Thanks, leeneia.

You either can or cannot get your muscles to obey your brain's commands.

Aye. There's the rub. My brain can't give the commands necessary to "follow the rhythm," because somewhere between the sound coming in, and the action going out, it's the command gets scrambled.

The online video game I linked to, above, provides precisely the sort of practice that you recommend. And it was through the process of trying out that sort of practice that I finally figured out that my difficulty with rhythm is all the way up at the command level.

I had a dorm mate in college, for example, with two "perfectly normal" eyes and optic nerves (apart from slight nearsightedness), but could not see in three dimensions, because the part of her brain that decodes the signals from her eyes didn't work normally. And now, I think I have a similar decoding difficulty with rhythm (and I now suspect that that decoding difficulty may have as much to do with my difficulty walking as with my funky muscle tone and range of motion).