I also seemed to be musically impaired for sight reading: took piano lessons for 3 years and could never read more than the melody line with any semblance of speed. At the same time I was able to work out tunes like Tiajuana taxi by ear...

By the way, my first instrument was whistling, which is a strictly "thought" instrument. Actually it's probably right brain stuff-- one can whistle pretty well while doing or thinking about other things. I sort of have a radio going in my head a lot of the time, whether I'm whistling or not.

When improvising with others, the trick is to listen and react to what you hear immediately, coming from the others and yourself. When you dive in on a harmonica, you don't necessarily know what the first sound will be that emerges, but if you want to sound good you have to figure how to get to the good notes in a hurry...