"Impulse" is the first track from Ameliorate by instrumental progressive-rock band An Endless Sporadic. You might recognize this song as being a bonus track on Guitar Hero III. The "progressive" label is not to be taken lightly. After a minute or two into the song, you will wonder if you are even listening to the same song. However, the beginning riff repeats itself later in the song to remind you what you're listening to. In the middle, there is the breakdown of all breakdowns. It might sound like absolute nonsense, but it's still relatively structured... I think. If you're into instrumental rock, give "Impulse" a listen, and if you like what you hear, also check out "Anything" from the same album.