Yeah, I just recently noticed that, too. Some of the visuals seem to be tuned specifically for bass, and do practically nothing with treble tones. It's really cool watching the patterns that are generated by different bass frequencies.

Here's a fun thing with the visuals: Turn on one of the linear spectrum analyzers, preferably one of the ones with really fine spectrum lines. Then play the Madonna song "Ray of Light". Starting around 3:28 there are some synthesizer filter sweeps that you can watch animated perfectly in the spectrum graph as they go from 22khz all the way down to low bass.



-- Tony Fabris -- Empeg #144 --
Caution: Do not look into laser with remaining good eye.