Auditory stream segregation is a phenomenon that I've been aware of for some time, but never knew the name of it. Basically, this is something that occurs when a listener hears a melody within a piece of music that consists of alternating high and low notes. When high notes and low notes are alternated (quickly) within a melody, the listener perceives two melodies being played rather than interpreting the notes as one continuous melody. In other words, when a melody like this is played on one instrument, the listener tends to hear the high notes as one independent phrase and the low notes as another independent phrase. There are other terms for this phenomena, such as implied polyphony (polyphony means more than one instrument) or compound melodic line.
I think that it is pretty cool how one performer can play a melody with one hand and their audience will perceive two different melodies. It makes the performer look twice as good in my opinion! As the textbook states, classical composers knew this too, and the example of auditory stream segregation given in the book is an excerpt from a Bach piece. As a musician, I am a huge Bach fan, and after listening to many of his pieces, I learned that he loved to put implied polyphonic lines in his music.
The best way to demonstrate this phenomenon is to play it and listen of course! So I recorded another Bach piece (one of his many preludes) for the purpose of showing exactly how auditory stream segregation works. As the melody is played, the ear begins to hear two different melodies. The only difference between the piece I demonstrated and the piece outlined in the text is that the text refers to a melody that only consists of high and low notes. In the video I play, the piece consists of "medium" notes too; this does not alter the effect of course, two melodies still "pop out" at the listener. I demonstrate both sets of melodies towards the end of the video.