Difference to Me
The fact is, when it comes to scenarios like this, the brain doesn't differentiate fantasy from reality. Imagining shooting baskets and actually doing the exercise has the same effect on our brain. The best part of doing the visualizations as opposed to the actual exercises is that in our brain, is we can execute the exercise perfectly. We can slow it down, speed it up, play the hardest parts with no effort at all. Best of all, to your mind, it's like you're really doing it. The same neural pathways and memory functions are being used. Much like reiterating your last chemistry lesson in your head, these mental exercises reinforce what you've already learned. It's one of the best ways to review and get the material completely ingrained in your mind.
The Practice Session
Obviously one of the best applications of this technique would be running through an upcoming performance, but there are other great uses. If a scale or some new chords have just been introduced, playing through them in your head is a great way to remember them. If you're having trouble with a difficult part, it's useful to go over it without your instrument. It may shorten the time it takes to learn it, eliminating the problem of practicing your mistakes. If you're learning new a new solo, being able to 'hear' and 'see' the solo in your head, makes it a lot easier to play it the next time you head to rehearsals. It's great for memorizing scales, chords, chord progressions, theory and of course, entire songs and performances. The applications really are unlimited.
Songwriting
One thing that may not be obvious is writing songs away from your instrument. After all the instrument is integral isn't it? In fact, you may find some interesting things happening when you start writing songs without any instrumentation. First of all it makes you focus entirely on the melody and lyrics. There is no harmony initially, there is only the melody. Instead of trying to find a melody to another framework, you focus entirely on the melody, making it as memorable as you can. You may find that after a while you may hear the harmonies and chords in your head. If fact, you may envision the entire arrangement before even touching an instrument. Initially you may want to start with something simple and work from there. I'm even suggesting that you start with no accompaniment at all, not even a beat. I mention the beat because it's so important in our music and there is a whole way of working where you write melodies and songs with only beats, and work on the chords and accompaniment later. It's important to note here that I'm talking about a bare beat and nothing with implied harmonies.
Not An Option
If you really want to make the most of your practice sessions, if you want to improve on your instrument and get better in a shorter time, this is something you have to do, The results can be amazing. Suddenly, things that were 'alright' and 'sort of there' are much more concrete. You'll find yourself getting a lot better between practice sessions. You may be able to practice a lot more and make use of time that would otherwise be wasted. Start simple. Try playing through your scales in your head. Then try your chords. Try and 'see' everything you would in your practice session; your hands, your instrument, the music in front of you, and the sound of your instrument. The more detail, the better. Try and hear as much as you see. If you're not used to visualizing, it may be tough to start. Find some material on doing visualizations and use those techniques in your sessions. Who knows, you're greatest performance, your greatest song, may be one visualization away.