In terms of ear training, visualization refers to the ability to mentally 'see' how you would play a fragment of music on your instrument. Once you are able to sing a phrase you hear, the next step is to visualize how you would actually play it.
For example, if somebody asked you to start from middle C and play the melody for 'Happy Birthday' on your instrument, can you first see clearly in your mind's eye where you would put your fingers? Advanced improvisors, composers and 'play-by-ear' musicians are able to do this for a wide range of melodies and chord progressions. They can 'see' exactly how to play a phrase before they actually play it. This allows them to take the music they hear in their minds and translate it almost instantaneously to their instruments.
Most of us, however, have not yet achieved this level of visualization prowess. We hear a phrase and then stumble around on our instruments until we find the right finger combination to play it. Or worse, we might not even know where to begin. (If you fall into this second category, be sure to see Play-by-ear Tip #5: Discover Your Voice).
Like most skills, visualization can be developed with practice. As always, the trick is to start with short, frequently used melodic patterns and build gradually from there. Repetition is key! Our game 'Parrot Phrases' is designed to help you develop your visualization skills. If you haven't already, I recommend you give it a try, and start a routine today for developing this important musical skill.