I listen to a lot of music.
Music is playing in the background most of the time whether I'm on the computer or not (thanks to my old, poorly-designed but still faithful Shuffle). However I've noticed that I find myself turning off iTunes when I'm writing.
Right now, for example, I have just put this song on pause because I'm writing this post (a post about why I paused that song - bit of a chicken-and-egg situation there.) I can't write with a song on, because the lyrics would be distracting.
However, I don't always do this when I'm typing. With some songs, and some kinds of writing, it's OK. I think this is how it works:
Instrumental songs are obviously OK. But I don't listen to many.
More interestingly, songs I've listened to many times are fine. I've just put on this which, according to iTunes, I have listened to no fewer than 140 times over the last three years. And this is fine. No distraction. I think the reason must be that I'm so used to the lyrics that the language part of my mind no longer needs to work out what they mean.
Some kinds of writing are compatible with songs. Blogging isn't and writing "important" emails aren't but a lot of emails are. Which I guess means that I'm not really putting much effort into writing them. I must be typing on auto-pilot, just repeating stock phrases ("Sounds good") rather than actually using my language areas, or at least, not using them very hard.
Psychologists are fond of using these kinds of selective distraction tasks to map out the architecture of the mind e.g. verbal ones distract verbal working memory but not spatial, and vice versa. So this is all pretty standard stuff, but what's interesting is that it's not intuitively obvious.
It doesn't feel like sometimes when I'm writing my language faculty is hard at work, and other times it's not. It feels like I'm thinking about what I type all the time. It's just typing. Sometimes I'll be replying to a bunch of emails, music on full blast, and then I'll find myself putting it on pause when I get to one particular email; but I couldn't tell you in advance which one it would be. It just feels right. Better turn the music off for this one, this one's serious - though even that's putting it too strongly. It doesn't feel serious, it just feels like the music needs to be off.
Our concious experience is smooth and seamless even though we're constantly switching between using different parts of our brains. This becomes all too evident in the case of brain lesions, which can rob us of capacities we never knew we had, because they were always there when we needed them. Some lesions, for example, render you completely unaware of anything that happens to your left. It doesn't seem like we're using a different part of our brain when dealing with stuff on the left as opposed to the right - but we are.