Polyphony and Perception

It is often stated by human physiology researchers that a human brain can only accurately follow two simultaneous independent musical voices (parts).

That means every piece of music with more than a two parts polyphony will be somewhat reduced in our perception to a two part structure by merging some voices together. Parts bearing the closest semblance to each other will be merged into one to form a two-note homophonic structure which then can be easily followed.