At the opening of the piece, where the oboe plays alone, you pay little attention to the percussion, in order to follow the oboe's melodic line. When two or three groups play together, of course you notice the interference patterns of the different percussion instruments, all the more so because each uses a different timbre; but their role remains in the background, since you are occupied with making out the melodic identity of the different groups, and following the heterophonic game through which they correspond with each other. As the number of groups increases, the density can no longer be perceived analytically, and you latch onto the rhythmic interference patterns of the percussion. The more the perception loses its footing in one realm, the more it latches onto a related realm which it had neglected up till then. The principal phenomena become the secondary, and vice versa.